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Effects of sodium sulfate on the hydration and properties of lime-based low carbon cementitious materials
Accepted Manuscript Effects of sodium sulfate on the hydration and properties of lime-based low carbon cementitious materials Meng Wu, Yunsheng Zhang, Yantao Jia, Wei She, Guojian Liu, Zhiqiang Yang, Yu Zhang, Wangtian Zhang, Wei Sun PII:
S0959-6526(19)30585-2
DOI:
https://doi.org/10.1016/j.jclepro.2019.02.186
Reference:
JCLP 15916
To appear in:
Journal of Cleaner Production
Received Date: 14 August 2018 Revised Date:
7 February 2019
Accepted Date: 17 February 2019
Please cite this article as: Wu M, Zhang Y, Jia Y, She W, Liu G, Yang Z, Zhang Y, Zhang W, Sun W, Effects of sodium sulfate on the hydration and properties of lime-based low carbon cementitious materials, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.02.186. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of sodium sulfate on the hydration and properties of
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Lime-based low carbon cementitious materials
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Meng Wua,b, Yunsheng Zhanga,b,*, Yantao Jiac,*, Wei Shea,b, Guojian Liua,b, Zhiqiang
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Yanga,b, Yu Zhanga,b, Wangtian Zhanga,b, Wei Suna,b
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School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China c College of Mechanics and Materials, Hohai University, Nanjing 211100, China b
* Corresponding author: Yunsheng Zhang, Email: [email protected]
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Abstract: To reduce the carbon footprint and energy consumption from the cement
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manufacturing industry, lime-based low carbon cementitious materials (LCM) has
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caught strong attention. LCM as a novel low-carbon cement shows impressive
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performance and promising prospects; however, its mechanical properties are inferior
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to those of ordinary Portland cement (OPC). In this study, different dosages of sulfate
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sodium was incorporated into LCM to investigate the effects of sodium sulfate on
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LCM performance. The properties and hydration of LCM with and without sulfate
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sodium were systemically investigated and analyzed. The results revealed that LCM
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blending with 2-3 wt% sodium sulfate showed the best mechanical performance. The
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compressive strength of LCM containing 3 wt% sodium sulfate was increased by 57.0%
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and 20.8% relative to the plain LCM at 3 d and 90 d, respectively. Microstructural
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characterization showed that a great amount of ettringite had formed at 3 d, which
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effectively improved the mechanical performance of LCM at early stage. Moreover,
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the addition of sodium sulfate effectively accelerated the hydration of the solid waste
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in LCM, and more hydrated lime was consumed in the hydration process. The
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ettringite became embedded in C(A)SH gel with increasing curing age, which resulted
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in a dense microstructure of hydrated paste with fewer coarse pores and an
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enhancement in the mechanical performance of LCM. Thus, the sodium sulfate
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effectively increased the strength of LCM at both early and later stage.
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Keywords: low carbon emissions; green cement; sulfate sodium; hydration products;
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microstructure;
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1. Introduction The cement industry contributes approximately 8%-9% anthropogenic carbon
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emission and 2%-3% energy use worldwide (Monteiro et al., 2017), mainly because
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the production of Portland cement requires the burning of raw materials at a high
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temperature of 1450 °C, during which a large amount of CO2 is released to the
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atmosphere. One ton of cement manufactured releases nearly one ton CO2 (Xu et al.,
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2015; Xiao et al., 2018). Nevertheless, Portland cement is widely used in construction
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and the output of global cement was approximately 4000 million tons in 2016.
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Therefore, reducing the carbon emissions of the cement industry is an issue of interest
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in the field of building materials. In recent years, increasingly more scholars have
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focused on designing and preparing low carbon cementitious materials to reduce the
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carbon emissions of cement. One of the most promising approaches is to decrease the
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Portland cement fraction in cement-based materials by replacement with
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supplementary cementitious materials (SCMs) (Avet and Scrivener, 2018; Latifi et al.,
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2018; Wu et al., 2018a; Yang et al., 2016). A large part of SCMs are currently derived
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from industrial solid waste such as blast furnace slag, fly ash, nickel slag and red mud
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(Tan et al., 2018; She et al., 2018 a). Ground blast furnace slag and fly ash, which are
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the most commonly used SCMs, have been broadly used in cement-based materials
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worldwide.
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Ground blast furnace slag is a latent hydraulic material that react directly with
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water to form a minor amount of amorphous gel; fly ash is a manmade pozzolanic
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material. Slag and fly ash can be reacted with calcium hydroxide from hydrated
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cement clinker to form hydrated calcium aluminate (C-A-H) and calcium silicate gel
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(C-S-H) (Lothenbach et al., 2011; Liu et al., 2016; She et al., 2018 b). Thus, a part of
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the cement clinker can be replaced with nickel slag, blast furnace slag, calcined clay,
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fly ash, red mud and other solid waste to prepare blended cement and other cements.
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Recently, lime-based low carbon cementitious materials (LCM) have been designed
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and prepared to further enlarge the content of industrial solid waste in cementitious
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materials (Jeong et al., 2016; Wu et al., 2018a). LCM consisted of a minor amount of
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Portland cement (typically less than 20 wt%), a moderate content of lime and a large
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amount of industrial solid waste; the resulting novel green cement shows strong
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performance according to previous report (Wu et al., 2018b). Nevertheless, the
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mechanical strength of LCM is inferior to that of cement-based materials because of
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the low activity of industrial solid waste. Therefore, the mechanical properties of
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LCM require enhancement. According to previous reports, freely soluble sulfates such
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ACCEPTED MANUSCRIPT as sulfate sodium can effectively increase the strength of binder materials containing
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fly ash or slag (Qian et al., 2001; Rashad et al., 2013; Velandia et al., 2018). It has
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reported that the soluble sulfates in cementitious materials are beneficial for ettringite
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formation and improve the mechanical properties of blends (Shi and Day, 1995; Sahin
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et al., 2016). Thus, soluble sulfates can increase the mechanical strength of LCM in
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theory. However, the optimal dosage of soluble sulfate used in LCM is unknown, and
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the specific effects of soluble sulfate on the mechanical performance, compositions of
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hydration products and microstructure of LCM are not clear. Moreover, the optimal
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amount, formation process and effects of ettringite in the hydration process of LCM
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with and without soluble sulfate are also uncertain. Therefore, the effects of soluble
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sulfates on the performance of LCM require systematically investigation.
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In this study, sodium sulfate is used as freely soluble sulfate to improve the
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performance of LCM. The sodium sulfate is easily accessible and features low carbon
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emissions and cost in production. Different dosages of sodium sulfate as an activator
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are blended with LCM to compare the effects of sodium sulfate on LCM performance.
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The strength, hydration and drying shrinkage of LCM with and without sodium
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sulfate are systematically studied and analyzed. We anticipate that this study will
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promote the development of low carbon cementitious materials.
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2. Raw materials and test methods
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2.1 Raw materials
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A commercial Portland cement (52.5 grade) that conforms to Chinese standard
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GB/T 175-2007 (similar to EN 197-1 and ASTM C150) was used in this study. Fly
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ash (type F), ground blast furnace slag, hydrated lime and gypsum were used as raw
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materials. To increase the utilization ratio of fly ash, the type F fly ash was used in
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this work to prepare LCM. River sand meeting the ASTM C778 standard was selected
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as fine aggregates to prepare mortar specimens. Sodium sulfate powder (analytical
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reagent) was used as an activator. The chemical compositions of the materials used in
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this work are presented in Table 1. The industrial grade hydrated lime that was
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approximately composed of 87% portlandite and 13% calcite was used in this study.
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The hydrated lime was significantly more stable on the specimen volume and reaction
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rate of LCM compared to the quick lime.
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Based on the previous literatures, the mineral admixtures used in this study were
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obtained via mixing with FA, GGBS and gypsum at a ratio of 0.475, 0.475 and 0.05
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by mass. Fig. 1 displays the particle size distributions of the raw materials used in this
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study.
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Table 1 Oxide compositions of raw materials PC
GGBS
FA
Lime
Gypsum
CaO Al2O3 SiO2 MgO Fe2O3 Na2O K2O SO3 LOI
64.38 4.38 21.60 3.43 3.42 0.51 2.23 2.54
37.12 15.51 32.72 5.50 0.24 0.40 0.30 2.61 0.36
4.40 30.41 51.53 0.91 6.90 0.62 1.37 0.91 1.52
97.30 0.41 0.47 1.00 0.23 0.32 0.1 26.75
46.89 0.14 0.30 0.20 0.07 0.11 52.09 7.01
80 60 40 20 0 0.1
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PC Fly ash Slag Lime
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Cumulative Volume %
100
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Material
1
10
100
Particle Size (µm)
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Fig. 1. Particle size distributions of raw materials.
2.2 Test methods
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2.2.1 Specimen preparation and curing Table 2 presents the mixture percentage of LCM activated with different amounts
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of sodium sulfate in this study. The preparation method of mortar specimens
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conformed to Chinese standard GB/T 17671-1999 (similar to EN 196-1 and ASTM
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C109); the mass ratio of river sands to LCM was fixed at 3. The sodium sulfate was
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dissolved in pre-weighed water before mixing. The mortar specimens were prepared
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in 40 * 40 *160 mm moulds, and the paste specimens were also prepared in 40 * 40 *
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40 mm moulds for hydration studies. The specimens were cured in a moist room at 20
118
± 2 °C and relative humidity (RH) ≥ 95% for 24 h before demoulding. The demoulded
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specimens were allowed to cure in a moist room until designated ages.
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Portland cement
Mineral admixtures
Hydrated lime
Sodium sulfate
w/b
N-0 N-1S N-2S N-3S N-4S
10 10 10 10 10
80 80 80 80 80
10 10 10 10 10
0 1 2 3 4
0.5 0.5 0.5 0.5 0.5
2.2.2 Mechanical properties
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Table 2 Details of mixture proportions (wt %)
According to Chinese standard GB/T 17671-1999, the strength of LCM mortar
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specimens was tested at 3 d, 7 d, 28 d, and 90 d. Three mortar specimens were used
127
for determination of flexural strength and compressive strength. The test results were
128
adopted the average value from the three measured specimens.
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2.2.3 Workability
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The fresh mortar flowability was obtained according to Chinese standard GB/T
131
2419-2005 (similar to ASTM C1437). The setting times of LCM mixtures were
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measured based on Chinese standard GB/T 1346-2011 (similar to ASTM C191).
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2.2.4 Heat of hydration
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The heat of hydration of the tested mixtures was recorded at 20 °C using a TAM
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Air isothermal calorimeter (TA Instruments, USA). An ampoule bottle filled with
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10-12 g fresh paste was put into the channel of an isothermal calorimeter, and the data
137
of heat flow were automatically measured by isothermal calorimeter.
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2.2.5 Drying shrinkage
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The drying shrinkage of mortar specimens was measured according to Chinese
140
standard JC/T603-2004 (similar to ASTM C596). After curing for 24 h in the moist
141
room, the specimens were demoulded and cured in lime-saturated water for two days.
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Subsequently, the initial length of specimens (L0) was recorded, and all the specimens
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were placed in room at 20 ± 2 °C and RH50% ± 3%. The length (Lt) of specimens in
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the drying room was recorded at the designated ages, and the drying shrinkage (εt) of
145
specimens were calculated according to Eq. (1).
146 147
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εt =
L0 − Lt L0
×100%
(1)
2.2.6 Hydration studies
148
Hydration studies were conducted on LCM paste at the designated curing age. The
149
small paste samples after crushing were immersed in ethanol for 72 h to prevent
150
further hydration. Subsequently, all the samples were dried at 50°C for 24 h in a
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vacuum drying oven, and some samples were carefully ground into very fine powder
152
samples (≤75 µm) by hand in a mortar . The mineral phases of the hydration products were characterized via a Bruker D8
154
Discovery diffractometer using a CuKα anode operating at 40 kV, 30 mA, 4°/min, and
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a range of 5° to 80° with steps of 0.02°. Before the XRD test, the fine powder samples
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were homogenously mixed with corundum (α-Al2O3≥99.99%) at a mass ratio of 9:1.
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The quantitative analysis of XRD data was performed on TOPAS software v4.0.
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Fourier transform infrared spectroscopy (FTIR) tests were performed on a Nicolet
159
iS10 infrared spectrometer via the KBr pellet method. All the samples were scanned
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32 times with a resolution of 4 cm-1 in the spectral range of 4000-400 cm-1.
Thermogravimetric analysis (TGA) tests were conducted on a NETZSCH STA449
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F3 thermogravimetric analyzer. The fine powder samples were heated from room
163
temperature to 1000 °C at a heating rate of 10 °C /min in a N2 atmosphere. The
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portlandite content in hydrated paste was computed from TGA curves by the
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tangential method, and the portlandite content was normalized to the dry sample
166
weight at 550 °C according to Eq. (2) (Adu-Amankwah et al., 2017), in which CHw is
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the wt% loss from water in portlandite as calculated by the tangent method.
M 550°C
18 ×100%
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CH =
CH w ⋅ 74
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Scanning electronic microscope (SEM) and energy dispersive spectroscopy (EDS)
170
tests were carried on the small paste samples coated with a Pt conductive film after
171
being dried. The SEM tests were perfumed on an FEI Navo Nano SEM 450 fitted
172
with a Thermo Fisher NS7 EDS analyzer.
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The small paste samples were used for mercury intrusion porosimetry (MIP) tests
174
using a Micromeritics AutoPore IV 9500. The measured pore size of sample was from
175
4 nm to 350 µm with an assumed mercury contact angle of 130 °.
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3. Test results
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3.1 Compressive and Flexural strength
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The mechanical property evolution of LCM with and without sodium sulfate is
179
shown in Fig. 2. From Fig. 2, minor amounts of sodium sulfate are found to
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significantly improve the mechanical properties of LCM mortar specimens at both
181
early and later ages. The mechanical properties of LCM improve with an increasing
182
content of sodium sulfate (from 1 to 3 %). However, the mechanical properties of
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LCM are degraded slightly when the sulfate sodium content in LCM is increased to
ACCEPTED MANUSCRIPT 4 %. The compressive strength of N-3S specimens reaches 14.6 MPa at 3 days and
185
39.5 MPa at 90 days, however, the compressive strength of N-0 specimens (control
186
specimens) reaches only 9.3 MPa and 32.7 MPa for the same curing ages, respectively.
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The compressive strength of N-3S specimens increases by 24% compared to N-0
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specimens, which indicates that the LCM blending with 3 % sulfate sodium shows the
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best mechanical properties. Note that the mechanical properties of LCM at 90 days
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are superior to the mechanical properties of LCM at 28 days, which indicates that the
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continuous hydration of GGBS and FA are beneficial to the improvement of
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mechanical strength of LCM.
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In addition, the LCM shows a satisfactory flexural strength compared to OPC. The
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flexural strength of N-3S specimens reaches 9.2 MPa at 90 days, and the compressive
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and flexural strength of Portland cement used in this study at 90 days is 63.8 MPa and
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9.3 MPa, respectively. Therefore, the ratio of flexural strength to compressive strength
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of LCM is higher than that of Portland cement. The ratio of flexural strength to
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compressive strength of Portland cement and N-3S at 90 days is 0.146 and 0.227,
199
respectively.
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28d 90d
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20
10
0 N-0
N-1S
N-2S
N-3S
(1) Compressive strength
201 202
3d 7d
28d 90d
8 6 4 2 0
N-4S
N-0
N-1S
N-2S
N-3S
N-4S
(2) Flexural strength
Fig. 2. Mechanical properties evolution of LCM with different dosages of sodium sulfate
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Flexural Strength /MPa
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Compreesive strength /MPa
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3.2 Workability
The basic properties of LCM are listed in the Table 3. The consistency of LCM is a
203
litter higher than Portland cement due to the fine hydrate lime particles. The silica
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ratio, alumina ratio and lime saturation ratio of LCM is 1.63, 5.82 and 0.28,
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respectively. The flowability of fresh LCM mortar with different content of sodium
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sulfate is presented in Fig. 3 (a). It can be founded that the flowability of fresh mortar
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with different dosages of sulfate sodium is approximately 220 mm, which meets
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construction requirements. The high sulfate dosage in LCM shows a little negative
209
influence on the flowability of fresh LCM mortar. It should be pointed that the added
ACCEPTED MANUSCRIPT sodium sulfate to fresh binder materials with a low w/b ratio may show a negative
211
effect on the dispersion of superplasticizers, particularly in polycarboxylate
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superplasticizer; because a large number of sulfate ions reduce the adsorption capacity
213
of the cement particle surface on the superplasticizer (Yamada et al., 2001). The
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setting time of LCM is shortened in the presence of sulfate sodium. From Fig. 3, the
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final setting time of N-0 and N-3S mixture is approximately 245 and 210 min,
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respectively. Therefore, the sodium sulfate improves the workability of LCM due to
217
the shortened final setting time.
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Table 3 Basic properties of LCM Soundness
Silica ratio
Alumina ratio
Lime saturation ratio
0.33
qualified
1.63
5.82
0.28
Flowability
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220
Flowability /mm
240
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Consistency
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200 40
Initial Setting Time Final Setting Time
180
120
60
0 N-0
N-1S
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20 N-2S
N-3S
0
N-4S
(a) Flowability 220
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210
N-0
N-1S
N-2S
N-3S
N-4S
(b) Setting Time
Fig. 3. Mortar flowability and setting time of LCM with sodium sulfate
3.3 Compositions of hydration products
222 223
3.3.1 XRD analyses The XRD test results of hydration products from N-0 and N-3S paste after different
224
ages are shown in Fig. 4. As shown in Fig. 4, the compositions of hydration products
225
in LCM with and without sodium sulfate are essentially the same at different curing
226
ages. The mineralogical compositions of N-0 and N-3S paste are composed of
227
portlandite, gypsum, ettringite, quartz, and mullite. Note that the mullite and quartz
228
phase in the XRD patterns are due to remnant fly ash particles in paste. The peak at
229
28.9°is assigned to the weakly crystalline C(A)SH gel, which is close to the structure
230
of aluminium-containing tobermorite (Guo et al., 2017). The corresponding peaks of
231
ettringite in Fig. 4 are detectable after 2 d, and the peaks of gypsum are only observed
232
in XRD patterns of N-0 and N-3S paste at 1 d, which suggests that a large quantity of
233
ettringite has formed at the early stage of hydration, particularly in the N-3S paste,
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because the gypsum in LCM reacts with the active aluminum phase in mineral
235
admixtures
236
(Ca4Al2(SO4)(OH)12·6H2O) was not been founded in the hydration products of LCM.
237
The monosulfoaluminate can not been formed in the pore solution with sufficient
238
sulfate ions (Matschei et al., 2007). As shown in Fig. 4, the full-width at
239
half-maximum (FWHM) values of ettringite in N-0 and N-3S paste are stable after 3 d.
240
Moreover, FWHM values of ettringite in N-3S paste are greater than that of N-0 paste
241
at all age, which verifies that the presence of sodium sulfate in LCM mixture
242
effectively increases the amount of ettringite in LCM. Therefore, the mechanical
243
performance of LCM are improved during the hydration process, particularly at 3 d.
form
ettringite.
Meanwhile,
the
monosulfoaluminate
Q: Quartz P: Portlandite O: Corundum G:Gypsum E: Ettringite M: Mullite C: C-(A)-S-H
E
E M E OQ C
PO
MO
P O
M E OQ C
E P M
90d
O
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Q: Quartz P: Portlandite O: Corundum G:Gypsum E: Ettringite M: Mullite C: C-(A)-S-H E P M
OO
PO
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28d
MO
P
O
90d O
OO
28d
7d
7d
3d
3d
2d
G
10
P M
M G G
20
OQ
G
30
P
P O
M O
40
P
O
50
1d
O
60
70
M
G
OO
2θ /°
10
20
OQ
2d
P
G
G
30
1d
O
M
40
O
P O
O
50
OO
60
70
2θ /°
(a) N-0 (b) N-3S Fig. 4. XRD patterns of N-0 and N-3S mixture at different curing age
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phase
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to
The FWHM values of portlandite in XRD patterns are substantially decreased with
246
increasing hydration time, which establishes that the glassy phase from mineral
247
admixtures react with hydrated lime and forms additional CAH and C(A)SH gel. The
248
quantitative analysis of the hydration products of LCM is computed by XRD data
249
refinement via the Rietveld method.
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Fig. 5 shows the evolution in hydration products of N-0 and N-3S paste based on
251
the refined XRD data. The amount of C(A)SH gel cannot be qualified directly
252
because a majority of the C(A)SH gel exists in amorphous phase. From Fig. 5, the
253
phases of calcium silicate consist of weakly crystalline calcium silicate hydrates
254
composed of tobermorite and rosenhahnite (Kupwade-Patil et al., 2018). The
255
undydrated mineral admixtures are composed of mullite, quartz, akermanite, gehlenite
256
and anorthite phase from unreacted FA and GGBS paticles. From Fig. 5, the
257
portlandite in the matrix is gradually reacted during the LCM hydration process. The
258
amount of portlandite of N-0 and N-3S paste at 90 d is 7.2% and 6.4% (by mass),
259
respectively, which demonstrates that the reaction degree of FA and GGBS in LCM
260
containing 3 wt% sodium sulfate is higher compared to that of the conventional LCM.
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The mineral admixtures react with portlandite and provide additional hydration
262
products in the matrix, particularly in the later period of hydration. The quantity of ettringite in the N-3S mixture is greater than that in the N-0 mixture
264
at each curing age. The content of ettringite in the hydration products of the N-3 paste
265
is 18. 2% and 17.5% at 3 d and 90 d, respectively, and the corresponding content of
266
ettringite from N-0 paste is 13.9% and 12.9%. Thus, the sodium sulfate content in
267
LCM effectively increases the formation of ettringite, which is beneficial for
268
improving the mechanical performance of LCM. Moreover, the formation of ettringite
269
crystals occurs at the early stage of hydration, particularly at the second and third day.
270
Besides, a large number of unhydrated mineral admixtures particles remain in the N-0
271
and N-3S paste after curing for 90 d. 80 Others
60
Ettringite
40
Portlandite
Calcium silicate
20
Unhydrated mineral admixtures Unhydrated cement
0 1
40
20
100
100
Amorphous
80
60
Others
80
60
Ettringite
40
Portlandite
40
Calcium silicate
20
20
Unhydrated mineral admixture
0 Unhydrated cement 1
10
0 100
Curing age (day)
TE D
Curing age (day)
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60
0
10
Hydration products (g/100g)
Amorphous
80
100
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100
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(a) N-0 (b) N-3S Fig. 5. Evolution in the hydration products of N-0 and N-3S paste
275
was performed to study the bands of S−O, O−H, Al−O, Si−O, and Si−O−T (T is
276
tetrahedral Si or Al unit), corresponding to portlandite, ettringite, gypsum and
277
C(A)SH gel, respectively (Li et al., 2017; Nath and Kumar, 2017). The test results of
278
FTIR spectrum of paste samples are shwon in Fig. 6. It can be observed that a
279
prominent absorption peak appears at approximately 3640cm-1 that represents the
280
stretching vibration of O−H in calcium hydroxide, and the corresponding peak of
281
calcium hydroxide is weakened with increasing of the curing age due to the
282
portlandite consumed in the hydration of LCM. The bands at approximately at 540
283
cm-1, 610 cm-1 and 1110 cm-1 are related to the bending vibration and stretching
284
vibration of S−O in the structure of ettringite or gypsum. The peak at approximately
285
670 cm-1 relates to the bending vibration of gypsum phase. From Fig. 6, the
286
characteristic vibration peak of gypsum at 670 cm-1 is in the 1d spectrum, and
287
corresponding peak of gypsum is weakened in the 2d spectrum, which indicates that
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3.3.2 FTIR analyses To further analyze the phases change in LCM with and without sodium sulfate, FTIR
273
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the gypsum is rapidly consumed in the early age of hydration. The vibration peak at
289
870 cm-1 corresponding to the band of Al−O−H appears after curing 1 d and the
290
vibration is enhanced in the 2 d and 3 d spectrum, which indicates that a great amount
291
of ettringite has formed due to the gypsum being consumed by the aluminum phase
292
from the mineral admixtures (Scholtzova et al., 2015). The evolution of ettringite as
293
identified from the FTIR test is in accord with the XRD analysis results. The band at around 970 cm-1 is attributed to the stretching vibration of Si−O−T in
295
the C(A)SH gel (Li et al., 2017). From Fig. 7, the vibration peak at 970 cm-1 in the
296
spectrum is enhanced with increasing curing time, which indicates that a large
297
quantity of C(A)SH gel forms in the paste. In addition, the stretching vibration of
298
Si−O−T appearing around 970 cm-1 is generated by Q2 units in the C(A)SH gel. The
299
vibration of Si−O−T due to Q2 units is enhanced and shifted to lower wavenumbers in
300
the hydration process of N-3S mixture, which indicates that the degree of the
301
polymerization of C(A)SH gel is improved (Lodeiro et al., 2009; Yu et al., 1999).
M AN U
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294
The band at 725 cm-1 is attributed to symmetric stretching vibration of T–O–Si,
303
corresponding to the aluminosilicate components of mineral admixtures (Nath and
304
Kumar, 2017). The band at approximately 450 cm-1 is assigned to the bending
305
vibrations of Si−O−Si and O−Si−O which is caused by the inert quartz in the fly ash
306
particles (Zhang et al., 2017). The absorption peaks observed at approximately 1480
307
cm-1 and 1420 cm-1 are related to the stretching vibrations of calcium carbonates. The
308
minor calcium carbonates originate mainly from the carbonation that occurs during
309
specimen curing and the sample preparation.
4000 3600 1600
310 311
28d
7d
7d
3d
3d
2d
2d
1d
1200
450
725 670
28d
610 540
870
90d
967
1480 1420
3640
540
725 670 610
450
1110
90d
970
1110
870
EP
1480 1420
AC C
3640
TE D
302
1d 800
400
4000 3600 1600
1200
Wavenumber /cm-1
Wavenumber /cm-1
(a) N-0
(b) N-3S
800
400
Fig. 6. FTIR spectra of N-0 and N-3S paste at different curing age
3.3.3 TGA analyses
312
In this study, TGA method is used to follow the reaction of LCM and identify
313
hydration phases. The thermo-gravimetric (TG) and derivative thermogravimetric
314
(DTG) curves of the LCM with and without sulfate sodium are shown in Fig. 7. Three
ACCEPTED MANUSCRIPT prominent endothermic peaks appear in the DTG curves of LCM paste at
316
approximately 50-200, 400-500 and 600-800 degrees and are related to dehydration of
317
C(A)SH gel and ettringite crystal, calcium hydroxide and carbonates, respectively
318
(Park et al., 2016). The carbonates are formed during specimen curing and sample
319
preparation due to the carbonation of portlandite and possibly C(A)SH gel. The bound
320
water from ettringite and C(A)SH gel is easily lost at approximately 100-200 °C. The
321
area size of the first endothermic peak in DTG curve of N-3S at 3 d is greater than
322
that of N-0 at 3 d, which proves that the hydration of mineral admixtures in LCM is
323
accelerated in the presence of sodium sulfate.
RI PT
315
The content of portlandite in hydrated paste can be obtained based on the mass loss
325
of Ca(OH)2 from TG and DTG curves, and the corresponding results are compared to
326
the QXRD results for the portlandite content shown in Fig. 8. From Fig. 8, the
327
addition of 3 wt% sulfate sodium in LCM substantially accelerates the consumption
328
of Ca(OH)2 in matrix after 3 d of curing. Thus the content of Ca(OH)2 is rapidly
329
consumed at early age, which results in the improvement of mechanical properties of
330
LCM due to additional hydration products formed. Moreover, a certain amount of
331
portlandite appears in the matrix at 90 d, which suggests that the mineral admixtures
332
continue to hydrate slowly over 90 d of curing. Additionally, the relative error of
333
Portlandite content measured by QXRD and TG-DTG is relatively small, which
334
supports the high accuracy of the QXRD results in this study. 3d 28d 90d -0.03
Carbonates
95
90
EP
Weight /%
Portlandite
AC C
85
Ettringite & C-(A)-S-H
80
0
200
400
600
Temperature /℃
335
800
-0.06
-0.09
-0.12 1000
Derivative weight %/℃
0.00
100
0.00 3d 28d -0.03 90d
Carbonates Portlandite
95
-0.06 90 -0.09 85
-0.12 Ettringite & C-(A)-S-H
80 0
200
-0.15 400
600
Temperature /℃
(a) N-0 (b) N-3S Fig. 7. TG and DTG curves of N-0 and N-3S paste samples
800
1000
Derivative weight %/℃
100
Weight /%
TE D
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ACCEPTED MANUSCRIPT QXRD TGA
10 8 6 4 2
RI PT
Portlandite content wt%
12
0
N-0-3d N-3S-3d N0-28d N-3S-28d N0-90d N-3S-90d
336 337
Fig. 8. Portlandite content in N-0 and N-3S pastes
3.5 Hydration heat
The hydration heat flow and cumulative hydration heat of LCM for different
339
dosages of sodium sulfate are shown in Fig. 9. From Fig. 9(a), three prominent
340
exothermic peaks appear in the hydration heat flow curve after the induction period.
341
The first exothermic peak is caused by the hydration of the minor content (10 wt%) of
342
Portland cement in the LCM. The Portland cement in the LCM reacts first due to its
343
high activity. Subsequently, the blast furnace slag starts to react with portlandite and
344
the hydration of slag will be accelerated in the alkaline environment (Myers et al.,
345
2017). Moreover, the activity of slag in mineral admixtures is far greater than that of
346
the fly ash. Therefore, the second exothermic peak is due to the hydration of GGBS
347
particles in the mineral admixtures. The area of the third exothermic peak in the LCM
348
is enlarged with increasing sodium sulfate amount, which indicates that the sodium
349
sulfate effectively improves the formation of a specified phase in the hydration
350
products. Considering the test results of XRD and FTIR, the third exothermic peak in
351
the heat flow curve presents the formation of ettringite during the early hydration
352
process. The curve of hydration heat flow of N-4S mixture is very close to that of the
353
N-3S mixture, which suggests that the 3 wt% sodium sulfate in the LCM is sufficient
354
to achieve a good activation effect in the early period. Considering the test results of
355
the mechanical properties, an excessive amount of sodium sulfate may affect the
356
hydration of LCM in the later period.
M AN U
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EP
AC C
357
SC
338
The sulfate sodium increases the cumulative hydration heat of LCM mainly due to
358
additional ettringite formation. The cumulative hydration heat of N-3S mixture
359
reaches approximately 253J/g after 120 h, 22% higher than that of the N-0 mixture,
360
which is well agree with the mechanical performance measurements. The cumulative
361
heat of hydration of all mixes is far below that of Portland cement, which is
362
meaningful for LCM used in massive concrete engineering.
N-0 N-1S N-2S N-3S N-4S
1.0
0.5
0.0
N-0 N-1S N-2S N-3S N-4S
250 200 150 100 50 0
0
24
48
72
96
120
Time (hour)
0
24
48
72
96
120
Time (hour)
(b) Cumulative heat of hydration
(a) Hydration heat flow
363
300
RI PT
Heat flow (mW/g)
1.5
Cumulative heat of hydration (J/g)
ACCEPTED MANUSCRIPT
Fig. 9. Hydration heat evolution curves of LCM with different dosages of sodium sulfate
3.6 Microstructure
365 366
3.6.1 MIP The pore size distribution and total porosity of N-0 and N-3S hydrated paste at 90 d
367
measured by the MIP test are presented in Fig. 10. It should be noted that the pore size
368
distribution measured by MIP test differs somewhat from the real pore size
369
distribution of sample according to previous studies (Galle, 2001). The pores in
370
hydrated paste are classified as gel pores (micropores, 0-10 nm), mesopores (10-50
371
nm), and capillary pores (over 50 nm) according to the pore size in this study (Mehta
372
and Monteiro, 2006). The total porosity of N-3S and N-0 paste after curing 90 d is
373
29.47% and 31.26%, respectively, which suggests that the sodium sulfate in LCM
374
reduces the total porosity during the hydration process to some extent. More
375
importantly, N-3S paste shows more gel pores in the range of 4-10 nm (green area)
376
compared to N-0 paste. In addition, the amount of capillary pores of N-0 paste in the
377
range of 50-100 nm (yellow area) is greater than that of N-3S paste. The critical pore
378
entry radius, recorded from the peak of the cumulative intrusion curves of N-0 and
379
N-3S paste, is 40.4 nm and 50.4 nm, respectively. The number of capillary pore from
380
N-3S and N-0 paste in the range of 50-100 nm reaches 16.6% and 32.2% of the
381
number of total pores, respectively, which proves that the sodium sulfate in the
382
hydration process effectively refines the pore size distribution of LCM. The improved
383
pore size distribution of the LCM at micro-scale ultimately augments the mechanical
384
properties of the LCM at macro-scale.
AC C
EP
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ACCEPTED MANUSCRIPT
20
N-0 N-3S 80% N-0 N-3S
10-50 nm 60%
Percentage /%
Porosity /%
25
Capillary pores
30
Mesopores
Gel pores
35
15
40%
20%
over 50 nm
0-10 nm
10 0%
Gel pores
Mesopores Capillary pores
0 1
10
RI PT
5
100
1000
10000
100000
Pore diameter /nm
385
(a) Pore size distribution (b) Total porosity Fig. 10. Pore size distribution and total porosity of N-0 and N-3S paste at 90 d
388
amount of needle-like and lath-like ettringite (AFt) has formed on the surface of the
389
mineral admixtures, which coincides with the previous test results. However, the large
390
number of microspores in the matrix results in a loose microstructure of hydrated
391
paste, particularly in N-3S paste, because the hydration degree of mineral admixtures
392
is relatively low so that the hydration products are not enough to fill the coarse pores
393
in the matrix. In addition, the bonding of mineral admixtures in N-3S past is more
394
compact than that of N-0 paste due to additional amount of formed and mutually
395
intermixed ettringite, which is beneficial for improving the mechanical properties at
396
early age.
AC C
EP
TE D
M AN U
SC
387
3.6.2 SEM-EDS The SEM micrographs of N-0 and N-3S paste at 3 d are shown in Fig. 11. A large
386
397
(a) N-0-3 d
(b) N-0-3 d
(c) N-3S-3 d
(d) N-3S-3 d
Fig. 11. The SEM micrographs of N-0 and N-3S paste at 3 d
ACCEPTED MANUSCRIPT Representative SEM micrographs of the microstructure of N-0 and N-3S paste at 90
399
d are represented in Fig. 12. The ettringite crystals (which appear lath-like), C(A)SH
400
gel and incompletely hydrated mineral admixtures (composed mainly of fly ash) can
401
be observed in the microstructure of N-0 and N-3S paste. In addition, a large number
402
of coarse pores appear in N-0 paste compared to N-3S paste, which is consistent with
403
the results of the MIP test. Moreover, the amount of ettringite crystals in the N-0
404
matrix is lower than in the N-3S matrix. The C(A)SH gel features an uncompact
405
microstructure with many coarse pores, as shown in the dashed area. Compared to
406
N-0 mixture, a sufficient quantity of ettringite crystals formed in the pores in the
407
matrix, refining the pore structure and reducing the amount of capillary pores in the
408
hydrated paste.
(b) N-0-90 d
EP
TE D
(a) N-0-90 d
M AN U
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RI PT
398
(c) N-3S-90 d
410
Fig. 12. The SEM micrographs of N-0 and N-3S paste at 90d
AC C
409
(d) N-3S-90 d
To investigate the elemental composition of hydration products, an EDS test is
411
conducted on the N-0 and N-3S sample on the basis of the acquired SEM micrographs.
412
The test results of element mapping of Fig. 12 (b) and Fig. 12(d) (corresponding to the
413
N-0 and N-3S samples, respectively) are presented in Fig. 13(a) and Fig. 13(b),
414
respectively. The Ca/Si ratio of hydration products of N-0 and N-3S paste in a
415
selected area is 1.362 and 1.128, respectively, and the Al/Si ratio of hydration
416
products of N-0 and N-3S paste in the same area is 0.228 and 0.287, respectively. To
417
further analyze the Ca/Si ratio and Al/Si ratio of CS(A)H gel, an EDS test is
418
performed to six selected points from C(A)SH gel, and the corresponding results are
419
illustrated in Fig. 14(c) and Fig. 14(d). It should be mentioned that the aluminum
ACCEPTED MANUSCRIPT phase can substitute silicon in C-S-H gel and form CASH gel in the hydration process.
421
(Lothenbach and Nonat, 2015). Therefore, the aluminum phase should be taken into
422
account in this study, and a more proper definition for the ratio of Ca/Si is generally
423
Ca/(Si+Al) due to substitution of silicon by aluminum. From Fig. 13(c) and Fig. 13(d),
424
the Ca/(Si+Al) ratio (average value of six points) of C(A)SH gel from N-0 and N-3S
425
paste is 1.322 and 1.075, respectively. The Al phase cotent in C(A)SH gel from N-3S
426
mixture is slightly higher than that of N-0 mixture at curing 90 d, which suggests that
427
the additional aluminum from mineral admixtures in N-3S paste is incorporated in the
428
bridging tetrahedra of the silicate chain (L'Hopital et al., 2016; Richardson, 2008).
429
Therefore, the sodium sulfate as an activator used in LCM decreases the Ca/Si ratio of
430
C(A)SH gel in the hydrated paste. 18
At % of element mapping
At % of element mapping
18
12 9 Ca
6
Si + Al
= 1.053
Al Si
3 0
15
M AN U
15
= 0.228
12
9 6
Al
Mg
S
Fe
TE D
Si
Ca
431 432
3.4 Drying shrinkage
5 0
Point1 Point2 Point3 Point4 Point5 Point6
Si
Al
Si
= 0.287
Mg
S
Fe
Ca Si Al
25
At % of element
EP
10
Al
(b) N-3S
Ca Si Al
AC C
At % of element
15
= 0.852
3
(a) N-0
20
Ca Si + Al
0
Ca
25
SC
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420
20 15 10 5 0 Point1 Point2 Point3 Point4 Point5 Point6
(c) N-0 (d) N-3S Fig. 13. The EDS test results of N-0 and N-3S paste at 90 d
433
The drying shrinkage of LCM mortar specimens for different contents sodium
434
sulfate is shown in Fig. 14. The phenomenon of drying shrinkage from cement-based
435
materials results from the evaporation of the free water from the microscale pores in
436
cement hardened paste at a low-humidity environment. Therefore, the drying
ACCEPTED MANUSCRIPT shrinkage is mainly influenced by the composition of binding materials. From Fig.
438
15, the LCM specimens with sodium sulfate show a lower drying shrinkage than that
439
of the control specimens. The drying shrinkage of LCM specimens with 3 wt% and 4
440
wt% sodium sulfate at 90 d decreases to 578 µε and 573 µε, respectively, while the
441
drying shrinkage of control specimens reaches 653 µε. Therefore, the drying
442
shrinkage of LCM is reduced when the sodium sulfate as activator is added into the
443
LCM, mainly because additional ettringite in the form of crystal generates in the
444
paste, further restricting the drying shrinkage of LCM. N-0 N-1S N-2S N-3S N-4S
SC
600
500
400
300
200 7
M AN U
Drying shrinkage µm/m (10-6)
700
RI PT
437
14
21
28
56
90
Age /day
445 446
Fig. 14. Evolution of drying shrinkage of LCM with different amounts of sodium sulfate
4. Discussion
As seen in equation (1) and (2), the major effect of sodium sulfate in LCM
448
hydration is that the soluble sulfate provides the sulfate ions to form additional
449
ettringite in the hydration process. In addition, the sodium sulfate reacts with calcium
450
hydroxide and generates sodium hydroxide in the pore solution of the matrix, which is
451
beneficial for the dissolution of active alumina and silica from mineral admixtures due
452
to the increased pH value of the pore solution. According to the previous literature,
453
the presence of 4 wt% sodium sulfate can raise the pH value of saturated Ca(OH)2
454
solution from 12.50 to 12.75 at 23 °C (Shi and Day, 2000a). Thus, the final setting
455
time of LCM with sodium sulfate is shorten as a certain extant because the sodium
456
sulfate accelerates the hydration of LCM.
EP
AC C
457
TE D
447
Na2SO4 + Ca(OH)2 + 2H2O＝2NaOH + CaSO4·2H2O
(1)
458
3(CaO·Al2O3) + 3(CaSO4·2H2O) + 26 H2O＝3CaO·Al2O3·3CaSO4·32H2O (2)
459
A large quantity of ettringite is formed in LCM at the early stages of hydration,
460
practically in N-3S paste. It should be noted that the amount of ettringite formed in 1
461
d is limited; most of the ettringite is formed over the subsequent two days because the
462
aluminum phase in pore solution is mainly from hydrated Portland cement at the
463
initial stage of hydration. However, the amount of Portland cement in LCM accounts
ACCEPTED MANUSCRIPT 464
for only 10 wt%, which limits the amount of calcium aluminum hydrates in the matrix.
465
Therefore, only a small amount of ettringite is formed in the first 24 hours even
466
though the sulfate content in the paste is sufficient. The active aluminum phase from the mineral admixtures accelerates dissolution
468
with an increase in the alkalinity in the pore solution due to the hydration of Portland
469
cement. According to the kinetic analysis, the presence of sodium sulfate effectively
470
accelerates both the dissolution of the glassy phase derived from the mineral
471
admixtures and the reaction between Ca(OH)2 and mineral admixtures (Shi and Day,
472
2000b). Subsequently, the aluminum phase reacts with sulfates, forming a great
473
amount of ettringite until the sulfates are consumed. The XRD test results indicate
474
that the content of ettringite in paste no longer increases after curing for 3 d. Note that
475
the ettringite formed in the early stage of hydration does not cause expansion damage,
476
because the space in the matrix can accommodate the formation of a great amount of
477
ettringite at the early stage of hydration. The formed ettringite serves as a mechanical
478
frame and increases the volume of hydration products in the hydrated paste, which
479
effectively enhances the mechanical performance of LCM at 3 d. Meanwhile, the
480
hydration heat flow of LCM significantly decreases after 3 d when a great number of
481
hydrates (C(A)SH gel and ettringite) form in the paste.
M AN U
SC
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467
With increasing curing age, the C(A)SH gel that originates from the hydrated
483
mineral admixtures is generated continuously, and ettringite crystals are inserted in
484
the C(A)SH gel, which results in a dense microstructure of hydrated paste with fewer
485
pores due to the void in the matrix occupied by C(A)SH gel. Thus, the mechanical
486
property of LCM activated with sodium sulfate increases stably with prolonged curing
487
age. Furthermore, a large quantity of ettringite as crystals and unreacted mineral
488
admixtures as micro-aggregate in the matrix restrict the drying shrinkage of LCM.
489
Thus, the drying shrinkage of the LCM containing sulfate sodium is reduced by
490
comparison with the plain LCM specimens.
EP
AC C
491
TE D
482
Based on the previous investigation, the energy intensity and carbon emissions of
492
the mixture of LCM used in this work are 1.80 MJ/kg and 0.21kg/kg, respectively;
493
however, the energy intensity and carbon emissions of Portland cement is 5.5 MJ/kg
494
and 0.93 kg/kg, respectively (Wu et al., 2018a). Furthermore, sodium sulfate can be
495
obtained as a secondary product from the manufacture of hydrochloric acid, battery
496
acid, viscose rayon, silica pigments and even from natural salt lake or brines.
497
Therefore, LCM activated with a minor amount of sodium sulfate offers a bright
498
prospect in the context of green production.
ACCEPTED MANUSCRIPT 499
4. Conclusions In this work, we used the different dosages of sodium sulfate as an activator to
501
improve the performance of LCM. An experimental study was conducted to
502
investigate the performance of LCM in the presence of sodium sulfate. The
503
mechanical properties, workability and drying shrinkage of LCM containing different
504
amounts of sodium sulfate were studied, and the evolution in microstructure and
505
compositions of hydration products of LCM paste with and without sulfate sodium
506
were analyzed. Finally, the effects of sodium sulfate as an activator in the hydration of
507
LCM were also discussed according to the test results. The conclusions of the test
508
results and discussion can be summarized as follows:
SC
RI PT
500
(1) The mechanical properties of LCM were substantially increased in the presence
510
of sodium sulfate, particularly in the early stage. In this study, LCM containing 3 wt%
511
sodium sulfate showed the best mechanical properties, reaching 14.6 MPa and 39.5
512
MPa at 3d and 90 d, respectively, corresponding to an increase of 57.0% and 20.8%
513
compared to that of conventional LCM.
M AN U
509
(2) Compared to the plain LCM, a small amount of sodium sulfate in the LCM
515
effectively accelerated the hydration of solid waste and consumed more portlandite
516
during the hydration process. Furthermore, a large amount of ettringite and the
517
C(A)SH gel with a low Ca/Si ratio formed in hydrated LCM with sodium sulfate.
TE D
514
(3) The quantity of ettringite crystals in the LCM containing sodium sulfate at 1 d
519
was limited, and a majority of ettringite formed at 2 d and 3 d, which effectively
520
increased the mechanical performance of the LCM at 3 d. Moreover, ettringite crystals
521
were inserted in C(A)SH gel from the hydrated mineral admixtures, which resulted in
522
a dense microstructure of hydrated paste with fewer pores and increased the
523
mechanical performance of the LCM in the later period.
525
AC C
524
EP
518
526
Acknowledgements
527
This work was financially supported by National 973 Program (No. 2015CB655102),
528
National Natural Science Foundation of China (No. 51678143, No. 51878153, No.
529
51808189, and No. 51508090) and Postgraduate Research &Practice Innovation
530
Program of Jiangsu Province (KYCX18_0078)
531
ACCEPTED MANUSCRIPT 532
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574
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S0959-6526(19)30585-2
DOI:
https://doi.org/10.1016/j.jclepro.2019.02.186
Reference:
JCLP 15916
To appear in:
Journal of Cleaner Production
Received Date: 14 August 2018 Revised Date:
7 February 2019
Accepted Date: 17 February 2019
Please cite this article as: Wu M, Zhang Y, Jia Y, She W, Liu G, Yang Z, Zhang Y, Zhang W, Sun W, Effects of sodium sulfate on the hydration and properties of lime-based low carbon cementitious materials, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.02.186. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of sodium sulfate on the hydration and properties of
2
Lime-based low carbon cementitious materials
3
Meng Wua,b, Yunsheng Zhanga,b,*, Yantao Jiac,*, Wei Shea,b, Guojian Liua,b, Zhiqiang
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Yanga,b, Yu Zhanga,b, Wangtian Zhanga,b, Wei Suna,b
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School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China c College of Mechanics and Materials, Hohai University, Nanjing 211100, China b
* Corresponding author: Yunsheng Zhang, Email: [email protected]
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Abstract: To reduce the carbon footprint and energy consumption from the cement
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manufacturing industry, lime-based low carbon cementitious materials (LCM) has
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caught strong attention. LCM as a novel low-carbon cement shows impressive
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performance and promising prospects; however, its mechanical properties are inferior
16
to those of ordinary Portland cement (OPC). In this study, different dosages of sulfate
17
sodium was incorporated into LCM to investigate the effects of sodium sulfate on
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LCM performance. The properties and hydration of LCM with and without sulfate
19
sodium were systemically investigated and analyzed. The results revealed that LCM
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blending with 2-3 wt% sodium sulfate showed the best mechanical performance. The
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compressive strength of LCM containing 3 wt% sodium sulfate was increased by 57.0%
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and 20.8% relative to the plain LCM at 3 d and 90 d, respectively. Microstructural
23
characterization showed that a great amount of ettringite had formed at 3 d, which
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effectively improved the mechanical performance of LCM at early stage. Moreover,
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the addition of sodium sulfate effectively accelerated the hydration of the solid waste
26
in LCM, and more hydrated lime was consumed in the hydration process. The
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ettringite became embedded in C(A)SH gel with increasing curing age, which resulted
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in a dense microstructure of hydrated paste with fewer coarse pores and an
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enhancement in the mechanical performance of LCM. Thus, the sodium sulfate
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effectively increased the strength of LCM at both early and later stage.
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Keywords: low carbon emissions; green cement; sulfate sodium; hydration products;
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microstructure;
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1. Introduction The cement industry contributes approximately 8%-9% anthropogenic carbon
38
emission and 2%-3% energy use worldwide (Monteiro et al., 2017), mainly because
39
the production of Portland cement requires the burning of raw materials at a high
40
temperature of 1450 °C, during which a large amount of CO2 is released to the
41
atmosphere. One ton of cement manufactured releases nearly one ton CO2 (Xu et al.,
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2015; Xiao et al., 2018). Nevertheless, Portland cement is widely used in construction
43
and the output of global cement was approximately 4000 million tons in 2016.
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Therefore, reducing the carbon emissions of the cement industry is an issue of interest
45
in the field of building materials. In recent years, increasingly more scholars have
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focused on designing and preparing low carbon cementitious materials to reduce the
47
carbon emissions of cement. One of the most promising approaches is to decrease the
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Portland cement fraction in cement-based materials by replacement with
49
supplementary cementitious materials (SCMs) (Avet and Scrivener, 2018; Latifi et al.,
50
2018; Wu et al., 2018a; Yang et al., 2016). A large part of SCMs are currently derived
51
from industrial solid waste such as blast furnace slag, fly ash, nickel slag and red mud
52
(Tan et al., 2018; She et al., 2018 a). Ground blast furnace slag and fly ash, which are
53
the most commonly used SCMs, have been broadly used in cement-based materials
54
worldwide.
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Ground blast furnace slag is a latent hydraulic material that react directly with
56
water to form a minor amount of amorphous gel; fly ash is a manmade pozzolanic
57
material. Slag and fly ash can be reacted with calcium hydroxide from hydrated
58
cement clinker to form hydrated calcium aluminate (C-A-H) and calcium silicate gel
59
(C-S-H) (Lothenbach et al., 2011; Liu et al., 2016; She et al., 2018 b). Thus, a part of
60
the cement clinker can be replaced with nickel slag, blast furnace slag, calcined clay,
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fly ash, red mud and other solid waste to prepare blended cement and other cements.
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Recently, lime-based low carbon cementitious materials (LCM) have been designed
63
and prepared to further enlarge the content of industrial solid waste in cementitious
64
materials (Jeong et al., 2016; Wu et al., 2018a). LCM consisted of a minor amount of
65
Portland cement (typically less than 20 wt%), a moderate content of lime and a large
66
amount of industrial solid waste; the resulting novel green cement shows strong
67
performance according to previous report (Wu et al., 2018b). Nevertheless, the
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mechanical strength of LCM is inferior to that of cement-based materials because of
69
the low activity of industrial solid waste. Therefore, the mechanical properties of
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LCM require enhancement. According to previous reports, freely soluble sulfates such
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ACCEPTED MANUSCRIPT as sulfate sodium can effectively increase the strength of binder materials containing
72
fly ash or slag (Qian et al., 2001; Rashad et al., 2013; Velandia et al., 2018). It has
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reported that the soluble sulfates in cementitious materials are beneficial for ettringite
74
formation and improve the mechanical properties of blends (Shi and Day, 1995; Sahin
75
et al., 2016). Thus, soluble sulfates can increase the mechanical strength of LCM in
76
theory. However, the optimal dosage of soluble sulfate used in LCM is unknown, and
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the specific effects of soluble sulfate on the mechanical performance, compositions of
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hydration products and microstructure of LCM are not clear. Moreover, the optimal
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amount, formation process and effects of ettringite in the hydration process of LCM
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with and without soluble sulfate are also uncertain. Therefore, the effects of soluble
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sulfates on the performance of LCM require systematically investigation.
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In this study, sodium sulfate is used as freely soluble sulfate to improve the
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performance of LCM. The sodium sulfate is easily accessible and features low carbon
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emissions and cost in production. Different dosages of sodium sulfate as an activator
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are blended with LCM to compare the effects of sodium sulfate on LCM performance.
86
The strength, hydration and drying shrinkage of LCM with and without sodium
87
sulfate are systematically studied and analyzed. We anticipate that this study will
88
promote the development of low carbon cementitious materials.
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2. Raw materials and test methods
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2.1 Raw materials
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A commercial Portland cement (52.5 grade) that conforms to Chinese standard
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GB/T 175-2007 (similar to EN 197-1 and ASTM C150) was used in this study. Fly
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ash (type F), ground blast furnace slag, hydrated lime and gypsum were used as raw
94
materials. To increase the utilization ratio of fly ash, the type F fly ash was used in
95
this work to prepare LCM. River sand meeting the ASTM C778 standard was selected
96
as fine aggregates to prepare mortar specimens. Sodium sulfate powder (analytical
97
reagent) was used as an activator. The chemical compositions of the materials used in
98
this work are presented in Table 1. The industrial grade hydrated lime that was
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approximately composed of 87% portlandite and 13% calcite was used in this study.
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The hydrated lime was significantly more stable on the specimen volume and reaction
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rate of LCM compared to the quick lime.
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Based on the previous literatures, the mineral admixtures used in this study were
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obtained via mixing with FA, GGBS and gypsum at a ratio of 0.475, 0.475 and 0.05
104
by mass. Fig. 1 displays the particle size distributions of the raw materials used in this
105
study.
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Table 1 Oxide compositions of raw materials PC
GGBS
FA
Lime
Gypsum
CaO Al2O3 SiO2 MgO Fe2O3 Na2O K2O SO3 LOI
64.38 4.38 21.60 3.43 3.42 0.51 2.23 2.54
37.12 15.51 32.72 5.50 0.24 0.40 0.30 2.61 0.36
4.40 30.41 51.53 0.91 6.90 0.62 1.37 0.91 1.52
97.30 0.41 0.47 1.00 0.23 0.32 0.1 26.75
46.89 0.14 0.30 0.20 0.07 0.11 52.09 7.01
80 60 40 20 0 0.1
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PC Fly ash Slag Lime
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Cumulative Volume %
100
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Material
1
10
100
Particle Size (µm)
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Fig. 1. Particle size distributions of raw materials.
2.2 Test methods
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2.2.1 Specimen preparation and curing Table 2 presents the mixture percentage of LCM activated with different amounts
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of sodium sulfate in this study. The preparation method of mortar specimens
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conformed to Chinese standard GB/T 17671-1999 (similar to EN 196-1 and ASTM
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C109); the mass ratio of river sands to LCM was fixed at 3. The sodium sulfate was
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dissolved in pre-weighed water before mixing. The mortar specimens were prepared
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in 40 * 40 *160 mm moulds, and the paste specimens were also prepared in 40 * 40 *
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40 mm moulds for hydration studies. The specimens were cured in a moist room at 20
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± 2 °C and relative humidity (RH) ≥ 95% for 24 h before demoulding. The demoulded
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specimens were allowed to cure in a moist room until designated ages.
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ACCEPTED MANUSCRIPT 122 123 Code
Portland cement
Mineral admixtures
Hydrated lime
Sodium sulfate
w/b
N-0 N-1S N-2S N-3S N-4S
10 10 10 10 10
80 80 80 80 80
10 10 10 10 10
0 1 2 3 4
0.5 0.5 0.5 0.5 0.5
2.2.2 Mechanical properties
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Table 2 Details of mixture proportions (wt %)
According to Chinese standard GB/T 17671-1999, the strength of LCM mortar
126
specimens was tested at 3 d, 7 d, 28 d, and 90 d. Three mortar specimens were used
127
for determination of flexural strength and compressive strength. The test results were
128
adopted the average value from the three measured specimens.
129
2.2.3 Workability
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The fresh mortar flowability was obtained according to Chinese standard GB/T
131
2419-2005 (similar to ASTM C1437). The setting times of LCM mixtures were
132
measured based on Chinese standard GB/T 1346-2011 (similar to ASTM C191).
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2.2.4 Heat of hydration
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The heat of hydration of the tested mixtures was recorded at 20 °C using a TAM
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Air isothermal calorimeter (TA Instruments, USA). An ampoule bottle filled with
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10-12 g fresh paste was put into the channel of an isothermal calorimeter, and the data
137
of heat flow were automatically measured by isothermal calorimeter.
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2.2.5 Drying shrinkage
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The drying shrinkage of mortar specimens was measured according to Chinese
140
standard JC/T603-2004 (similar to ASTM C596). After curing for 24 h in the moist
141
room, the specimens were demoulded and cured in lime-saturated water for two days.
142
Subsequently, the initial length of specimens (L0) was recorded, and all the specimens
143
were placed in room at 20 ± 2 °C and RH50% ± 3%. The length (Lt) of specimens in
144
the drying room was recorded at the designated ages, and the drying shrinkage (εt) of
145
specimens were calculated according to Eq. (1).
146 147
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εt =
L0 − Lt L0
×100%
(1)
2.2.6 Hydration studies
148
Hydration studies were conducted on LCM paste at the designated curing age. The
149
small paste samples after crushing were immersed in ethanol for 72 h to prevent
150
further hydration. Subsequently, all the samples were dried at 50°C for 24 h in a
ACCEPTED MANUSCRIPT 151
vacuum drying oven, and some samples were carefully ground into very fine powder
152
samples (≤75 µm) by hand in a mortar . The mineral phases of the hydration products were characterized via a Bruker D8
154
Discovery diffractometer using a CuKα anode operating at 40 kV, 30 mA, 4°/min, and
155
a range of 5° to 80° with steps of 0.02°. Before the XRD test, the fine powder samples
156
were homogenously mixed with corundum (α-Al2O3≥99.99%) at a mass ratio of 9:1.
157
The quantitative analysis of XRD data was performed on TOPAS software v4.0.
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Fourier transform infrared spectroscopy (FTIR) tests were performed on a Nicolet
159
iS10 infrared spectrometer via the KBr pellet method. All the samples were scanned
160
32 times with a resolution of 4 cm-1 in the spectral range of 4000-400 cm-1.
Thermogravimetric analysis (TGA) tests were conducted on a NETZSCH STA449
162
F3 thermogravimetric analyzer. The fine powder samples were heated from room
163
temperature to 1000 °C at a heating rate of 10 °C /min in a N2 atmosphere. The
164
portlandite content in hydrated paste was computed from TGA curves by the
165
tangential method, and the portlandite content was normalized to the dry sample
166
weight at 550 °C according to Eq. (2) (Adu-Amankwah et al., 2017), in which CHw is
167
the wt% loss from water in portlandite as calculated by the tangent method.
M 550°C
18 ×100%
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CH w ⋅ 74
(2)
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Scanning electronic microscope (SEM) and energy dispersive spectroscopy (EDS)
170
tests were carried on the small paste samples coated with a Pt conductive film after
171
being dried. The SEM tests were perfumed on an FEI Navo Nano SEM 450 fitted
172
with a Thermo Fisher NS7 EDS analyzer.
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The small paste samples were used for mercury intrusion porosimetry (MIP) tests
174
using a Micromeritics AutoPore IV 9500. The measured pore size of sample was from
175
4 nm to 350 µm with an assumed mercury contact angle of 130 °.
176
3. Test results
177
3.1 Compressive and Flexural strength
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178
The mechanical property evolution of LCM with and without sodium sulfate is
179
shown in Fig. 2. From Fig. 2, minor amounts of sodium sulfate are found to
180
significantly improve the mechanical properties of LCM mortar specimens at both
181
early and later ages. The mechanical properties of LCM improve with an increasing
182
content of sodium sulfate (from 1 to 3 %). However, the mechanical properties of
183
LCM are degraded slightly when the sulfate sodium content in LCM is increased to
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185
39.5 MPa at 90 days, however, the compressive strength of N-0 specimens (control
186
specimens) reaches only 9.3 MPa and 32.7 MPa for the same curing ages, respectively.
187
The compressive strength of N-3S specimens increases by 24% compared to N-0
188
specimens, which indicates that the LCM blending with 3 % sulfate sodium shows the
189
best mechanical properties. Note that the mechanical properties of LCM at 90 days
190
are superior to the mechanical properties of LCM at 28 days, which indicates that the
191
continuous hydration of GGBS and FA are beneficial to the improvement of
192
mechanical strength of LCM.
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In addition, the LCM shows a satisfactory flexural strength compared to OPC. The
194
flexural strength of N-3S specimens reaches 9.2 MPa at 90 days, and the compressive
195
and flexural strength of Portland cement used in this study at 90 days is 63.8 MPa and
196
9.3 MPa, respectively. Therefore, the ratio of flexural strength to compressive strength
197
of LCM is higher than that of Portland cement. The ratio of flexural strength to
198
compressive strength of Portland cement and N-3S at 90 days is 0.146 and 0.227,
199
respectively.
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28d 90d
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20
10
0 N-0
N-1S
N-2S
N-3S
(1) Compressive strength
201 202
3d 7d
28d 90d
8 6 4 2 0
N-4S
N-0
N-1S
N-2S
N-3S
N-4S
(2) Flexural strength
Fig. 2. Mechanical properties evolution of LCM with different dosages of sodium sulfate
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Flexural Strength /MPa
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Compreesive strength /MPa
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3.2 Workability
The basic properties of LCM are listed in the Table 3. The consistency of LCM is a
203
litter higher than Portland cement due to the fine hydrate lime particles. The silica
204
ratio, alumina ratio and lime saturation ratio of LCM is 1.63, 5.82 and 0.28,
205
respectively. The flowability of fresh LCM mortar with different content of sodium
206
sulfate is presented in Fig. 3 (a). It can be founded that the flowability of fresh mortar
207
with different dosages of sulfate sodium is approximately 220 mm, which meets
208
construction requirements. The high sulfate dosage in LCM shows a little negative
209
influence on the flowability of fresh LCM mortar. It should be pointed that the added
ACCEPTED MANUSCRIPT sodium sulfate to fresh binder materials with a low w/b ratio may show a negative
211
effect on the dispersion of superplasticizers, particularly in polycarboxylate
212
superplasticizer; because a large number of sulfate ions reduce the adsorption capacity
213
of the cement particle surface on the superplasticizer (Yamada et al., 2001). The
214
setting time of LCM is shortened in the presence of sulfate sodium. From Fig. 3, the
215
final setting time of N-0 and N-3S mixture is approximately 245 and 210 min,
216
respectively. Therefore, the sodium sulfate improves the workability of LCM due to
217
the shortened final setting time.
218
Table 3 Basic properties of LCM Soundness
Silica ratio
Alumina ratio
Lime saturation ratio
0.33
qualified
1.63
5.82
0.28
Flowability
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220
Flowability /mm
240
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Consistency
219
200 40
Initial Setting Time Final Setting Time
180
120
60
0 N-0
N-1S
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20 N-2S
N-3S
0
N-4S
(a) Flowability 220
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210
N-0
N-1S
N-2S
N-3S
N-4S
(b) Setting Time
Fig. 3. Mortar flowability and setting time of LCM with sodium sulfate
3.3 Compositions of hydration products
222 223
3.3.1 XRD analyses The XRD test results of hydration products from N-0 and N-3S paste after different
224
ages are shown in Fig. 4. As shown in Fig. 4, the compositions of hydration products
225
in LCM with and without sodium sulfate are essentially the same at different curing
226
ages. The mineralogical compositions of N-0 and N-3S paste are composed of
227
portlandite, gypsum, ettringite, quartz, and mullite. Note that the mullite and quartz
228
phase in the XRD patterns are due to remnant fly ash particles in paste. The peak at
229
28.9°is assigned to the weakly crystalline C(A)SH gel, which is close to the structure
230
of aluminium-containing tobermorite (Guo et al., 2017). The corresponding peaks of
231
ettringite in Fig. 4 are detectable after 2 d, and the peaks of gypsum are only observed
232
in XRD patterns of N-0 and N-3S paste at 1 d, which suggests that a large quantity of
233
ettringite has formed at the early stage of hydration, particularly in the N-3S paste,
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ACCEPTED MANUSCRIPT 234
because the gypsum in LCM reacts with the active aluminum phase in mineral
235
admixtures
236
(Ca4Al2(SO4)(OH)12·6H2O) was not been founded in the hydration products of LCM.
237
The monosulfoaluminate can not been formed in the pore solution with sufficient
238
sulfate ions (Matschei et al., 2007). As shown in Fig. 4, the full-width at
239
half-maximum (FWHM) values of ettringite in N-0 and N-3S paste are stable after 3 d.
240
Moreover, FWHM values of ettringite in N-3S paste are greater than that of N-0 paste
241
at all age, which verifies that the presence of sodium sulfate in LCM mixture
242
effectively increases the amount of ettringite in LCM. Therefore, the mechanical
243
performance of LCM are improved during the hydration process, particularly at 3 d.
form
ettringite.
Meanwhile,
the
monosulfoaluminate
Q: Quartz P: Portlandite O: Corundum G:Gypsum E: Ettringite M: Mullite C: C-(A)-S-H
E
E M E OQ C
PO
MO
P O
M E OQ C
E P M
90d
O
SC
Q: Quartz P: Portlandite O: Corundum G:Gypsum E: Ettringite M: Mullite C: C-(A)-S-H E P M
OO
PO
M AN U
28d
MO
P
O
90d O
OO
28d
7d
7d
3d
3d
2d
G
10
P M
M G G
20
OQ
G
30
P
P O
M O
40
P
O
50
1d
O
60
70
M
G
OO
2θ /°
10
20
OQ
2d
P
G
G
30
1d
O
M
40
O
P O
O
50
OO
60
70
2θ /°
(a) N-0 (b) N-3S Fig. 4. XRD patterns of N-0 and N-3S mixture at different curing age
TE D
244
phase
RI PT
to
The FWHM values of portlandite in XRD patterns are substantially decreased with
246
increasing hydration time, which establishes that the glassy phase from mineral
247
admixtures react with hydrated lime and forms additional CAH and C(A)SH gel. The
248
quantitative analysis of the hydration products of LCM is computed by XRD data
249
refinement via the Rietveld method.
EP
245
Fig. 5 shows the evolution in hydration products of N-0 and N-3S paste based on
251
the refined XRD data. The amount of C(A)SH gel cannot be qualified directly
252
because a majority of the C(A)SH gel exists in amorphous phase. From Fig. 5, the
253
phases of calcium silicate consist of weakly crystalline calcium silicate hydrates
254
composed of tobermorite and rosenhahnite (Kupwade-Patil et al., 2018). The
255
undydrated mineral admixtures are composed of mullite, quartz, akermanite, gehlenite
256
and anorthite phase from unreacted FA and GGBS paticles. From Fig. 5, the
257
portlandite in the matrix is gradually reacted during the LCM hydration process. The
258
amount of portlandite of N-0 and N-3S paste at 90 d is 7.2% and 6.4% (by mass),
259
respectively, which demonstrates that the reaction degree of FA and GGBS in LCM
260
containing 3 wt% sodium sulfate is higher compared to that of the conventional LCM.
AC C
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ACCEPTED MANUSCRIPT 261
The mineral admixtures react with portlandite and provide additional hydration
262
products in the matrix, particularly in the later period of hydration. The quantity of ettringite in the N-3S mixture is greater than that in the N-0 mixture
264
at each curing age. The content of ettringite in the hydration products of the N-3 paste
265
is 18. 2% and 17.5% at 3 d and 90 d, respectively, and the corresponding content of
266
ettringite from N-0 paste is 13.9% and 12.9%. Thus, the sodium sulfate content in
267
LCM effectively increases the formation of ettringite, which is beneficial for
268
improving the mechanical performance of LCM. Moreover, the formation of ettringite
269
crystals occurs at the early stage of hydration, particularly at the second and third day.
270
Besides, a large number of unhydrated mineral admixtures particles remain in the N-0
271
and N-3S paste after curing for 90 d. 80 Others
60
Ettringite
40
Portlandite
Calcium silicate
20
Unhydrated mineral admixtures Unhydrated cement
0 1
40
20
100
100
Amorphous
80
60
Others
80
60
Ettringite
40
Portlandite
40
Calcium silicate
20
20
Unhydrated mineral admixture
0 Unhydrated cement 1
10
0 100
Curing age (day)
TE D
Curing age (day)
272
60
0
10
Hydration products (g/100g)
Amorphous
80
100
SC
100
M AN U
Hydration products (g/100g)
100
RI PT
263
(a) N-0 (b) N-3S Fig. 5. Evolution in the hydration products of N-0 and N-3S paste
275
was performed to study the bands of S−O, O−H, Al−O, Si−O, and Si−O−T (T is
276
tetrahedral Si or Al unit), corresponding to portlandite, ettringite, gypsum and
277
C(A)SH gel, respectively (Li et al., 2017; Nath and Kumar, 2017). The test results of
278
FTIR spectrum of paste samples are shwon in Fig. 6. It can be observed that a
279
prominent absorption peak appears at approximately 3640cm-1 that represents the
280
stretching vibration of O−H in calcium hydroxide, and the corresponding peak of
281
calcium hydroxide is weakened with increasing of the curing age due to the
282
portlandite consumed in the hydration of LCM. The bands at approximately at 540
283
cm-1, 610 cm-1 and 1110 cm-1 are related to the bending vibration and stretching
284
vibration of S−O in the structure of ettringite or gypsum. The peak at approximately
285
670 cm-1 relates to the bending vibration of gypsum phase. From Fig. 6, the
286
characteristic vibration peak of gypsum at 670 cm-1 is in the 1d spectrum, and
287
corresponding peak of gypsum is weakened in the 2d spectrum, which indicates that
AC C
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274
3.3.2 FTIR analyses To further analyze the phases change in LCM with and without sodium sulfate, FTIR
273
ACCEPTED MANUSCRIPT 288
the gypsum is rapidly consumed in the early age of hydration. The vibration peak at
289
870 cm-1 corresponding to the band of Al−O−H appears after curing 1 d and the
290
vibration is enhanced in the 2 d and 3 d spectrum, which indicates that a great amount
291
of ettringite has formed due to the gypsum being consumed by the aluminum phase
292
from the mineral admixtures (Scholtzova et al., 2015). The evolution of ettringite as
293
identified from the FTIR test is in accord with the XRD analysis results. The band at around 970 cm-1 is attributed to the stretching vibration of Si−O−T in
295
the C(A)SH gel (Li et al., 2017). From Fig. 7, the vibration peak at 970 cm-1 in the
296
spectrum is enhanced with increasing curing time, which indicates that a large
297
quantity of C(A)SH gel forms in the paste. In addition, the stretching vibration of
298
Si−O−T appearing around 970 cm-1 is generated by Q2 units in the C(A)SH gel. The
299
vibration of Si−O−T due to Q2 units is enhanced and shifted to lower wavenumbers in
300
the hydration process of N-3S mixture, which indicates that the degree of the
301
polymerization of C(A)SH gel is improved (Lodeiro et al., 2009; Yu et al., 1999).
M AN U
SC
RI PT
294
The band at 725 cm-1 is attributed to symmetric stretching vibration of T–O–Si,
303
corresponding to the aluminosilicate components of mineral admixtures (Nath and
304
Kumar, 2017). The band at approximately 450 cm-1 is assigned to the bending
305
vibrations of Si−O−Si and O−Si−O which is caused by the inert quartz in the fly ash
306
particles (Zhang et al., 2017). The absorption peaks observed at approximately 1480
307
cm-1 and 1420 cm-1 are related to the stretching vibrations of calcium carbonates. The
308
minor calcium carbonates originate mainly from the carbonation that occurs during
309
specimen curing and the sample preparation.
4000 3600 1600
310 311
28d
7d
7d
3d
3d
2d
2d
1d
1200
450
725 670
28d
610 540
870
90d
967
1480 1420
3640
540
725 670 610
450
1110
90d
970
1110
870
EP
1480 1420
AC C
3640
TE D
302
1d 800
400
4000 3600 1600
1200
Wavenumber /cm-1
Wavenumber /cm-1
(a) N-0
(b) N-3S
800
400
Fig. 6. FTIR spectra of N-0 and N-3S paste at different curing age
3.3.3 TGA analyses
312
In this study, TGA method is used to follow the reaction of LCM and identify
313
hydration phases. The thermo-gravimetric (TG) and derivative thermogravimetric
314
(DTG) curves of the LCM with and without sulfate sodium are shown in Fig. 7. Three
ACCEPTED MANUSCRIPT prominent endothermic peaks appear in the DTG curves of LCM paste at
316
approximately 50-200, 400-500 and 600-800 degrees and are related to dehydration of
317
C(A)SH gel and ettringite crystal, calcium hydroxide and carbonates, respectively
318
(Park et al., 2016). The carbonates are formed during specimen curing and sample
319
preparation due to the carbonation of portlandite and possibly C(A)SH gel. The bound
320
water from ettringite and C(A)SH gel is easily lost at approximately 100-200 °C. The
321
area size of the first endothermic peak in DTG curve of N-3S at 3 d is greater than
322
that of N-0 at 3 d, which proves that the hydration of mineral admixtures in LCM is
323
accelerated in the presence of sodium sulfate.
RI PT
315
The content of portlandite in hydrated paste can be obtained based on the mass loss
325
of Ca(OH)2 from TG and DTG curves, and the corresponding results are compared to
326
the QXRD results for the portlandite content shown in Fig. 8. From Fig. 8, the
327
addition of 3 wt% sulfate sodium in LCM substantially accelerates the consumption
328
of Ca(OH)2 in matrix after 3 d of curing. Thus the content of Ca(OH)2 is rapidly
329
consumed at early age, which results in the improvement of mechanical properties of
330
LCM due to additional hydration products formed. Moreover, a certain amount of
331
portlandite appears in the matrix at 90 d, which suggests that the mineral admixtures
332
continue to hydrate slowly over 90 d of curing. Additionally, the relative error of
333
Portlandite content measured by QXRD and TG-DTG is relatively small, which
334
supports the high accuracy of the QXRD results in this study. 3d 28d 90d -0.03
Carbonates
95
90
EP
Weight /%
Portlandite
AC C
85
Ettringite & C-(A)-S-H
80
0
200
400
600
Temperature /℃
335
800
-0.06
-0.09
-0.12 1000
Derivative weight %/℃
0.00
100
0.00 3d 28d -0.03 90d
Carbonates Portlandite
95
-0.06 90 -0.09 85
-0.12 Ettringite & C-(A)-S-H
80 0
200
-0.15 400
600
Temperature /℃
(a) N-0 (b) N-3S Fig. 7. TG and DTG curves of N-0 and N-3S paste samples
800
1000
Derivative weight %/℃
100
Weight /%
TE D
M AN U
SC
324
ACCEPTED MANUSCRIPT QXRD TGA
10 8 6 4 2
RI PT
Portlandite content wt%
12
0
N-0-3d N-3S-3d N0-28d N-3S-28d N0-90d N-3S-90d
336 337
Fig. 8. Portlandite content in N-0 and N-3S pastes
3.5 Hydration heat
The hydration heat flow and cumulative hydration heat of LCM for different
339
dosages of sodium sulfate are shown in Fig. 9. From Fig. 9(a), three prominent
340
exothermic peaks appear in the hydration heat flow curve after the induction period.
341
The first exothermic peak is caused by the hydration of the minor content (10 wt%) of
342
Portland cement in the LCM. The Portland cement in the LCM reacts first due to its
343
high activity. Subsequently, the blast furnace slag starts to react with portlandite and
344
the hydration of slag will be accelerated in the alkaline environment (Myers et al.,
345
2017). Moreover, the activity of slag in mineral admixtures is far greater than that of
346
the fly ash. Therefore, the second exothermic peak is due to the hydration of GGBS
347
particles in the mineral admixtures. The area of the third exothermic peak in the LCM
348
is enlarged with increasing sodium sulfate amount, which indicates that the sodium
349
sulfate effectively improves the formation of a specified phase in the hydration
350
products. Considering the test results of XRD and FTIR, the third exothermic peak in
351
the heat flow curve presents the formation of ettringite during the early hydration
352
process. The curve of hydration heat flow of N-4S mixture is very close to that of the
353
N-3S mixture, which suggests that the 3 wt% sodium sulfate in the LCM is sufficient
354
to achieve a good activation effect in the early period. Considering the test results of
355
the mechanical properties, an excessive amount of sodium sulfate may affect the
356
hydration of LCM in the later period.
M AN U
TE D
EP
AC C
357
SC
338
The sulfate sodium increases the cumulative hydration heat of LCM mainly due to
358
additional ettringite formation. The cumulative hydration heat of N-3S mixture
359
reaches approximately 253J/g after 120 h, 22% higher than that of the N-0 mixture,
360
which is well agree with the mechanical performance measurements. The cumulative
361
heat of hydration of all mixes is far below that of Portland cement, which is
362
meaningful for LCM used in massive concrete engineering.
N-0 N-1S N-2S N-3S N-4S
1.0
0.5
0.0
N-0 N-1S N-2S N-3S N-4S
250 200 150 100 50 0
0
24
48
72
96
120
Time (hour)
0
24
48
72
96
120
Time (hour)
(b) Cumulative heat of hydration
(a) Hydration heat flow
363
300
RI PT
Heat flow (mW/g)
1.5
Cumulative heat of hydration (J/g)
ACCEPTED MANUSCRIPT
Fig. 9. Hydration heat evolution curves of LCM with different dosages of sodium sulfate
3.6 Microstructure
365 366
3.6.1 MIP The pore size distribution and total porosity of N-0 and N-3S hydrated paste at 90 d
367
measured by the MIP test are presented in Fig. 10. It should be noted that the pore size
368
distribution measured by MIP test differs somewhat from the real pore size
369
distribution of sample according to previous studies (Galle, 2001). The pores in
370
hydrated paste are classified as gel pores (micropores, 0-10 nm), mesopores (10-50
371
nm), and capillary pores (over 50 nm) according to the pore size in this study (Mehta
372
and Monteiro, 2006). The total porosity of N-3S and N-0 paste after curing 90 d is
373
29.47% and 31.26%, respectively, which suggests that the sodium sulfate in LCM
374
reduces the total porosity during the hydration process to some extent. More
375
importantly, N-3S paste shows more gel pores in the range of 4-10 nm (green area)
376
compared to N-0 paste. In addition, the amount of capillary pores of N-0 paste in the
377
range of 50-100 nm (yellow area) is greater than that of N-3S paste. The critical pore
378
entry radius, recorded from the peak of the cumulative intrusion curves of N-0 and
379
N-3S paste, is 40.4 nm and 50.4 nm, respectively. The number of capillary pore from
380
N-3S and N-0 paste in the range of 50-100 nm reaches 16.6% and 32.2% of the
381
number of total pores, respectively, which proves that the sodium sulfate in the
382
hydration process effectively refines the pore size distribution of LCM. The improved
383
pore size distribution of the LCM at micro-scale ultimately augments the mechanical
384
properties of the LCM at macro-scale.
AC C
EP
TE D
M AN U
SC
364
ACCEPTED MANUSCRIPT
20
N-0 N-3S 80% N-0 N-3S
10-50 nm 60%
Percentage /%
Porosity /%
25
Capillary pores
30
Mesopores
Gel pores
35
15
40%
20%
over 50 nm
0-10 nm
10 0%
Gel pores
Mesopores Capillary pores
0 1
10
RI PT
5
100
1000
10000
100000
Pore diameter /nm
385
(a) Pore size distribution (b) Total porosity Fig. 10. Pore size distribution and total porosity of N-0 and N-3S paste at 90 d
388
amount of needle-like and lath-like ettringite (AFt) has formed on the surface of the
389
mineral admixtures, which coincides with the previous test results. However, the large
390
number of microspores in the matrix results in a loose microstructure of hydrated
391
paste, particularly in N-3S paste, because the hydration degree of mineral admixtures
392
is relatively low so that the hydration products are not enough to fill the coarse pores
393
in the matrix. In addition, the bonding of mineral admixtures in N-3S past is more
394
compact than that of N-0 paste due to additional amount of formed and mutually
395
intermixed ettringite, which is beneficial for improving the mechanical properties at
396
early age.
AC C
EP
TE D
M AN U
SC
387
3.6.2 SEM-EDS The SEM micrographs of N-0 and N-3S paste at 3 d are shown in Fig. 11. A large
386
397
(a) N-0-3 d
(b) N-0-3 d
(c) N-3S-3 d
(d) N-3S-3 d
Fig. 11. The SEM micrographs of N-0 and N-3S paste at 3 d
ACCEPTED MANUSCRIPT Representative SEM micrographs of the microstructure of N-0 and N-3S paste at 90
399
d are represented in Fig. 12. The ettringite crystals (which appear lath-like), C(A)SH
400
gel and incompletely hydrated mineral admixtures (composed mainly of fly ash) can
401
be observed in the microstructure of N-0 and N-3S paste. In addition, a large number
402
of coarse pores appear in N-0 paste compared to N-3S paste, which is consistent with
403
the results of the MIP test. Moreover, the amount of ettringite crystals in the N-0
404
matrix is lower than in the N-3S matrix. The C(A)SH gel features an uncompact
405
microstructure with many coarse pores, as shown in the dashed area. Compared to
406
N-0 mixture, a sufficient quantity of ettringite crystals formed in the pores in the
407
matrix, refining the pore structure and reducing the amount of capillary pores in the
408
hydrated paste.
(b) N-0-90 d
EP
TE D
(a) N-0-90 d
M AN U
SC
RI PT
398
(c) N-3S-90 d
410
Fig. 12. The SEM micrographs of N-0 and N-3S paste at 90d
AC C
409
(d) N-3S-90 d
To investigate the elemental composition of hydration products, an EDS test is
411
conducted on the N-0 and N-3S sample on the basis of the acquired SEM micrographs.
412
The test results of element mapping of Fig. 12 (b) and Fig. 12(d) (corresponding to the
413
N-0 and N-3S samples, respectively) are presented in Fig. 13(a) and Fig. 13(b),
414
respectively. The Ca/Si ratio of hydration products of N-0 and N-3S paste in a
415
selected area is 1.362 and 1.128, respectively, and the Al/Si ratio of hydration
416
products of N-0 and N-3S paste in the same area is 0.228 and 0.287, respectively. To
417
further analyze the Ca/Si ratio and Al/Si ratio of CS(A)H gel, an EDS test is
418
performed to six selected points from C(A)SH gel, and the corresponding results are
419
illustrated in Fig. 14(c) and Fig. 14(d). It should be mentioned that the aluminum
ACCEPTED MANUSCRIPT phase can substitute silicon in C-S-H gel and form CASH gel in the hydration process.
421
(Lothenbach and Nonat, 2015). Therefore, the aluminum phase should be taken into
422
account in this study, and a more proper definition for the ratio of Ca/Si is generally
423
Ca/(Si+Al) due to substitution of silicon by aluminum. From Fig. 13(c) and Fig. 13(d),
424
the Ca/(Si+Al) ratio (average value of six points) of C(A)SH gel from N-0 and N-3S
425
paste is 1.322 and 1.075, respectively. The Al phase cotent in C(A)SH gel from N-3S
426
mixture is slightly higher than that of N-0 mixture at curing 90 d, which suggests that
427
the additional aluminum from mineral admixtures in N-3S paste is incorporated in the
428
bridging tetrahedra of the silicate chain (L'Hopital et al., 2016; Richardson, 2008).
429
Therefore, the sodium sulfate as an activator used in LCM decreases the Ca/Si ratio of
430
C(A)SH gel in the hydrated paste. 18
At % of element mapping
At % of element mapping
18
12 9 Ca
6
Si + Al
= 1.053
Al Si
3 0
15
M AN U
15
= 0.228
12
9 6
Al
Mg
S
Fe
TE D
Si
Ca
431 432
3.4 Drying shrinkage
5 0
Point1 Point2 Point3 Point4 Point5 Point6
Si
Al
Si
= 0.287
Mg
S
Fe
Ca Si Al
25
At % of element
EP
10
Al
(b) N-3S
Ca Si Al
AC C
At % of element
15
= 0.852
3
(a) N-0
20
Ca Si + Al
0
Ca
25
SC
RI PT
420
20 15 10 5 0 Point1 Point2 Point3 Point4 Point5 Point6
(c) N-0 (d) N-3S Fig. 13. The EDS test results of N-0 and N-3S paste at 90 d
433
The drying shrinkage of LCM mortar specimens for different contents sodium
434
sulfate is shown in Fig. 14. The phenomenon of drying shrinkage from cement-based
435
materials results from the evaporation of the free water from the microscale pores in
436
cement hardened paste at a low-humidity environment. Therefore, the drying
ACCEPTED MANUSCRIPT shrinkage is mainly influenced by the composition of binding materials. From Fig.
438
15, the LCM specimens with sodium sulfate show a lower drying shrinkage than that
439
of the control specimens. The drying shrinkage of LCM specimens with 3 wt% and 4
440
wt% sodium sulfate at 90 d decreases to 578 µε and 573 µε, respectively, while the
441
drying shrinkage of control specimens reaches 653 µε. Therefore, the drying
442
shrinkage of LCM is reduced when the sodium sulfate as activator is added into the
443
LCM, mainly because additional ettringite in the form of crystal generates in the
444
paste, further restricting the drying shrinkage of LCM. N-0 N-1S N-2S N-3S N-4S
SC
600
500
400
300
200 7
M AN U
Drying shrinkage µm/m (10-6)
700
RI PT
437
14
21
28
56
90
Age /day
445 446
Fig. 14. Evolution of drying shrinkage of LCM with different amounts of sodium sulfate
4. Discussion
As seen in equation (1) and (2), the major effect of sodium sulfate in LCM
448
hydration is that the soluble sulfate provides the sulfate ions to form additional
449
ettringite in the hydration process. In addition, the sodium sulfate reacts with calcium
450
hydroxide and generates sodium hydroxide in the pore solution of the matrix, which is
451
beneficial for the dissolution of active alumina and silica from mineral admixtures due
452
to the increased pH value of the pore solution. According to the previous literature,
453
the presence of 4 wt% sodium sulfate can raise the pH value of saturated Ca(OH)2
454
solution from 12.50 to 12.75 at 23 °C (Shi and Day, 2000a). Thus, the final setting
455
time of LCM with sodium sulfate is shorten as a certain extant because the sodium
456
sulfate accelerates the hydration of LCM.
EP
AC C
457
TE D
447
Na2SO4 + Ca(OH)2 + 2H2O＝2NaOH + CaSO4·2H2O
(1)
458
3(CaO·Al2O3) + 3(CaSO4·2H2O) + 26 H2O＝3CaO·Al2O3·3CaSO4·32H2O (2)
459
A large quantity of ettringite is formed in LCM at the early stages of hydration,
460
practically in N-3S paste. It should be noted that the amount of ettringite formed in 1
461
d is limited; most of the ettringite is formed over the subsequent two days because the
462
aluminum phase in pore solution is mainly from hydrated Portland cement at the
463
initial stage of hydration. However, the amount of Portland cement in LCM accounts
ACCEPTED MANUSCRIPT 464
for only 10 wt%, which limits the amount of calcium aluminum hydrates in the matrix.
465
Therefore, only a small amount of ettringite is formed in the first 24 hours even
466
though the sulfate content in the paste is sufficient. The active aluminum phase from the mineral admixtures accelerates dissolution
468
with an increase in the alkalinity in the pore solution due to the hydration of Portland
469
cement. According to the kinetic analysis, the presence of sodium sulfate effectively
470
accelerates both the dissolution of the glassy phase derived from the mineral
471
admixtures and the reaction between Ca(OH)2 and mineral admixtures (Shi and Day,
472
2000b). Subsequently, the aluminum phase reacts with sulfates, forming a great
473
amount of ettringite until the sulfates are consumed. The XRD test results indicate
474
that the content of ettringite in paste no longer increases after curing for 3 d. Note that
475
the ettringite formed in the early stage of hydration does not cause expansion damage,
476
because the space in the matrix can accommodate the formation of a great amount of
477
ettringite at the early stage of hydration. The formed ettringite serves as a mechanical
478
frame and increases the volume of hydration products in the hydrated paste, which
479
effectively enhances the mechanical performance of LCM at 3 d. Meanwhile, the
480
hydration heat flow of LCM significantly decreases after 3 d when a great number of
481
hydrates (C(A)SH gel and ettringite) form in the paste.
M AN U
SC
RI PT
467
With increasing curing age, the C(A)SH gel that originates from the hydrated
483
mineral admixtures is generated continuously, and ettringite crystals are inserted in
484
the C(A)SH gel, which results in a dense microstructure of hydrated paste with fewer
485
pores due to the void in the matrix occupied by C(A)SH gel. Thus, the mechanical
486
property of LCM activated with sodium sulfate increases stably with prolonged curing
487
age. Furthermore, a large quantity of ettringite as crystals and unreacted mineral
488
admixtures as micro-aggregate in the matrix restrict the drying shrinkage of LCM.
489
Thus, the drying shrinkage of the LCM containing sulfate sodium is reduced by
490
comparison with the plain LCM specimens.
EP
AC C
491
TE D
482
Based on the previous investigation, the energy intensity and carbon emissions of
492
the mixture of LCM used in this work are 1.80 MJ/kg and 0.21kg/kg, respectively;
493
however, the energy intensity and carbon emissions of Portland cement is 5.5 MJ/kg
494
and 0.93 kg/kg, respectively (Wu et al., 2018a). Furthermore, sodium sulfate can be
495
obtained as a secondary product from the manufacture of hydrochloric acid, battery
496
acid, viscose rayon, silica pigments and even from natural salt lake or brines.
497
Therefore, LCM activated with a minor amount of sodium sulfate offers a bright
498
prospect in the context of green production.
ACCEPTED MANUSCRIPT 499
4. Conclusions In this work, we used the different dosages of sodium sulfate as an activator to
501
improve the performance of LCM. An experimental study was conducted to
502
investigate the performance of LCM in the presence of sodium sulfate. The
503
mechanical properties, workability and drying shrinkage of LCM containing different
504
amounts of sodium sulfate were studied, and the evolution in microstructure and
505
compositions of hydration products of LCM paste with and without sulfate sodium
506
were analyzed. Finally, the effects of sodium sulfate as an activator in the hydration of
507
LCM were also discussed according to the test results. The conclusions of the test
508
results and discussion can be summarized as follows:
SC
RI PT
500
(1) The mechanical properties of LCM were substantially increased in the presence
510
of sodium sulfate, particularly in the early stage. In this study, LCM containing 3 wt%
511
sodium sulfate showed the best mechanical properties, reaching 14.6 MPa and 39.5
512
MPa at 3d and 90 d, respectively, corresponding to an increase of 57.0% and 20.8%
513
compared to that of conventional LCM.
M AN U
509
(2) Compared to the plain LCM, a small amount of sodium sulfate in the LCM
515
effectively accelerated the hydration of solid waste and consumed more portlandite
516
during the hydration process. Furthermore, a large amount of ettringite and the
517
C(A)SH gel with a low Ca/Si ratio formed in hydrated LCM with sodium sulfate.
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(3) The quantity of ettringite crystals in the LCM containing sodium sulfate at 1 d
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was limited, and a majority of ettringite formed at 2 d and 3 d, which effectively
520
increased the mechanical performance of the LCM at 3 d. Moreover, ettringite crystals
521
were inserted in C(A)SH gel from the hydrated mineral admixtures, which resulted in
522
a dense microstructure of hydrated paste with fewer pores and increased the
523
mechanical performance of the LCM in the later period.
525
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EP
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Acknowledgements
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This work was financially supported by National 973 Program (No. 2015CB655102),
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National Natural Science Foundation of China (No. 51678143, No. 51878153, No.
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51808189, and No. 51508090) and Postgraduate Research &Practice Innovation
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Program of Jiangsu Province (KYCX18_0078)
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