The influence of chemical admixtures on the strength and hydration behavior of lime-based composite cementitious materials

Cement and Concrete Composites 103 (2019) 353–364

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The influence of chemical admixtures on the strength and hydration behavior of lime-based composite cementitious materials

T

Meng Wua,b, Yunsheng Zhanga,b,∗, Yantao Jiac,∗∗, Wei Shea,b, Guojian Liua,b, Yonggan Yanga,b, Zhidan Ronga,b, Wei Suna,b a

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

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical admixtures Lime Hydration products Cementitious materials

This study focused on the modification of a mixture of a lime-based composite cementitious material (LCM) by using chemical admixtures to increase the mechanical performance of the LCM. The different types and amounts of chemical admixtures including NaOH and Na2SO4 were used as activators to prepare the novel modified LCM. The mechanical strength, hydration kinetics, assemblage of hydrates and microstructure of the modified LCM were systemically explored and compared to conventional LCM. The experimental results indicated that Na2SO4 had better effects on the improvement of LCM strength compared to NaOH when used alone as the activator. The composite activator that was composed of Na2SO4 and NaOH showed the optimal effects on the increase of the LCM strength. The composite activator effectively increased the hydration rate of the LCM, and more hydrates were formed during the hydration process of the LCM. Furthermore, the U-phase was formed only in the hydration products of the LCM containing the composite activator. A large amount of U-phase as crystals effectively occupied and filled the space in the LCM paste and led to the formation of a compact microstructure with fewer macro-pores. Thus, the presence of U-phase in the paste increased the strength of the LCM.

1. Introduction Ordinary Portland cement (OPC) is widely used in the construction industry across the world because of its good performance and low cost. In 2017, the production of cement from the Chinese cement industry was 2.32 billion tons, and the total production of cement in worldwide is 4.13 billion tons. However, the large output of the cement industry brings a series of problems, including the consumption of the nonrenewable resources, environmental pollution, and carbon emissions, especially in China [1,2]. For instance, the carbon emission and energy intensity of per kg Portland cement is 0.93 kg and 5.5 MJ, respectively [3]. Cement production accounts for 8–9% of global carbon footprint and 2–3% of energy consumption [4]. Thus, greenhouse gas emissions from the cement industry cannot be ignored. Carbon emissions from the cement industry are mainly derived from the calcination of cement clinker in kilns at a very high temperature (approximately 1450 °C) and result in a significant amount of pulverized coal that is burned. The decomposition of calcite in the process of cement production also releases carbon dioxide. Therefore, low-carbon

∗

cementitious materials are an essential issue in the field of building materials research. Currently, one of the most promising methods to diminish carbon emissions from cement-based materials is the replacement of partial Portland cement by mineral admixtures in cement or supplementary cementitious materials (SCMs) in concrete [5–9]. Meanwhile, there are numerous of byproducts and solid waste from industrial production, such as slag, red mud, flue gas desulfurization gypsum, coal fly ash and so on. For instance, coal fly ash is one of the largest sources of solid waste in China. The output of coal fly ash from China was 0.6 billion tons in 2017, and a large quantity of coal fly ash is derived from thermal power plants. Nevertheless, the recycling of coal fly ash in China only reaches 70%, and a substantial amount of it is stored as ash ponds and tailing hills. Thus, solid waste that is prepared as mineral admixtures to reduce the use of Portland cement or cement clinkers is an attractive approach. Many studies have considered novel low carbon binder materials by using mineral admixtures to prepare blended cement, where the content of the mineral admixtures in the blended cement is usually less than 50 wt% [10–13]. Therefore, to further decrease the amount of OPC

Corresponding author.School of Materials Science and Engineering, Southeast University, Nanjing, 211189, China. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Jia).

∗∗

https://doi.org/10.1016/j.cemconcomp.2019.05.008 Received 21 January 2019; Received in revised form 6 May 2019; Accepted 7 May 2019 Available online 11 May 2019 0958-9465/ © 2019 Elsevier Ltd. All rights reserved.

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in binder materials and increase the amount of mineral admixtures in them, a novel green binder material, referred to as a lime-based composite cementitious material (LCM) has attracted attention and interest from researchers in the area of building materials [14–16]. The previous studies suggest that the LCM can be prepared by lime and blast furnace slag or metakaolin and show good performance [17–19]. The suitable amount of lime as a moderate alkaline activator is added to the binder materials to react with mineral admixtures and form hydrates during the hydration process. At present, the novel type of LCM is mainly composed of a small amount of OPC and a substantial amount of mineral admixtures as well as lime. The minor amount (10–20 wt%) of OPC in LCM can effectively improve the mechanical strength of LCM and increase the hydration rate of mineral admixtures [16]. Consequently, carbon emissions and energy intensity of the LCM is much lower than those of OPC. However, it should be noted that the mechanical performance of LCM is lower than that of OPC due to the low activity and reaction rate of mineral admixtures, especially when fly ash is added into the LCM. Thus, the study focused on the modification of the mechanical strength of LCM need to be carried out. Based on previous findings, the hydration rate of mineral admixtures will be increased when the chemical admixtures including strong alkali (such as NaOH) and soluble sulfate (such as Na2SO4) are added in the binder materials [20–23]. The strong alkali can accelerate the depolymerization of glassy phase from slag or fly ash to improve the hydration rate of them in binder materials, and the soluble sulfate is beneficial for the formation of ettringite in hydrates to improve the strength of binder materials. Thus, the strong alkali and soluble sulfate can effectively enhance the strength of blended cement containing high-content solid waste, especially in the blended cement using slag or fly ash as mineral admixtures [23,24]. Thus, in this work, the chemical admixtures, including NaOH and Na2SO4, were used as activators to prepare the novel modified LCM. The influence of different activators on the development of the strength and hydration process of the LCM was studied and compared. Thus, this study will provide new perspectives to prepare novel modified LCM with high performance.

Table 2 The material composition of the LCM (by mass).

Portland cement GGBFS FA Gypsum Hydrated lime

MgO

SO3

K2 O

Na2O

LOI

21.60

64.38

4.38

3.42

3.43

2.23

–

0.51

2.54

32.72 51.53 0.30 0.47

37.12 4.40 46.89 97.30

15.51 30.41 0.14 0.41

0.24 6.90 0.07 0.23

5.50 0.91 0.20 1.00

2.61 0.91 52.09 0.10

0.30 1.37 – 0.32

0.40 0.62 0.11 –

0.36 1.52 7.01 26.75

10

10

80

Binder materials (LCM)

100 100 100 100 100 100 100 100 100 100 100 100 100

Chemical admixture Sodium sulfate

Sodium hydroxide

– 1 2 3 – – – 1 2 2 3 3 3

– – – – 1 2 3 1 1 2 1 2 3

w/b

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

2.2. Specimen preparation In this study, the LCM was prepared by Portland cement, hydrated lime and mineral admixtures at 1: 1: 8 (by mass). The chemical admixtures were used as activators for the LCM, and the details of all LCM mixtures tested in this study were listed in Table 2 and Table 3. The LCM mortar specimens were prepared at a fixed river sand to binder material ratio of 3 and a water to binder material ratio (w/b) of 0.5. The chemical admixtures were pre-dissolved in the water. All the LCM fresh mortar were prepared based on Chinese standard GB/T 17671-1999, and the fresh mortar were immediately cast into 40 × 40 × 160 mm molds. Meanwhile, the fresh paste were also prepared with the same mixture and procedure and cast into plastic cylindrical molds. All the specimens were cured in a moist curing room (RH ≥ 95%) at 20 °C for 24 h. After demolding, the specimens were cured in the same environment for 3, 7, 28, and 90 days. 2.3. Test methods 2.3.1. Compressive strength The compressive strength of the LCM mixtures was determined on mortar specimens based on China standard GB/T 17671-1999. The average value of three parallel specimens was reported for each mixture. 2.3.2. Heat of hydration The heat flow data of LCM mixtures were determined at 20 °C by using a eight channel TAM Air isothermal calorimeter from TA Instruments, USA. The fresh LCM paste sample (approximately 10 g) was poured into a small ampoule bottle and the sealed bottle was quickly placed into the isothermal calorimeter. The heat of hydration data were automatically recorded by a calorimeter for 120 h.

Chemical composition (%) Fe2O3

LCM

Note: In this study, “S” represents Na2SO4 and “N” represents NaOH.

Table 1 Chemical compositions of raw materials.

Al2O3

Mineral admixtures

L-control L-1S L-2S L-3S L-1N L-2N L-3N L-1S1N L-2S1N L-2S2N L-3S1N L-3S2N L-3S3N

P·II 52.5 Portland cement, hydrated lime, ground granulated blast furnace slag (GGBFS), Type F fly ash (FA) and gypsum were used to prepare the LCM specimens. The chemical compositions of the raw materials determined by X-ray fluorescence (XRF) are presented in Table 1. The mineral admixtures in the present study were prepared by blending GGBFS, FA and gypsum at mass ratios of 0.475, 0.475, and 0.05, respectively, based on the previous findings [14]. Clean river sand with a fineness modulus of 2.8 was used as the fine aggregate to prepare LCM mortar specimens. The chemical admixtures (NaOH and Na2SO4) used in this work were analytical reagent (purity over 99 wt%) from Sinopharm Chemical Reagent Co., Ltd.

CaO

Hydrated lime

No.

2.1. Materials

SiO2

Portland cement

Table 3 Mix proportions of the modified LCM (by mass).

2. Experimental

Raw materials

No.

2.3.3. X-ray powder diffraction (XRD) The LCM paste specimens were crushed and immersed in ethanol for approximately three days to arrest hydration, and then the small paste samples were vacuum-dried at 50° for 12 h. The fine powder samples (< 75 μm) were prepared by carefully grinding a portion of the dried paste samples. XRD was conducted on a Bruker D8 Discovery diffractometer with 354

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Compreesive strength /MPa

Cu-Kα radiation and at 40 kV and 30 mA. The scanning range was 5°–80° and the step size was 0.02°/step. The 10 wt% corundum powder as an internal standard was mixed with fine powder samples for the quantification analysis. The Rietveld method was used to quantify the hydration products of the LCM via TOPAS v4.0 software. 2.3.4. Thermogravimetric analysis (TGA) TGA was carried out on a STA449F3 thermogravimetric analyzer from NETZSCH company. The test was determined at a heating rate of 10 °C/min from ambient temperature to 1000 °C in a pure N2 atmosphere. The calcium hydroxide and chemically bound water content in the hydrated paste were calculated according to equations (1) and (2).

CH % =

CBW % =

(M50°C − M1000°C ) × 100% M1000°C

Compreesive strength /MPa

Compreesive strength /MPa

28d 90d

30

20

10

L-2S

L-3S1N

L-3S2N

L-3S3N

Thus, Na2SO4, as an activator used alone, shows a better effect on the improvement of compressive strength compared to NaOH. Furthermore, in this study, the LCM mix containing 3 wt% NaOH leads to a lower strength at the later age compared to the control mixture, and the compressive strength of L-3N mixture at 28 days and 90 days is 26.7 MPa and 30.5 MPa, respectively. Thus, it should be note that a high dosage of NaOH hinders the strength development of the LCM after longer curing and results in a lower strength at a long-term. To further enhance the strength of the LCM, composite chemical admixtures that consisted of Na2SO4 and NaOH as activators at a certain mass ratio were used, and the corresponding test results are displayed in Fig. 2. Compared to Figs. 1 and 2, it can be found that the composite chemical admixtures show more efficient improvement of compressive strength compared to the Na2SO4 or NaOH used alone. The compressive strength of L-3S1N and L-3S2N at 3 d curing-age is 15.9 MPa and 18.2 MPa, respectively. Moreover, the strength of the LCM activated by composite chemical admixtures increases in a stable manner with prolonged curing. The compressive strength of the L3S1N, L-3S2N, and control mixtures after curing 90 days is 41.7 MPa, 42.8 MPa, 32.7 MPa, respectively. Thus, the effects of composite chemical admixtures as activators on the enhancement of the strength of LCM are better than those of Na2SO4 or NaOH used alone. From the test results, the L-3S2N mixture has the best compressive strength compared to the other mixtures. The compressive strength of the L-3S2N mixture at 90 days is approximately 31% greater than that of the control mixture. In addition, the excessive amount of NaOH in the composite chemical admixtures is not beneficial for the improvement of the strength of the LCM. The mechanical performance of the L-3S3N mixture at 90 days is lower than that of the L-3S1N and L-3S2N mixtures. According to the test results, the compressive strength contour plots of LCM mixtures containing different amounts of chemical admixture at 3 days and 90 days are shown in Fig. 3. From Fig. 3, the composite chemical admixture is more beneficial to the development of the LCM compressive strength. In this study, to obtain a high mechanical

The compressive strength values of the LCM specimens containing different amounts of NaOH and Na2SO4 are shown in Fig. 1. From Fig. 1, the mechanical strength of the LCM mixtures at 3 days is remarkably enhanced when NaOH and Na2SO4 are added into the LCM mixtures. The compressive strength of L-3S, L-2N and control mixture at curing for 3 days is 14.5 MPa, 14.6 MPa, and 9.3 MPa, respectively. Thus, the NaOH and Na2SO4 as chemical admixtures effectively improve the compressive strength of the LCM at the early stage. With increasing curing time, the strength of LCM specimens continues to grow due to the hydration of fly ash and GGBFS. It is worth noting that the L-group specimens activated by Na2SO4 have a good compressive strength at the later stage; however, the L-group mortar specimens containing NaOH show an insufficient increase in the compressive strength compared to the specimens containing Na2SO4. After 90 days of curing, the compressive strength of the L-3S, L-2N and L-control (control mixture) is 39.5 MPa, 33.5 MPa, and 32.7 MPa, respectively.

L-1S

L-2S2N

(2)

3.1. Compressive strength

L-control

L-2S1N

Fig. 2. Mechanical strength of LCM mixtures containing composite chemical admixtures.

3. Results and discussion

0

L-1S1N

10

(1)

2.3.6. Scanning electron microscope-energy dispersive spectrometer (SEMEDS) After vacuum drying, the selected LCM paste samples were coated with a platinum conductive film in a vacuum environment before the test, and the test was carried on an FEI Quanta 3D FEG SEM and an EDAX Genesis EDS analyzer.

3d 7d

L-control

20

0

2.3.5. Mercury intrusion porosimetry test (MIP) The MIP test was carried on an Autopore IV 9500 mercury porosimeter by using the small oven dried paste samples. The measurable pore size distributions of paste samples were from 4 to 350000 nm.

40

28d 90d

30

⋅ 74

CHw 18 × 100% M550°C

3d 7d

40

40

28d 90d

30

20

10

0

L-3S

3d 7d

L-control

L-1N

L-2N

L-3N

Fig. 1. Compressive strength of LCM mortar specimens containing different chemical admixtures. 355

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(b) 90 days

(a) 3 days

Fig. 3. Compressive strength contour plots of LCM mixtures containing chemical admixtures at 3 days and 90 days.

second exothermic peak appears after hydration for 24 h. This is because the hydraulic activity of GGBFS is enhanced in the alkaline condition because of the existing Ca(OH)2 [25]. The hydrated lime also provides sufficient calcium ions to form the C(A)SH gel. Thus, the second exothermic peak is mainly assigned to the hydration effect of the GGBFS in the LCM. From Fig. 4, with increasing the amount of Na2SO4 in LCM, the heat flow value of the third exothermic peak in the decelerating period is obviously enhanced, and this peak is attributed to the formation of ettringite in the paste. Thus. the peak value and the area of the exothermic peak are enlarged when Na2SO4 is added into the LCM mixture [22,23]. The active aluminum phase from GGBFS and FA reacts with hydrated lime and sulfates to form additional ettringite. The corresponding reactions are presented in Eq. (3) and Eq. (4).

property, the optimal activator composites for the LCM consist of approximately 2.7%–3% Na2SO4 and 1.3%–2.2% NaOH (mass ratio). Meanwhile, with the same amount of NaOH, it can be observed that the strength of the LCM is improved when the amount of Na2SO4 in the activators increases. However, with equal amounts of Na2SO4, the compressive strength of the LCM first increases and then decreases with increasing amounts of NaOH in the composite chemical admixtures. 3.2. Heat of hydration The heat flow value from the LCM mixtures containing chemical admixtures was determined via an isothermal calorimeter in this study. Four periods (including initial stage, induction stage, accelerating stage and decelerating stage) can be found in the hydration process of binder materials. Fig. 4 depicts the evolution of the heat of hydration of the LCM mixtures blending with different amounts of Na2SO4. From Fig. 4, it can be seen that three exothermic peaks were observed in the accelerating period and decelerating period of the plain LCM hydration. The first exothermic peak in the accelerating period appears approximately 4 h after hydration, which indicates the speedy hydration of the cement particles in the LCM. Note that the maximum value of LCM heat flow is smaller than other exothermic peaks due to the low content of Portland cement in the LCM [16]. The second exothermic peak can be observed approximately 12 h after hydration, and the peak value of the L-control L-1S L-2S L-3S

1.0

0.5

0.0

0

24

48

72

96

(3)

Al2 O3 + 3(CaSO4⋅2H2 O) + 3Ca (OH )2 + 23H2 O = 3CaO⋅Al2 O3⋅3CaSO4 (4)

⋅32H2 Oa

Therefore, the cumulative heat of the LCM mixtures containing Na2SO4 is notably increased due to the abundant ettringite formation in the paste. The peak value of the third exothermic peak is greater than that of the other exothermic peaks due to the positive effect of Na2SO4. Additionally, the cumulative hydration heat of the LCM is enlarged

Cumulative heat of hydration (J/g)

Heat flow (mW/g)

1.5

Na2 SO4 + Ca (OH)2 + 2H2 O = CaSO4⋅2H2 O + 2NaOH

120

300

L-control L-1S L-2S L-3S

250 200 150 100 50 0

0

24

Time (hour)

48

72

96

Time (hour)

(b) Cumulative heat of hydration

(a) Hydration heat release rate

Fig. 4. Calorimetric curve of LCM mixture with Na2SO4. 356

120

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3.0

L-control L-1N L-2N L-3N

Heat flow (mW/g)

2.5 2.0 1.5 1.0 0.5 0.0

0

24

48

72

96

Cumulative heat of hydration (J/g)

M. Wu, et al.

120

300

L-control L-1N L-2N L-3N

250 200 150 100 50 0

0

24

48

72

96

120

Time (hour)

Time (hour)

(b) Cumulative heat of hydration

(a) Hydration heat release rate

Fig. 5. Calorimetric curve of LCM mixture with NaOH.

of the heat flow of the LCM containing the composite activator is similar to that of the LCM containing NaOH, the area of the exothermic peak is enlarged compared to the LCM mixture with the same amount of NaOH. The cumulative hydration heat of the L-3S2N mixture is 260.8 J/ g at 5 days, however, the cumulative hydration heat of the L-2N mixture only reaches 229.9 J/g at 5 days. Thus, the composite chemical admixture shows better activation effects in accelerating LCM hydration from the test results, and the activity from the GGBFS and FA particles is effectively activated in the early stages of hydration. The contour plots of the cumulative hydration heat of LCM mixtures containing activators at 5 days are depicted in Fig. 7. As shown in Fig. 7, it suggests that the composite activators composed of 2.7%–3% Na2SO4 and 1.3%–2.4% NaOH has the optimal effects on accelerating the hydration of the LCM. From test results, the composite activator in the optimal range shows the best activating effect on the development of compressive strength of LCM. The heat of hydration has a close connection to the development of strength in a LCM. Therefore, the linear fitting between the compressive strength and total heat of hydration (at 5 days) is established in Fig. 8. From Fig. 8, the linear relationship between the strength and heat of hydration is established as y = 2.97x+133.13 (R2 = 0.87). It is clear that an increase in the heat of hydration has the benefit of improving the strength of the LCM.

because the additional ettringite crystals form in paste after hydration for 2 days. After hydration for 5 days, the total heat of hydration of the L-2S and L-3S mixtures is 244.8 J/g and 252.7 J/g, respectively. Fig. 5 illustrates the evolution of the rate of hydration of the LCM mixtures with NaOH. From Fig. 5, The first two exothermic peaks are caused by the hydration of cement particles and minerals admixtures (mainly of GGBFS particles), respectively, and the third exothermic peak is attributed to ettringite formation. With an increase in the amount of NaOH in LCM, the second and the third exothermic peak combine into a whole exothermic peak due to the increase of hydration rate, and the similar phenomenon can be observed when the blended cement cures at a high temperature [26]. Moreover, the peak value of the combined exothermic peak from the LCM mixture with NaOH is far greater than that of the exothermic peaks from the plain mixture. Therefore, it can be concluded that NaOH effectively promotes the hydration reaction of the LCM at the early period, which is in accordance with the results of mechanical strength at 3 days. The dissolved NaOH in the LCM rapidly enhances the pH value of the fresh LCM paste and effectively accelerate the dissolution of the glassy phase from the GGBFS particles. Thus, the hydration of GGBFS in the LCM mix with NaOH at an early period is faster than that of GGBFS in the control mix. Meanwhile, it should be noted that the value of heat flow of the L3N mixture is clearly lower than that of the plain mixture after hydration for 5 days, which suggests that the high amount of NaOH hinders the additional hydration of the LCM. The test results of compressive strength also proves that the high dosage of NaOH has a negative effect on the development of strength in the later period. From test results, the influence of NaOH in LCM is mainly on the hydration rate of mineral admixtures at the early stage, however, the presence of NaOH in LCM affects the hydration of LCM at the later stage at a certain extent. Even the high amount of NaOH impedes the hydration of LCM at the middle and later stage. Compare to NaOH, Na2SO4 as an activator effectively improve the formation the ettringite phase and increase the hydration heat of LCM at the early age. Moreover, the added Na2SO4 has little negative influence on the hydration rate of LCM at the later age, which is different from the NaOH as an activator used in the LCM. Thus, Na2SO4 shows better activating effects on the hydration of LCM compared to NaOH when they used alone. The evolution of the heat flow value of the LCM mixtures containing composite activators is shown in Fig. 6. The composite activators are composed of Na2SO4 fixed at 3% and NaOH fixed at 1–3%. From Fig. 6, the overlapped exothermic peak appears when the dosage of NaOH is up to 2%; moreover, the overlapped exothermic peak shifts to the early age with increasing dosage of NaOH in the LCM. Although the evolution

3.3. XRD analysis The assemblage of hydration products in L-2N, L-3S, L-3S2N and control paste at different curing ages were analyzed by XRD in this study. As seen in Fig. 9, ettringite (ICDD: 01-072-0646) as a crystal phase can be observed in the hydrates of all pastes. Moreover, the fullwidth-half-maximum (FWHM) value of ettringite phase from the L-3S mixture is enlarged compared to those in the L-control mixture; however, the corresponding area and intensity of ettringite phase in the XRD patterns of L-2N paste are shrunk compared to those in the Lcontrol paste. From the previous findings, the formation of ettringite is retarded with increasing pH. Moreover, the formed ettringite cannot remain stable, and will decompose in a high pH value environment [27]. Thus, the added NaOH in the LCM affects the generation of ettringite due to the increase of the alkalinity of the pore solution in the paste to some extent. From the XRD patterns of L-3S2N paste, it can be seen that a new mineral phase named U-phase appears in the hydrates of the L-3S2N paste at 3 days, 28 days and 90 days. The chemical composition of Uphase (ICDD: 00-044-0272) can be established as 4CaO·0.9Al2O3·1.1SO3·0.5Na2O·16H2O, and the crystal structure of U357

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3.0

L-control L-3S1N L-3S2N L-3S3N

2.5

Heat flow (mW/g)

Cumulative heat of hydration (J/g)

M. Wu, et al.

2.0 1.5 1.0 0.5 0.0 0

24

48

72

96

120

300

L-control L-3S1N L-3S2N L-3S3N

250 200 150 100 50 0 0

Time (hour)

24

48

72

96

120

Time (hour)

(a) Hydration heat release rate

(b) Cumulative heat of hydration

Fig. 6. Calorimetric curve of LCM mixture containing composite chemical admixtures.

phase is similar to the AFm-group crystals with hexagonal or pseudohexagonal layered structures [28–30]. Thus, the U-phase is regarded as a special form of the AFm phase due to the sodium bonded in the AFm structure. In this study, the U-phase is only found in the LCM mixture containing a composite activator consisting of NaOH and Na2SO4; however, the U-phase is not observed in the hydrates of LCM activated by NaOH or Na2SO4 alone. This is because the formation of U-phase requires a high alkalinity environment and sufficient sulfates and alumina. The chemical reaction to form U-phase is shown in Eq. (5). Previous studies indicate that U-phase appears in the hydration products of calcium sulfoaluminate cement that hydrates in a NaOH solution [31–33]. In addition, the formation of U-phase occurs prior to ettringite in theory, according to the solubility product of U-phase and ettringite [34].

Al2 O3 + 3.3Na2 SO4 + 6.9Ca (OH )2 + 42.9H2 O = 3.0U − phase + 3.6NaOH

From Fig. 9 (d), the FWHM values of the U-phase are almost unchanged from 3 days to 90 days, which suggests that the U-phase generates in the L-3S2N paste during the early-age hydration. The Uphase forms in the hydrated LCM paste and fills the pore and space in the microstructure of the paste at the early stage. Therefore, the formation of U-phase in the paste cannot result in the expansion and cracking of LCM specimens. Previous studies have shown that the delayed or secondary formation of the U-phase has a negative influence on the soundness of cementitious materials [33,35]. The hydrated lime reacts with active silica from GGBFS and FA and forms the C(A)SH gel. It should be noted that the alumina atom can substitute for the silicon atom in the unit structure of the CSH gel and lead to the formation of the C(A)SH gel. In the present study, the weak crystalline C(A)SH gel is observed in the XRD patterns. The microstructure of the C(A)SH gel shows similarity with Al-containing tobermorite (ICDD 00-019-0052) [36]. The calcium hydroxide is progressively consumed in the process of hydration; thus, the area and peak value of the diffraction peaks of portlandite decrease at 90 days. The quartz and mullite phases from unhydrated and incompletely hydrated FA particles are also found in XRD patterns due to the low activity of FA. To calculate the content of hydration products from the LCM paste containing different chemical admixtures, the Rietveld refinement method was used to further analyze the XRD patterns, and the results are presented in Fig. 10. In this work, calcium silicate represents the poorly crystalline C(A)SH gel, and the amorphous phase is mainly regarded as the C(A)SH gel without a crystalline structure. From Fig. 12, the amount of ettringite in the L-control, L-2N, L-3S and L-3S2N paste

Fig. 7. Heat of hydration contour plots of LCM mixtures containing chemical admixtures.

Heat of hydration (J/g)

270

y 2.97 x 133.13 (R 2 0.87)

260

250

240

230

220 30

35

40

(5)

45

Compressive strength (Mpa) Fig. 8. Relationship between compressive strength and heat of hydration.

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Q: Quartz P: Portlandite O: Corundum E: Ettringite M: Mullite C: C-(A)-S-H C: Calcite

Q: Quartz P: Portlandite O: Corundum E: Ettringite M: Mullite C: C-(A)-S-H C: Calcite

E

E M E OQCC

P

E M

E

P

M E O CC Q

E M E

P M E OQCC

E M

10

20

M P O

P

90d

PO

O

P

O

O

EM

OO

P

O

EM

28d

O

E

30

MO

40

P

3d

P

O

O

50

M E OQCC

P O

OO

P O

M

O

70

10

20

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Fig. 9. XRD patterns of LCM pastes at different curing age.

This result has some differences compared to the previous study, which shows that the partially synthesized ettringite immersed in the NaOH solution can convert into the U-phase [37]. This is probably because the pH value of the pore solution from the L-3S2N mixture is lower than that of the NaOH solution; meanwhile, the reaction temperature also

after curing 90 days is 12.9%, 8.9%, 17.5% and 9.8%, respectively, which proves the previous analysis. The amount of U-phase in the L3S2N paste at 90 days is 10.5%. Moreover, the content of ettringite and U-phase in the L-3S2N paste is stable from 3 days to 90 days, which indicates that the formed ettringite is not transformed into the U-phase.

Unhydrated cement Ettringite

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Mass fraction /%

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d d d d d 8d 8d 0d 3d -3d -3d -3d 90 28 28 90 90 l-2 l-9 olS-2 -3SNNN2N N-3S -2N 3 2 2 2 2 tro tro ntr S L L n n o S 3 S L L L L o o LL-c L-3 L-3 L-c L-c Fig. 10. Quantitative analysis of hydration products of LCM pastes. 359

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has a critical influence on the final test results. The mineral admixtures in the hydrated LCM paste show a lower hydration degree compared with Portland cement particles, especially fly ash, and the strength of the LCM at 90 days is slightly lower than the commercial OPC. However, from Fig. 6, the amount of calcium hydroxide in the L-control, L-2N, L-3S and L-3S2N paste at 90 days is 7.2%, 8.8%, 6.4% and 6.1%, respectively. It is suggested that the GGBFS and FA will continue hydration due to the presence of calcium hydroxide in the LCM paste. Therefore, the strength of the LCM mixture can continue to increase with prolonged the curing age.

3d 28d 90d

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3.4. TGA analysis

10 8 6 4 2

Fig. 13 shows the TG and DTG curves of four LCM paste samples at 3 days, 28 days, and 90 days, respectively. As shown in Fig. 11, it is clear that three endothermic peaks can be observed in the DTG curves, which correspond to peak values at approximately 100 °C, 400 °C and 600 °C. Moreover, the three endothermic peaks are attributed to weight loss of the C(A)SH gel and ettringite crystal, hydrated lime (calcium hydroxide) and calcite, respectively [38]. In addition, in the L-3S2N paste sample, the endothermic peak at approximately 270 °C is possibly caused by the decomposition of U-phase. The simultaneous dehydration of C(A)SH gel and ettringite results in a wide endothermic peak and a rapid weight loss from ambient temperature to 200 °C. The endothermic peak of calcium hydroxide is mainly derived from the unreacted hydration lime in the matrix, which is in accord with the results from the XRD test. The carbonates in the hydrated LCM paste are from the specimens prepared and cured due to the natural carbonation effect. Note that the sample preparation can also produce the carbonates, especially when the hydration is stopped by organic solvent exchange [39]. The amounts of calcium hydroxide are calculated from the TGA

0

L-control

L-2N

L-3S

L-3S2N

Fig. 12. Calcium hydroxide content in the LCM paste.

curve and the corresponding results are presented in Fig. 12. The content of calcium hydroxide in the L-control, L-2N, L-3S and L-3S2N pastes at 90 days is 7.18%, 8.76%, 6.13% and 6.67%, respectively, which is almost equal to the results from the Rietveld refinement of XRD data. The content of calcium hydroxide in the L-2N paste is greater than that in other LCM pastes and the amount of calcium hydroxide in the L-2N paste at 28 and 90 days is very close, which suggests that the hydration degree of GGBFS and FA in the L-2N pastes is limited at the later stage. To accurately determine the amount of hydrates in hydrated paste 360

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bound water in the L-control, L-2N, L-3S and L-3S2N pastes at 3 days is 12.40%, 14.78%, 18.53% and 18.35%, respectively, which suggests that NaOH and Na2SO4 both effectively increase the amount of hydrates in the LCM paste at an early age. While the quantity of chemically bound water in the L-control, L-2N, L-3S and L-3S2N pastes at 90 days is 20.28%, 18.17%, 22.99% and 25.53%, respectively. Thus, the test results prove that NaOH impedes the hydration of the LCM to a certain extent in the later period, which is agree with the results from the strength development of the LCM. The Na2SO4 and the composite activators both show sound effects on the hydration of the LCM, and more hydrates forming in the LCM paste results in an increase of chemically bound water content. Even the content of chemically bound water in the L-3S2N mixture increases by approximately 26% at 90 days compared to that in the control mixture. Thus, the composite activator efficiently accelerated the hydration of the LCM at both early and later ages. According to the strength test results, the linear relationship between the strength and chemically bound water can be described as y = 2.44–19.79 (R2 = 0.74) in Fig. 14. The increase in the chemically bound water is beneficial for the increase of the strength of LCM because of additional hydration products being formed in the matrix of LCM.

3d 28d 90d

25 20 15 10 5

L-control

L-2N

L-3S

L-3S2N

Fig. 13. Chemically bound water content in the LCM paste.

2.44 x 19.79 (R 2

0.74)

3.5. MIP analysis

40

The MIP technique was used to investigate the pore size distributions and total porosity of LCM pastes at 90-day curing age. As shown in Fig. 15, the critical pore entry radius is defined mathematically by the inflection point of the main mercury intrusion step, which can be recorded by the steepest slope from the cumulative mercury intrusion curve [40,41]. From Fig. 15(a), the critical pore entry radius of L-2N and L-Control paste is approximately 40 nm, and corresponding radius of L-3S and L3S2N paste is approximately 50 nm. Moreover, the ratio of mesopores (10–50 nm) in the L-2N3S paste is greater than that of mesopores in the L-control paste at 90 days. However, the ratio of capillary pores (50–1000 nm) in the L-2N3S paste is significantly lower than that of in the L-control paste. The ratio of capillary pores in the pore size distributions of L-2N3S and L-control paste is 10.4% and 28.7%, respectively. Therefore, the composite activator effectively improves the pore size distributions of LCM due to the decrease in the amount of capillary pores in the paste. At 90 day-curing age, the total porosity of L-2N3S and L-control paste is 28.41% and 31.26%, respectively, which indicates that the amount of total pores in the modified paste is reduced. The composite activator not only modifies the compositions of hydrates of LCM, but also promotes the evolution of the pore structure of LCM.

30

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Chemically bound water (wt%) Fig. 14. Relationship between compressive strength and chemically bound content.

at different curing ages, the content of chemically bound water is calculated in the present study, and Fig. 13 shows the content of chemically bound water for the LCM. From Fig. 13, the content of chemically

35 L-Control L-2N L-3S L-2N3S

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Fig. 15. Pore size distributions and porosity of LCM pastes at 90-day curing age. 361

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Fig. 16. SEM images and EDS analysis of LCM paste at 90-day curing age.

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formed in the paste during the hydration process in the presence of the composite activator. The XRD and SEM-EDS test results proved that the new hydrates U-phase was found only in the hydration products of LCM containing the composite activator. (5) When the composite activator was used in the LCM, large quantities of formed U-phase as crystals effectively occupied and filled the space in the hydrated paste and formed a compact microstructure with less macro-pores. Thus, the presence of U-phase in the paste improved the strength of the LCM. Additionally, the U-phase formed at the early stage of hydration and could not result in the expansion of LCM.

3.6. SEM and EDX analysis To characterize the morphology of hydrates in the LCM paste, SEM photos of the L-2N, L-3S, L-3S2N and L-control pastes at 90-day curing age are shown in Fig. 16. As shown in Fig. 16, a large quantities of hexagonal rod-like crystals can be found in the hydrates of all LCM pastes, which can be identified as ettringite crystals. Note that a larger number of small platelet-like crystals can be observed in the microstructure of L-3S2N paste. These small platelet-like crystals show a morphology that is hexagonal or pseudo-hexagonal, and the shape of these platelet-like crystals resembles the character of a “U” to some extent, which is consistent with the characteristics of the U-phase from previous studies [28,42,43]. Furthermore, from Fig. 16(c), the EDS elemental analysis indicates that the platelet-like crystals are dominantly composed of Na, Ca, Al, and S. Therefore, the small platelet-like crystals in the microstructure of L-3S2N are identified as U-phase. From SEM images, the U-phase is not be founded in the microstructure of other LCM pastes, which demonstrates the accuracy of the results from XRD test. Additionally, compared to the L-control paste, the L-3S and L3S2N paste show a more compact microstructure with little void, which is agree with the results from the compressive strength test. According to the results of the Rietveld refinement of the XRD data, the quantity of ettringite phase in the L-3S2N paste is lower than that in the L-control paste (12.9% and 9.8%, respectively) at 90 days. However, quantities of U-phase (10.5%) that formed as crystals at 90 days in the LCM paste has a substantial effect on the mechanical strength of the LCM, because the U-phase crystals are embedded in the LCM matrix and effectively fill the pores and spaces in the microstructure. Under the same w/b ratio, more hydrates formed in the L2N3S paste, which suggests that the paste has a lower porosity with finer micropores compared to the L-control paste. From Fig. 16(c), it can be seen that the U-phase and ettringite crystals are imbedded in the hydrated gel to form a more compact microstructure with less capillary pores, which is consistent with the MIP test results. Therefore, the L3S2N mixture has better mechanical performance than the control mixture.

Conflicts of interest None. Acknowledgements Authors gratefully acknowledge the financial support from National Basic Research Program of China (973 Program, Grant No. 2015CB655102), the National Natural Science Foundation of China (51508090, 51678142, 51678143, 51878153 and 51808189) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0078). References [1] D. Xu, Y.S. Cui, H. Li, K. Yang, W. Xu, Y.X. Chen, On the future of Chinese cement industry, Cement Concr. Res. 78 (2015) 2–13. [2] J. Yu, C. Lu, C.K.Y. Leung, G.Y. Li, Mechanical properties of green structural concrete with ultrahigh-volume fly ash, Constr. Build. Mater. 147 (2017) 510–518. [3] M. Wu, Y.S. Zhang, Y.S. Ji, G.J. Liu, C. Liu, W. She, W. Sun, Reducing environmental impacts and carbon emissions: study of effects of superfine cement particles on blended cement containing high volume mineral admixtures, J. Clean. Prod. 196 (2018) 358–369. [4] P.J.M. Monteiro, S.A. Miller, A. Horvath, Towards sustainable concrete, Nat. Mater. 16 (7) (2017) 698–699. [5] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cement Concr. Res. 41 (12) (2011) 1244–1256. [6] K.L. Scrivener, A. Nonat, Hydration of cementitious materials, present and future, Cement Concr. Res. 41 (7) (2011) 651–665. [7] I. Garcia-Lodeiro, A. Fernandez-Jimenez, A. Palomo, Variation in hybrid cements over time. Alkaline activation of fly ash-portland cement blends, Cement Concr. Res. 52 (2013) 112–122. [8] H.B. Tan, X. Zhang, X.Y. He, Y.L. Guo, X.F. Deng, Y. Su, J. Yang, Y.B. Wang, Utilization of lithium slag by wet-grinding process to improve the early strength of sulphoaluminate cement paste, J. Clean. Prod. 205 (2018) 536–551. [9] W. She, Y. Du, C.W. Miao, J.P. Liu, G.T. Zhao, J.Y. Jiang, Y.S. Zhang, Application of organic- and nanoparticle-modified foams in foamed concrete: reinforcement and stabilization mechanisms, Cement Concr. Res. 106 (2018) 12–22. [10] F. Avet, K. Scrivener, Investigation of the calcined kaolinite content on the hydration of limestone calcined clay cement (LC3), Cement Concr. Res. 107 (2018) 124–135. [11] D.F. Velandia, C.J. Lynsdale, J.L. Provis, F. Ramirez, A.C. Gomez, Evaluation of activated high volume fly ash systems using Na2SO4, lime and quicklime in mortars with high loss on ignition fly ashes, Constr. Build. Mater. 128 (2016) 248–255. [12] K.H. Yang, A.R. Cho, J.K. Song, S.H. Nam, Hydration products and strength development of calcium hydroxide-based alkali-activated slag mortars, Constr. Build. Mater. 29 (2012) 410–419. [13] W.G. Li, Z.Y. Huang, F.L. Cao, Z.H. Sun, S.P. Shah, Effects of nano-silica and nanolimestone on flowability and mechanical properties of ultra-high-performance concrete matrix, Constr. Build. Mater. 95 (2015) 366–374. [14] W.S. Yum, Y. Jeong, S. Yoon, D. Jeon, Y. Jun, J.E. Oh, Effects of CaCl2 on hydration and properties of lime(CaO)-activated slag/fly ash binder, Cement Concr. Compos. 84 (2017) 111–123. [15] D.O. Koteng, C.T. Chen, Strength development of lime-pozzolana pastes with silica fume and fly ash, Constr. Build. Mater. 84 (2015) 294–300. [16] M. Wu, Y.S. Zhang, G.J. Liu, Z.T. Wu, Y.G. Yang, W. Sun, Experimental study on the performance of lime-based low carbon cementitious materials, Constr. Build. Mater. 168 (2018) 780–793. [17] Y. Jeong, J.E. Oh, Y. Jun, J. Park, J.H. Ha, S.G. Sohn, Influence of four additional activators on hydrated-lime [Ca(OH)(2)] activated ground granulated blast-furnace slag, Cement Concr. Compos. 65 (2016) 1–10. [18] H. Park, Y. Jeong, Y. Jun, J.H. Jeong, J.E. Oh, Strength enhancement and pore-size refinement in clinker-free CaO-activated GGBFS systems through substitution with gypsum, Cement Concr. Compos. 68 (2016) 57–65. [19] A. Vimmrova, M. Keppert, O. Michalko, R. Cerny, Calcined gypsum-lime-

4. Conclusions In this study, the influence of chemical admixtures on the development of the strength and the evolution in the hydration of LCM was explored and discussed. The heat of hydration, compositions of hydrates and microstructure of modified LCM containing different types and amounts of chemical admixtures were analyzed and characterized by means of isothermal calorimetry, XRD, TGA, MIP and SEM-EDS. According to the test results presented in this study, the conclusions can be summarized as follows: (1) The NaOH as activators could improve the mechanical strength of LCM at the early period. However, the NaOH affected the rate of hydration of LCM at the later stage and finally resulted in a limited progress on the development of the strength. The recommended dosages for NaOH as an activator for LCM is 1–2 wt%. (2) The addition of Na2SO4 in the LCM increased the early and later mechanical strength due to more ettringite being formed in the paste. Compared to NaOH, the Na2SO4 shows little negative influence on the hydration of LCM at a long-term. The optimal amount of Na2SO4 as an activator added into the LCM was 3 wt% in this study. (3) The composite activator that was composed of NaOH and Na2SO4 showed optimal effects on the development of the mechanical strength of LCM at both early and later age. The optimized composite activator for the LCM was composed of 2 wt% NaOH and 3 wt% Na2SO4, which resulted in the strength of the LCM being increased by 31% at 90 days. (4) The hydration rate of LCM was accelerated and more hydrates were 363

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