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Effect of the activator on the performance of alkali-activated slag mortars with pottery sand as fine aggregate
Construction and Building Materials 197 (2019) 83–90
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Effect of the activator on the performance of alkali-activated slag mortars with pottery sand as fine aggregate Zhenzhen Jiao a,b,c, Ying Wang a,b,c, Wenzhong Zheng a,b,c,⇑, Wenxuan Huang a,b,c a
Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China c School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China b
h i g h l i g h t s A mixture of Na2CO3 and NaOH was used as alkaline activator for AAS mortars. Properties and microstructural characteristics of AAS mortars were investigated. Characteristics of AAS mortar depend on Na2CO3-to-NaOH ratios and Na2O contents. Na2CO3 can prolong the setting time and improve compressive strength of AAS mortar. Both Na2CO3-to-NaOH ratio and Na2O content obviously affected drying shrinkage.
a r t i c l e
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Article history: Received 14 August 2018 Received in revised form 8 November 2018 Accepted 23 November 2018
Keywords: Alkali-activated slag Pottery sand Sodium carbonate Sodium hydroxide Mechanical properties Microstructure
a b s t r a c t Recently, alkali-activated slag mortars have attracted considerable attention as environmentally friendly materials. The objective of this paper was to explore the influence of an alkaline activator (NaOH or NaOH/Na2CO3 solutions) on the performance of alkali-activated slag (AAS) mortars with pottery sand as fine aggregate. Five Na2CO3-to-NaOH ratios of 0/100, 20/80, 40/60, 60/40, and 80/20, and three Na2O contents of 4%, 6%, and 8%, were used to prepare AAS mortars. The water-to-slag ratio and slagto-aggregate ratio were kept constant, at 0.38 and 0.8, respectively. The setting time, fluidity, compressive strength, and drying shrinkage were measured. The reaction hydrates were analyzed using microscopy and elemental analysis, while the pore structures were characterized using mercury intrusion porosimetry. The experimental results showed that both the Na2CO3-to-NaOH ratio and Na2O content had a significant effect on the fresh and hardened properties of AAS mortars. A high Na2CO3-to-NaOH ratio and low Na2O content resulted in AAS mortars with longer initial and final setting times. The higher Na2CO3-to-NaOH ratios led to a higher late compressive strength of AAS mortars, where the 6% Na2O samples showed the best performance with Na2O contents of 4–8%. In addition, the drying shrinkage behavior of AAS mortars as a function of Na2CO3-to-NaOH ratio was dependent on the Na2O content. Moreover, the compressive strength and drying shrinkage have a direct relationship with the microstructure of AAS mortars. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Blast furnace slag is a by-product of blast furnace smelting and large amounts of slag are generated worldwide [1]. More than 2.4 million tons of slag was produced in 2017 in China; however, very
⇑ Corresponding author at: Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China. E-mail addresses: [email protected] (Z. Jiao), [email protected] (Y. Wang), [email protected] (W. Zheng). https://doi.org/10.1016/j.conbuildmat.2018.11.178 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
little of this (about 20–30%) can be efficiently used and the remainder is classed as solid industrial waste [2]. Because slag consists of silicates and aluminosilicate, an effective way to treat solid waste is to convert it into green building materials via alkali-activation processes [3,4]. These new types of building materials save resources by reutilizing a waste product that would otherwise pollute the environment. Hence, over the past few years, many different studies have been performed evaluating the effective application of solid waste [5–8].
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In general, the properties of AAS depend on the type and content of the alkaline activator, as well as the curing methods, composition of the raw materials, water-to-binder ratio, and processing temperature [9–13]. The main activators commonly used to activate slag are water glass (K2SiO3 or Na2SiO3), NaOH, and Na2CO3. In previous studies, slag activated using water glass achieved excellent mechanical properties compared with other activators [14,15]. The amount of reaction products, such as C-(A)-S-H and N-A-S-H, increased with increasing curing time, which was beneficial for the compressive strength [16–18]. NaOH is also widely used as an alkaline activator because of its high alkalinity. Li [19] observed that NaOH-based AAS performed better than Na2CO3based AAS in terms of the early compressive strength; however, NaOH-Na2CO3-based AAS had better properties than NaOH-based or Na2CO3-based AAS considering the late compressive strength. This could be due to the loose structure and coarse pore structure of NaOH-based AAS [20]. In addition, the low early strength of Na2CO3-based AAS limits its practical engineering applications, owing to lack of compressive strength at 3 d at room temperature [21,22]. The disadvantages of AAS materials include their fast setting [23,24] and substantial shrinkage [14,25,26], which can result in large problems in construction progress and the quality of the projects. Aydin et al. [23] reported that the setting rate of AAS specimens activated by water glass with a silicate modulus of 0.4 and 0.8 and Na2O contents of 6% and 8% was too fast due to initial CS-H formation. However, slag activated by Na2CO3 has a long setting time, which is mainly attributed to initial sodium calcium carbonate formation [14,27]. A significant challenge for AAS materials is optimization of the setting behavior; both short and long setting times can limit the applicability of the material. The type of alkaline activator critically affects the drying shrinkage of AAS mortars. AAS mortars with water glass exhibits the highest drying shrinkage and that with Na2CO3 showed the lowest compared with the alkaline activator for NaOH [14]. Collins et al. [28] reported that the drying shrinkage of AAS is four times larger than that of OPC at 50% relative humidity (RH) and 23 °C. Curing at high temperature is an effective way to reduce the drying shrinkage of AAS concrete [29,30], because the dense hydration products and the tortuosity of the pore network increase, and the air or water is not easily lost after heat-curing. However, heat-curing is not suitable for practical in-situ engineering, it may only be well-suited for precast application [30]. Pottery sand, a lightweight aggregate, primarily comprise fly ash and bauxite, with fly ash accounting for 50–80%. Fly ash is generated from thermal power plants as an industrial by-product; its annual production was 565 million tons in 2016 [31]. Therefore, pottery sand is very cheap owing to it being made of industrial solid waste raw material. Pottery sand is typically used as a fine aggregate for preparing lightweight mortar and concrete in China. It is highly dimensionally stable, which reduces the shrinkage of mortars compared with that of pastes. However, to the best of our knowledge, no studies describing the utilization of pottery sand in the alkali-activated slag mortars have been published. Consequently, it is necessary to investigate the properties of fresh and hardened AAS mortars using pottery sand as the fine aggregate. The objective of this study was to advance knowledge regarding the use of blends of NaOH and Na2CO3 as the alkaline activator for a new type of AAS mortar. Five Na2CO3-to-NaOH ratios and three Na2O contents were considered and their effects on the macro-properties and microstructure AAS mortars were investigated. The macro-properties included the setting times, fluidity, compressive strength, and drying shrinkage, while the microstructure was investigated using microscopy, elemental analysis, and mercury intrusion porosimetry (MIP).
2. Materials and methods 2.1. Raw materials Blast furnace slag from the Tangshan steel plant in China was used in this study. The slag was classified as S95 according to the GB/T 18046-2008 standard [32], with a specific surface area of 424 m2/kg. Pottery sand made from fly ash and bauxite calcined at a high temperature (1200–1600 °C) was used as the lightweight fine aggregate. The pottery sand had a particle size less than 1 mm, specific gravity of 1.8, and bulk density of 1.2 g/cm3. The chemical compositions of the slag and pottery sand are listed in Table 1, where photographs of these raw materials are shown in Fig. 1. The alkaline activator for the AAS mortar was prepared using only NaOH or blends of NaOH and Na2CO3. NaOH flakes (96% purity) were supplied by the Dalu Chemical Reagent Factory (Tianjin, China), while Na2CO3 pellets (99.8% purity) were supplied by the Zhiyuan Chemical Reagent Factory (Tianjin, China). 2.2. Sample preparation To prepare the alkaline activators, we used five different Na2CO3-to-NaOH mass ratios of 0/100, 20/80, 40/60, 60/40, and 80/20, and three different Na2O contents of 4%, 6%, and 8% (relative to the mass of slag). The water-to-slag ratio and slag-toaggregate mass ratio were kept constant at 0.38 and 0.8, respectively. The proportions of the mixtures are summarized in Table 2. For example, the code ‘‘C2N8-6M” represents the mortar with Na2CO3-to-NaOH mass ratio of 20/80 and 6% Na2O content. To prepare the AAS mortars, the slag and pottery sand were firstly premixed for 5 min, then the activator was added, and lastly, all mixtures were stirred for 1 min at a slow rate of 200 ± 5 rpm for another 1 min at a fast rate of 400 ± 10 rpm. 2.3. Test methods The setting time was measured in accordance with the JGJ/T 70–2009 standard [33]. The cross-sectional area of the stainless steel needle in the instrument used to measure the setting time of the mortar was 30 mm2. The setting time was defined as the time from the addition of the activator to the raw materials to the time when the penetration resistance reached 15 N. The fluidity test was performed with a jump table following standard GB/T 2419-2005 [34]. Firstly, the fresh mixtures were poured into truncated conical molds (h = 60 mm, dtop = 70 mm, dbottom = 100 mm). Then, the mold was lifted vertically and the jump table was vibrated for 25 s (the table vibration frequency was 1 s 1). Then, the diameters along the two perpendicular directions were measured after the vibration had finished. The average diameter was used to determine the fluidity of the mortar sample. In accordance with the standard JGJ/T 70-2009 [33], fresh mortar mixtures were poured into cubic molds with an edge of 70.7 mm, followed by mechanical shaking. Next, plastic foil was used to seal the top surface of the specimens to prevent water loss. Then, the samples were stored under standard ambient conditions (temperature of 20 ± 2 °C and relative humidity > 95%) for 24 h. After removal from the mold, the samples were kept under these standard conditions until required for testing at the various scheduled/predetermined ages (1, 3, 7, 14, 28 d). The compressive strength was measured using an electro-hydraulic system at 1.0 kN/s. An average value of three specimens was determined as the compressive strength of the AAS mortars for the corresponding curing time. According to the standard JC/T 603-2004 [35], the fresh mortars were poured into prismatic molds with 25 mm (length) 25 mm (width) 280 mm (height). Further treatment was as the same as described in the previous section until the specimens were removed from the molds. Firstly, the dimensions of the samples were measured; then, they were placed in a room with constant temperature (20 ± 2 °C) and relative humidity (50 ± 5%). The drying shrinkages of the samples were measured with a vertical length comparator (digital dial gauge) daily until 28 days. After compressive strength tests at the corresponding curing time, samples from the crushed prisms were kept in the compound of alcohol and acetone (volume ratio of 1/1) for 7 d to prevent hydration. Then, the specimens were removed from the compound and placed in an oven at 60 °C under vacuum to dry. In order to determine the hydration products and reaction mechanism of the AAS mortars, the morphology and elemental composition of the hydration products were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively (ZEISS MERLIN Compact FE-SEM, Germany). The pore structures of the AAS mortars were evaluated using MIP (Autopore IV 9500, Micromeritics Instrument Ltd., USA).
3. Results 3.1. Setting time Fig. 2 shows the setting time of AAS mortars for the different Na2CO3-to-NaOH ratios and Na2O contents. The setting behavior
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Z. Jiao et al. / Construction and Building Materials 197 (2019) 83–90 Table 1 Chemical compositions of the slag and pottery sand (%). Oxide
SiO2
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
Others
Slag Pottery sand
32.83 62.12
17.19 16.32
36.69 3.26
0.38 7.84
0.37 1.25
8.20 2.04
0.65 1.97
1.94 –
1.75 5.2
Fig. 1. Photographs of the (a) cementitious material (blast furnace slag) and (b) fine aggregate (pottery sand).
Table 2 Mix proportions used in this study. Code
Na2CO3-to-NaOH ratio
Na2O content (%)
Slag (g)
Pottery sand (g)
Na2CO3 (g)
NaOH (g)
H2O (g)
c(OH ) (mol/L)
c(CO23 ) (mol/L)
C0N10-4M C0N10-6M C0N10-8M C2N8-4M C2N8-6M C2N8-8M C4N6-4M C4N6-6M C4N6-8M C6N4-4M C6N4-6M C6N4-8M C8N2-4M C8N2-6M C8N2-8M
0/100 0/100 0/100 20/80 20/80 20/80 40/60 40/60 40/60 60/40 60/40 60/40 80/20 80/20 80/20
4 6 8 4 6 8 4 6 8 4 6 8 4 6 8
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250
– – – 10.9 16.3 21.7 22.9 34.3 45.8 36.3 54.5 72.6 51.4 77.1 102.7
51.6 77.4 103.2 43.4 65.1 86.8 34.3 51.5 68.7 24.2 36.3 48.4 12.8 19.3 25.7
368.4 362.6 356.8 370.2 365.3 360.5 372.3 368.4 364.5 374.6 371.8 369.1 377.1 375.7 374.2
3.50 5.34 7.23 2.93 4.46 6.02 2.30 3.49 4.71 1.62 2.44 3.28 0.85 1.28 1.72
– – – 0.28 0.42 0.57 0.58 0.88 1.19 0.91 1.38 1.86 1.29 1.94 2.59
Fig. 2. Setting time of AAS mortars with different Na2O contents and Na2CO3-toNaOH ratios.
was dependent on both the Na2CO3-to-NaOH ratios and Na2O contents. The setting times of the AAS mortars were within the range of 11.3–44.0 min. Previous studies [27,36,37] showed a dramatic difference in the range of 30 min to more than 3 days in the AAS
activated by Na2CO3. The setting time increased with an increasing of Na2CO3-to-NaOH ratios and decreasing of Na2O contents. The longest setting time (for C8N2-4M) was almost four times the shortest one (for C0N10-8M). Higher Na2CO3-to-NaOH ratios and lower Na2O contents in the alkaline activators resulted in longer setting times. The higher Na2CO3 content resulted in a higher content of initial calcium carbonate/sodium calcium carbonate, which was the primary reason for the reduced reaction rate [27,38]. They found that the initial formation of carbonated compounds resulted in loss of plasticity in specimens, however, which did not reach the setting stage, and the later formation of C-S-H was contributed to the setting of AAS. Moreover, the OH concentration of the alkaline activator played a key role in the setting behavior. Similar results were also reported by Cengiz et al. [14] and Puertas et al. [22], who showed that increasing the amount of Na2CO3 in the activator reduced the OH concentration of the alkaline activator. In addition, the OH ions in the activator accelerated the decomposition and hydration of the slag structure to rapidly form a C-S-H gel, and further accelerate setting [39]. The setting and hardening of specimens activated by NaOH were also attributed to the formation of extra polysilicate hydrates [40]. Therefore, the setting time can be adjusted via appropriate addition of Na2CO3.
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3.2. Fluidity The fluidity values of AAS mortars with different contents of alkaline activator are plotted in Fig. 3. In the AAS mortars with only NaOH activator, the fluidity increased as the Na2O contents increased. Puertas et al. [41] discovered that the slump value decreased with increasing of Na2O content from 3% to 4%; however, it increased as the Na2O content increased from 4% to 5% after 7 min. They indicated that the higher yield stress is, the lower is the slump. However, for all NaOH-Na2CO3 activated specimens, the fluidity decreased as the Na2O content increased. Higher Na2O contents resulted in higher OH concentrations, which can accelerate the dissolution of Ca2+. The initial precipitation of CaCO3 was responsible for a dramatic decrease in the fluidity when the Na2O content increased. Puertas et al. [41] showed that the slump value of AAS activated by Na2CO3/NaOH decreases with increasing of Na2O content from 3% to 4%. Mehdizadeh et al. [42] investigated rheology alkali-activated phosphorous slag based on water glass. They showed that the main reason for the reduction of fluidity is the increasing of phosphorous slag dissolution and more gel formation. Moreover, the highest fluidity was obtained for AAS mortars with a Na2CO3-to-NaOH ratio of 20/80, regardless of the Na2O content in all the NaOH-Na2CO3-activated specimens. Increasing the Na2CO3 content reduced the fluidity in NaOH-Na2CO3-activated samples, perhaps due to its higher stress [41]. Zivica et al. [40] reported that the workability of AAS mortar activated by only Na2CO3 decreased as the dosage of alkaline activator increased from 3% to 7%. More Na2CO3 resulted in more CO23 the more initial sodium calcium carbonate or calcium carbonate formed, the more reduced the workability. Furthermore, when the Na2O content was 4%, the fluidity of the AAS mortar activated by NaOH was lower than that by Na2CO3-NaOH. However, when the Na2O content increased to 8%, the fluidity of the AAS mortar activated by NaOH was higher than that by Na2CO3-NaOH. Meanwhile, Collins et al. [21] reported that the workability of AAS improved as the Na2CO3 content increased when the alkaline activator was NaOH in combination with Na2CO3. However, Puertas et al. [41] showed that the slump value of AAS with NaOH was higher than that with NaOH/Na2CO3 at the 4% Na2O. The fluidity of all AAS mortars varied between 184.5 and 211.5 mm. The lowest fluidity (of C6N4-8M) was about 87.2% of the highest fluidity (of C2N8-4M), where the lowest value was above 180 mm, conforming to the criteria given by the GB/T 8077-2012 standard [43]. Consequently, the Na2CO3-to-NaOH
Fig. 3. Fluidity values of AAS mortars with different Na2O contents and Na2CO3-toNaOH ratios.
ratios and Na2O contents are both important for the fluidity of AAS mortars. 3.3. Compressive strength The compressive strength of AAS mortars using the various Na2CO3-to-NaOH ratios and Na2O contents at curing times of 1, 3, 7, 14, and 28 d are shown in Fig. 4. Generally, there was an increase in the compressive strength as the curing time increased for all samples. Before 3 d, the compressive strength improved with increasing Na2CO3-to-NaOH ratios within the scope of 0/100– 40/60, and then decreased when it increased further from 60/40 to 80/20. This indicates that the highest Na2CO3-to-NaOH ratios resulted in the lowest compressive strengths in the early stage. However, we observed a completely different trend in the later stage. After 7 d, the compressive strength increased with increasing Na2CO3-to-NaOH ratio, regardless of the Na2O content. For Na2O contents of 4%, 6%, and 8%, the compressive strengths of the AAS mortars with a Na2CO3-to-NaOH ratio of 80/20 were 1.32, 1.33, and 1.27 times that of the 0/100 sample, respectively. The OH ion played an important role in dissolving the glassy structure of the slag and improving the hydration process [44]. The addition of Na2CO3 decreased the OH concentration of the alkaline activator, while sodium calcium carbonate decreased the early compressive strength [22]; however, with increasing curing age, the highly cross-linked structure formed by carbonated compounds can enhance the late compressive strength [15]. Moreover, the compressive strength of the AAS mortars with a Na2O content of 6% was higher than those of samples with contents of 4% and 8%. Higher Na2O contents resulted in higher compressive strengths; however, there was an optimum Na2O content in the alkaline activator as excess Na2O decreased the strength. Komnitsas et al. [45] stated that the optimum KOH activator concentration was 8M, and demonstrated that any excess will not be fully reacted. Hence, the compressive strength of slag activated by Na2CO3 and NaOH was significantly affected by the Na2O content and Na2CO3-to-NaOH ratio. 3.4. Drying shrinkage The drying shrinkage of AAS mortars in the first 28 d are shown in Fig. 5. For the lowest Na2O content (4%), the drying shrinkage of the AAS mortars increased with increasing Na2CO3-to-NaOH ratios. However, for the highest Na2O content (8%), the drying shrinkage showed the opposite behavior. On the other hand, Na2CO3 was attributed to reducing the drying shrinkage of AAS mortars at higher Na2O content. The highest drying shrinkage was 0.36% for the C8N2-4M sample, which was 3.27 times that of the lowest value of 0.11% (C8N2-8M). Similarly, there was a dramatic drop of the drying shrinkage for AAS activated by Na2CO3 as the Na2CO3 content increased from 4% to 6% [46]. The reaction product, the hydrotalcite-like phase, was contributed to reducing the drying shrinkage [46–48]. Moreover, the drying shrinkage decreased as the Na2O content increased for samples with higher Na2CO3-to-NaOH ratios (40/60, 60/40, and 80/20). This may be due to the lower OH concentration for samples with lower Na2O contents, which cannot be fully reacted and therefore, remaining moisture resulted in higher drying shrinkages [49]. However, there was an inverse trend of drying shrinkage for the AAS mortars with only NaOH or Na2CO3 [14], where the drying shrinkage increased as the sodium concentration (in the range of 4–8%) increased. The drying shrinkage mechanism for the AAS mortars is complex, as it depends on both the Na2CO3 to-NaOH ratio and the Na2O content. The effect of reaction production on drying shrinkage mainly was the amorphous phase of C-SH gel, rather than the crystalline phase of tobermorite [50]. Ye et al.
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Fig. 4. Compressive strength of AAS mortars with different Na2O contents and Na2CO3-to-NaOH ratios, (a) 4% Na2O, (b) 6% Na2O, (c) 8% Na2O.
Fig. 5. Drying shrinkage of AAS mortars with different Na2O contents and Na2CO3-to-NaOH ratios, (a) 4% Na2O, (b) 6% Na2O, (c) 8% Na2O.
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[51] discovered that the formation of C-A-S-H incorporating the alkali cations for AAS mortars results in the high drying shrinkage. In addition, Häkkinen [52] formed different conclusions about the tendency of the drying shrinkage of AAS pastes with only NaOH or Na2CO3. They found that the drying shrinkage of AAS pastes with 5% Na2CO3 was higher than that with 5% NaOH after 231 d. Wittmann and Collins [28,53] showed that the pore size distribution of mesopores and the C-S-H gel features had a significant effect on the drying shrinkage. The radius of the pores where the meniscus formed was the major factor determining the drying shrinkage due to the larger capillary tensile forces. A detailed explanation of the drying shrinkage behavior is discussed with respect to the pore structures in Section 3.6.
3.5. Microstructure and composition SEM/EDS was performed to study the reaction products and study the interfacial transition zone (ITZ) of hardened AAS mortars. Figs. 6 and 7 show the SEM and EDS micrographs of AAS mortars with different Na2CO3-to-NaOH ratios at 28 d, respectively. A series of amorphous particle products was observed for all AAS mortars, which were mainly comprised of Ca, Si, Na, Mg, Al, and O. The main reaction hydrates were C-S-H gels, hydrotalcite, and gaylussite [36,54]. Moreover, the microstructure became gradually denser with increasing Na2CO3-to-NaOH ratio, which was consistent with the compressive strength development shown in Section 3.3. A similar result was found where the hydration products became denser as the Na2CO3-to-NaOH ratios increased in AAS mortars [46]. In addition, the composition and morphology of the reaction products was closely related to the compressive strength. Previous studies [17,36,55] indicated that the CO23 anion produced to form carbonate compounds like Na2Ca(CO3)25H2O and the cross-linked structure of C-(A)-S-H enhanced the compressive strength. The ITZ between the aggregate and paste also plays a key role in the devel-
opment of compressive strength in the mortars [56]. It can be seen that there were some small cracks in the C0N10-6M and C4N6-6M samples, but none in C8N2-6M. However, the ITZ of all AAS mortars showed good adhesion and there was no clear distinction between the aggregates and paste for different Na2CO3-to-NaOH ratios. These microstructural properties can explain the development of the mechanical performance of our AAS mortars.
3.6. Pore-size distribution The classification of the pore structures of the AAS materials according to the standards of the International Union of Pure and Applied Chemistry showed micropores (d < 2.5 nm), mesopores (2.5 nm < d < 50 nm), macropores (50 nm < d < 10000 nm), and voids and microcracks (d > 10000 nm). The calculated pore diameters were in the range of 5 nm–360 mm. The effect of the Na2CO3to-NaOH ratio on the pore structures of AAS mortars is depicted in Fig. 8, where the total pore volumes of the AAS mortars decreased as the Na2CO3-to-NaOH ratio increased from 0/100 to 80/20 (Fig. 8(a)). With increasing Na2CO3-to-NaOH ratio, a higher fraction of carbonates formed a denser pore structure at the later stage. In addition, Fig. 8(b) shows that the two peaks (mesopores and macropores) both had a slightly higher intensity for the lower Na2CO3-to-NaOH ratios; with increasing Na2CO3-to-NaOH ratios, the peaks shifted towards smaller pore sizes. This shift in the mean pore size can be explained by gradual filling of the larger pores with reaction products with increasing Na2CO3-to-NaOH ratio. Fig. 8(c) shows that the porosity decreased from 14.2% to 10.9% as the Na2CO3-to-NaOH ratio increased from 0/100 to 80/20 in AAS mortars with 6% Na2O. The finer and denser pore structures indicated that more slag was hydrated and there was a stronger bonding between the aggregates and paste; this is consistent with the higher compressive strength of these samples (Section 3.3).
Fig. 6. SEM micrographs of the fracture surface of hardened AAS mortars cured for 28 d. (a) C0N10-6 M; (b) C4N6-6 M; and (c) C8N2-6 M.
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Fig. 7. EDS spectra for the regions labeled in Fig. 6. (a) Spectrum 1; and (b) Spectrum 2.
Fig. 8. Pore size distribution of AAS mortars with different Na2CO3-to-NaOH ratios. (a) Cumulative intrusion data as a function of pore size; (b) differential intrusion data as a function of pore size; (c) porosity.
Meanwhile, the mesopore volume decreased with increasing Na2CO3-to-NaOH ratio. Wittmann [53] indicated that there was an important relationship between the pore size distribution and drying shrinkage. The larger mesopores volumes resulted in a higher drying shrinkage due to the higher contracting stress [12,28], which was consistent with the trend in the drying shrinkage behavior (Section 3.4). A similar relationship between the refined pore structure and shrinkage was reported by Gao et al. [57]. Collins et al. [28] stated that a lower porosity and narrower pore size distribution usually results in a higher sample strength. They concluded that the water loss through mesopores and a macropore size distribution had a positive impact on the drying shrinkage. 4. Conclusions The effects of Na2O content and Na2CO3-to-NaOH ratio on the setting time, fluidity, mechanical performance, and drying shrinkage of AAS mortars were investigated. The main conclusions of this study are as follows. (1) Both the Na2O content and Na2CO3-to-NaOH ratio played a significant role in determining the properties of fresh and hardened AAS mortars.
(2) The setting time increased with the increasing Na2CO3-toNaOH ratio and Na2O content. The setting time of the AAS mortars was in the range of 11.3–44.0 min. (3) In the early stage of curing, the compressive strength increased as the Na2CO3-to-NaOH ratio increased from 0/100 to 40/60, and then decreased as it increased from 60/40 to 80/20; however, in the later stage, the higher Na2CO3-to-NaOH ratio resulted in a higher compressive strength. (4) For lower Na2O contents, the higher Na2CO3-to-NaOH ratios resulted in higher drying shrinkage; however, increasing the Na2CO3-to-NaOH ratios effectively reduced the drying shrinkage of AAS mortars with higher Na2O contents. (5) With increasing Na2CO3-to-NaOH ratios, the microstructure of the AAS mortars became denser and the mesopore volume decreased, which were consistent with the higher late compressive strength and lower drying shrinkage.
Conflicts of interest None.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of the activator on the performance of alkali-activated slag mortars with pottery sand as fine aggregate Zhenzhen Jiao a,b,c, Ying Wang a,b,c, Wenzhong Zheng a,b,c,⇑, Wenxuan Huang a,b,c a
Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China c School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China b
h i g h l i g h t s A mixture of Na2CO3 and NaOH was used as alkaline activator for AAS mortars. Properties and microstructural characteristics of AAS mortars were investigated. Characteristics of AAS mortar depend on Na2CO3-to-NaOH ratios and Na2O contents. Na2CO3 can prolong the setting time and improve compressive strength of AAS mortar. Both Na2CO3-to-NaOH ratio and Na2O content obviously affected drying shrinkage.
a r t i c l e
i n f o
Article history: Received 14 August 2018 Received in revised form 8 November 2018 Accepted 23 November 2018
Keywords: Alkali-activated slag Pottery sand Sodium carbonate Sodium hydroxide Mechanical properties Microstructure
a b s t r a c t Recently, alkali-activated slag mortars have attracted considerable attention as environmentally friendly materials. The objective of this paper was to explore the influence of an alkaline activator (NaOH or NaOH/Na2CO3 solutions) on the performance of alkali-activated slag (AAS) mortars with pottery sand as fine aggregate. Five Na2CO3-to-NaOH ratios of 0/100, 20/80, 40/60, 60/40, and 80/20, and three Na2O contents of 4%, 6%, and 8%, were used to prepare AAS mortars. The water-to-slag ratio and slagto-aggregate ratio were kept constant, at 0.38 and 0.8, respectively. The setting time, fluidity, compressive strength, and drying shrinkage were measured. The reaction hydrates were analyzed using microscopy and elemental analysis, while the pore structures were characterized using mercury intrusion porosimetry. The experimental results showed that both the Na2CO3-to-NaOH ratio and Na2O content had a significant effect on the fresh and hardened properties of AAS mortars. A high Na2CO3-to-NaOH ratio and low Na2O content resulted in AAS mortars with longer initial and final setting times. The higher Na2CO3-to-NaOH ratios led to a higher late compressive strength of AAS mortars, where the 6% Na2O samples showed the best performance with Na2O contents of 4–8%. In addition, the drying shrinkage behavior of AAS mortars as a function of Na2CO3-to-NaOH ratio was dependent on the Na2O content. Moreover, the compressive strength and drying shrinkage have a direct relationship with the microstructure of AAS mortars. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Blast furnace slag is a by-product of blast furnace smelting and large amounts of slag are generated worldwide [1]. More than 2.4 million tons of slag was produced in 2017 in China; however, very
⇑ Corresponding author at: Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China. E-mail addresses: [email protected] (Z. Jiao), [email protected] (Y. Wang), [email protected] (W. Zheng). https://doi.org/10.1016/j.conbuildmat.2018.11.178 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
little of this (about 20–30%) can be efficiently used and the remainder is classed as solid industrial waste [2]. Because slag consists of silicates and aluminosilicate, an effective way to treat solid waste is to convert it into green building materials via alkali-activation processes [3,4]. These new types of building materials save resources by reutilizing a waste product that would otherwise pollute the environment. Hence, over the past few years, many different studies have been performed evaluating the effective application of solid waste [5–8].
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In general, the properties of AAS depend on the type and content of the alkaline activator, as well as the curing methods, composition of the raw materials, water-to-binder ratio, and processing temperature [9–13]. The main activators commonly used to activate slag are water glass (K2SiO3 or Na2SiO3), NaOH, and Na2CO3. In previous studies, slag activated using water glass achieved excellent mechanical properties compared with other activators [14,15]. The amount of reaction products, such as C-(A)-S-H and N-A-S-H, increased with increasing curing time, which was beneficial for the compressive strength [16–18]. NaOH is also widely used as an alkaline activator because of its high alkalinity. Li [19] observed that NaOH-based AAS performed better than Na2CO3based AAS in terms of the early compressive strength; however, NaOH-Na2CO3-based AAS had better properties than NaOH-based or Na2CO3-based AAS considering the late compressive strength. This could be due to the loose structure and coarse pore structure of NaOH-based AAS [20]. In addition, the low early strength of Na2CO3-based AAS limits its practical engineering applications, owing to lack of compressive strength at 3 d at room temperature [21,22]. The disadvantages of AAS materials include their fast setting [23,24] and substantial shrinkage [14,25,26], which can result in large problems in construction progress and the quality of the projects. Aydin et al. [23] reported that the setting rate of AAS specimens activated by water glass with a silicate modulus of 0.4 and 0.8 and Na2O contents of 6% and 8% was too fast due to initial CS-H formation. However, slag activated by Na2CO3 has a long setting time, which is mainly attributed to initial sodium calcium carbonate formation [14,27]. A significant challenge for AAS materials is optimization of the setting behavior; both short and long setting times can limit the applicability of the material. The type of alkaline activator critically affects the drying shrinkage of AAS mortars. AAS mortars with water glass exhibits the highest drying shrinkage and that with Na2CO3 showed the lowest compared with the alkaline activator for NaOH [14]. Collins et al. [28] reported that the drying shrinkage of AAS is four times larger than that of OPC at 50% relative humidity (RH) and 23 °C. Curing at high temperature is an effective way to reduce the drying shrinkage of AAS concrete [29,30], because the dense hydration products and the tortuosity of the pore network increase, and the air or water is not easily lost after heat-curing. However, heat-curing is not suitable for practical in-situ engineering, it may only be well-suited for precast application [30]. Pottery sand, a lightweight aggregate, primarily comprise fly ash and bauxite, with fly ash accounting for 50–80%. Fly ash is generated from thermal power plants as an industrial by-product; its annual production was 565 million tons in 2016 [31]. Therefore, pottery sand is very cheap owing to it being made of industrial solid waste raw material. Pottery sand is typically used as a fine aggregate for preparing lightweight mortar and concrete in China. It is highly dimensionally stable, which reduces the shrinkage of mortars compared with that of pastes. However, to the best of our knowledge, no studies describing the utilization of pottery sand in the alkali-activated slag mortars have been published. Consequently, it is necessary to investigate the properties of fresh and hardened AAS mortars using pottery sand as the fine aggregate. The objective of this study was to advance knowledge regarding the use of blends of NaOH and Na2CO3 as the alkaline activator for a new type of AAS mortar. Five Na2CO3-to-NaOH ratios and three Na2O contents were considered and their effects on the macro-properties and microstructure AAS mortars were investigated. The macro-properties included the setting times, fluidity, compressive strength, and drying shrinkage, while the microstructure was investigated using microscopy, elemental analysis, and mercury intrusion porosimetry (MIP).
2. Materials and methods 2.1. Raw materials Blast furnace slag from the Tangshan steel plant in China was used in this study. The slag was classified as S95 according to the GB/T 18046-2008 standard [32], with a specific surface area of 424 m2/kg. Pottery sand made from fly ash and bauxite calcined at a high temperature (1200–1600 °C) was used as the lightweight fine aggregate. The pottery sand had a particle size less than 1 mm, specific gravity of 1.8, and bulk density of 1.2 g/cm3. The chemical compositions of the slag and pottery sand are listed in Table 1, where photographs of these raw materials are shown in Fig. 1. The alkaline activator for the AAS mortar was prepared using only NaOH or blends of NaOH and Na2CO3. NaOH flakes (96% purity) were supplied by the Dalu Chemical Reagent Factory (Tianjin, China), while Na2CO3 pellets (99.8% purity) were supplied by the Zhiyuan Chemical Reagent Factory (Tianjin, China). 2.2. Sample preparation To prepare the alkaline activators, we used five different Na2CO3-to-NaOH mass ratios of 0/100, 20/80, 40/60, 60/40, and 80/20, and three different Na2O contents of 4%, 6%, and 8% (relative to the mass of slag). The water-to-slag ratio and slag-toaggregate mass ratio were kept constant at 0.38 and 0.8, respectively. The proportions of the mixtures are summarized in Table 2. For example, the code ‘‘C2N8-6M” represents the mortar with Na2CO3-to-NaOH mass ratio of 20/80 and 6% Na2O content. To prepare the AAS mortars, the slag and pottery sand were firstly premixed for 5 min, then the activator was added, and lastly, all mixtures were stirred for 1 min at a slow rate of 200 ± 5 rpm for another 1 min at a fast rate of 400 ± 10 rpm. 2.3. Test methods The setting time was measured in accordance with the JGJ/T 70–2009 standard [33]. The cross-sectional area of the stainless steel needle in the instrument used to measure the setting time of the mortar was 30 mm2. The setting time was defined as the time from the addition of the activator to the raw materials to the time when the penetration resistance reached 15 N. The fluidity test was performed with a jump table following standard GB/T 2419-2005 [34]. Firstly, the fresh mixtures were poured into truncated conical molds (h = 60 mm, dtop = 70 mm, dbottom = 100 mm). Then, the mold was lifted vertically and the jump table was vibrated for 25 s (the table vibration frequency was 1 s 1). Then, the diameters along the two perpendicular directions were measured after the vibration had finished. The average diameter was used to determine the fluidity of the mortar sample. In accordance with the standard JGJ/T 70-2009 [33], fresh mortar mixtures were poured into cubic molds with an edge of 70.7 mm, followed by mechanical shaking. Next, plastic foil was used to seal the top surface of the specimens to prevent water loss. Then, the samples were stored under standard ambient conditions (temperature of 20 ± 2 °C and relative humidity > 95%) for 24 h. After removal from the mold, the samples were kept under these standard conditions until required for testing at the various scheduled/predetermined ages (1, 3, 7, 14, 28 d). The compressive strength was measured using an electro-hydraulic system at 1.0 kN/s. An average value of three specimens was determined as the compressive strength of the AAS mortars for the corresponding curing time. According to the standard JC/T 603-2004 [35], the fresh mortars were poured into prismatic molds with 25 mm (length) 25 mm (width) 280 mm (height). Further treatment was as the same as described in the previous section until the specimens were removed from the molds. Firstly, the dimensions of the samples were measured; then, they were placed in a room with constant temperature (20 ± 2 °C) and relative humidity (50 ± 5%). The drying shrinkages of the samples were measured with a vertical length comparator (digital dial gauge) daily until 28 days. After compressive strength tests at the corresponding curing time, samples from the crushed prisms were kept in the compound of alcohol and acetone (volume ratio of 1/1) for 7 d to prevent hydration. Then, the specimens were removed from the compound and placed in an oven at 60 °C under vacuum to dry. In order to determine the hydration products and reaction mechanism of the AAS mortars, the morphology and elemental composition of the hydration products were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively (ZEISS MERLIN Compact FE-SEM, Germany). The pore structures of the AAS mortars were evaluated using MIP (Autopore IV 9500, Micromeritics Instrument Ltd., USA).
3. Results 3.1. Setting time Fig. 2 shows the setting time of AAS mortars for the different Na2CO3-to-NaOH ratios and Na2O contents. The setting behavior
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Z. Jiao et al. / Construction and Building Materials 197 (2019) 83–90 Table 1 Chemical compositions of the slag and pottery sand (%). Oxide
SiO2
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
Others
Slag Pottery sand
32.83 62.12
17.19 16.32
36.69 3.26
0.38 7.84
0.37 1.25
8.20 2.04
0.65 1.97
1.94 –
1.75 5.2
Fig. 1. Photographs of the (a) cementitious material (blast furnace slag) and (b) fine aggregate (pottery sand).
Table 2 Mix proportions used in this study. Code
Na2CO3-to-NaOH ratio
Na2O content (%)
Slag (g)
Pottery sand (g)
Na2CO3 (g)
NaOH (g)
H2O (g)
c(OH ) (mol/L)
c(CO23 ) (mol/L)
C0N10-4M C0N10-6M C0N10-8M C2N8-4M C2N8-6M C2N8-8M C4N6-4M C4N6-6M C4N6-8M C6N4-4M C6N4-6M C6N4-8M C8N2-4M C8N2-6M C8N2-8M
0/100 0/100 0/100 20/80 20/80 20/80 40/60 40/60 40/60 60/40 60/40 60/40 80/20 80/20 80/20
4 6 8 4 6 8 4 6 8 4 6 8 4 6 8
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250
– – – 10.9 16.3 21.7 22.9 34.3 45.8 36.3 54.5 72.6 51.4 77.1 102.7
51.6 77.4 103.2 43.4 65.1 86.8 34.3 51.5 68.7 24.2 36.3 48.4 12.8 19.3 25.7
368.4 362.6 356.8 370.2 365.3 360.5 372.3 368.4 364.5 374.6 371.8 369.1 377.1 375.7 374.2
3.50 5.34 7.23 2.93 4.46 6.02 2.30 3.49 4.71 1.62 2.44 3.28 0.85 1.28 1.72
– – – 0.28 0.42 0.57 0.58 0.88 1.19 0.91 1.38 1.86 1.29 1.94 2.59
Fig. 2. Setting time of AAS mortars with different Na2O contents and Na2CO3-toNaOH ratios.
was dependent on both the Na2CO3-to-NaOH ratios and Na2O contents. The setting times of the AAS mortars were within the range of 11.3–44.0 min. Previous studies [27,36,37] showed a dramatic difference in the range of 30 min to more than 3 days in the AAS
activated by Na2CO3. The setting time increased with an increasing of Na2CO3-to-NaOH ratios and decreasing of Na2O contents. The longest setting time (for C8N2-4M) was almost four times the shortest one (for C0N10-8M). Higher Na2CO3-to-NaOH ratios and lower Na2O contents in the alkaline activators resulted in longer setting times. The higher Na2CO3 content resulted in a higher content of initial calcium carbonate/sodium calcium carbonate, which was the primary reason for the reduced reaction rate [27,38]. They found that the initial formation of carbonated compounds resulted in loss of plasticity in specimens, however, which did not reach the setting stage, and the later formation of C-S-H was contributed to the setting of AAS. Moreover, the OH concentration of the alkaline activator played a key role in the setting behavior. Similar results were also reported by Cengiz et al. [14] and Puertas et al. [22], who showed that increasing the amount of Na2CO3 in the activator reduced the OH concentration of the alkaline activator. In addition, the OH ions in the activator accelerated the decomposition and hydration of the slag structure to rapidly form a C-S-H gel, and further accelerate setting [39]. The setting and hardening of specimens activated by NaOH were also attributed to the formation of extra polysilicate hydrates [40]. Therefore, the setting time can be adjusted via appropriate addition of Na2CO3.
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3.2. Fluidity The fluidity values of AAS mortars with different contents of alkaline activator are plotted in Fig. 3. In the AAS mortars with only NaOH activator, the fluidity increased as the Na2O contents increased. Puertas et al. [41] discovered that the slump value decreased with increasing of Na2O content from 3% to 4%; however, it increased as the Na2O content increased from 4% to 5% after 7 min. They indicated that the higher yield stress is, the lower is the slump. However, for all NaOH-Na2CO3 activated specimens, the fluidity decreased as the Na2O content increased. Higher Na2O contents resulted in higher OH concentrations, which can accelerate the dissolution of Ca2+. The initial precipitation of CaCO3 was responsible for a dramatic decrease in the fluidity when the Na2O content increased. Puertas et al. [41] showed that the slump value of AAS activated by Na2CO3/NaOH decreases with increasing of Na2O content from 3% to 4%. Mehdizadeh et al. [42] investigated rheology alkali-activated phosphorous slag based on water glass. They showed that the main reason for the reduction of fluidity is the increasing of phosphorous slag dissolution and more gel formation. Moreover, the highest fluidity was obtained for AAS mortars with a Na2CO3-to-NaOH ratio of 20/80, regardless of the Na2O content in all the NaOH-Na2CO3-activated specimens. Increasing the Na2CO3 content reduced the fluidity in NaOH-Na2CO3-activated samples, perhaps due to its higher stress [41]. Zivica et al. [40] reported that the workability of AAS mortar activated by only Na2CO3 decreased as the dosage of alkaline activator increased from 3% to 7%. More Na2CO3 resulted in more CO23 the more initial sodium calcium carbonate or calcium carbonate formed, the more reduced the workability. Furthermore, when the Na2O content was 4%, the fluidity of the AAS mortar activated by NaOH was lower than that by Na2CO3-NaOH. However, when the Na2O content increased to 8%, the fluidity of the AAS mortar activated by NaOH was higher than that by Na2CO3-NaOH. Meanwhile, Collins et al. [21] reported that the workability of AAS improved as the Na2CO3 content increased when the alkaline activator was NaOH in combination with Na2CO3. However, Puertas et al. [41] showed that the slump value of AAS with NaOH was higher than that with NaOH/Na2CO3 at the 4% Na2O. The fluidity of all AAS mortars varied between 184.5 and 211.5 mm. The lowest fluidity (of C6N4-8M) was about 87.2% of the highest fluidity (of C2N8-4M), where the lowest value was above 180 mm, conforming to the criteria given by the GB/T 8077-2012 standard [43]. Consequently, the Na2CO3-to-NaOH
Fig. 3. Fluidity values of AAS mortars with different Na2O contents and Na2CO3-toNaOH ratios.
ratios and Na2O contents are both important for the fluidity of AAS mortars. 3.3. Compressive strength The compressive strength of AAS mortars using the various Na2CO3-to-NaOH ratios and Na2O contents at curing times of 1, 3, 7, 14, and 28 d are shown in Fig. 4. Generally, there was an increase in the compressive strength as the curing time increased for all samples. Before 3 d, the compressive strength improved with increasing Na2CO3-to-NaOH ratios within the scope of 0/100– 40/60, and then decreased when it increased further from 60/40 to 80/20. This indicates that the highest Na2CO3-to-NaOH ratios resulted in the lowest compressive strengths in the early stage. However, we observed a completely different trend in the later stage. After 7 d, the compressive strength increased with increasing Na2CO3-to-NaOH ratio, regardless of the Na2O content. For Na2O contents of 4%, 6%, and 8%, the compressive strengths of the AAS mortars with a Na2CO3-to-NaOH ratio of 80/20 were 1.32, 1.33, and 1.27 times that of the 0/100 sample, respectively. The OH ion played an important role in dissolving the glassy structure of the slag and improving the hydration process [44]. The addition of Na2CO3 decreased the OH concentration of the alkaline activator, while sodium calcium carbonate decreased the early compressive strength [22]; however, with increasing curing age, the highly cross-linked structure formed by carbonated compounds can enhance the late compressive strength [15]. Moreover, the compressive strength of the AAS mortars with a Na2O content of 6% was higher than those of samples with contents of 4% and 8%. Higher Na2O contents resulted in higher compressive strengths; however, there was an optimum Na2O content in the alkaline activator as excess Na2O decreased the strength. Komnitsas et al. [45] stated that the optimum KOH activator concentration was 8M, and demonstrated that any excess will not be fully reacted. Hence, the compressive strength of slag activated by Na2CO3 and NaOH was significantly affected by the Na2O content and Na2CO3-to-NaOH ratio. 3.4. Drying shrinkage The drying shrinkage of AAS mortars in the first 28 d are shown in Fig. 5. For the lowest Na2O content (4%), the drying shrinkage of the AAS mortars increased with increasing Na2CO3-to-NaOH ratios. However, for the highest Na2O content (8%), the drying shrinkage showed the opposite behavior. On the other hand, Na2CO3 was attributed to reducing the drying shrinkage of AAS mortars at higher Na2O content. The highest drying shrinkage was 0.36% for the C8N2-4M sample, which was 3.27 times that of the lowest value of 0.11% (C8N2-8M). Similarly, there was a dramatic drop of the drying shrinkage for AAS activated by Na2CO3 as the Na2CO3 content increased from 4% to 6% [46]. The reaction product, the hydrotalcite-like phase, was contributed to reducing the drying shrinkage [46–48]. Moreover, the drying shrinkage decreased as the Na2O content increased for samples with higher Na2CO3-to-NaOH ratios (40/60, 60/40, and 80/20). This may be due to the lower OH concentration for samples with lower Na2O contents, which cannot be fully reacted and therefore, remaining moisture resulted in higher drying shrinkages [49]. However, there was an inverse trend of drying shrinkage for the AAS mortars with only NaOH or Na2CO3 [14], where the drying shrinkage increased as the sodium concentration (in the range of 4–8%) increased. The drying shrinkage mechanism for the AAS mortars is complex, as it depends on both the Na2CO3 to-NaOH ratio and the Na2O content. The effect of reaction production on drying shrinkage mainly was the amorphous phase of C-SH gel, rather than the crystalline phase of tobermorite [50]. Ye et al.
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Fig. 4. Compressive strength of AAS mortars with different Na2O contents and Na2CO3-to-NaOH ratios, (a) 4% Na2O, (b) 6% Na2O, (c) 8% Na2O.
Fig. 5. Drying shrinkage of AAS mortars with different Na2O contents and Na2CO3-to-NaOH ratios, (a) 4% Na2O, (b) 6% Na2O, (c) 8% Na2O.
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[51] discovered that the formation of C-A-S-H incorporating the alkali cations for AAS mortars results in the high drying shrinkage. In addition, Häkkinen [52] formed different conclusions about the tendency of the drying shrinkage of AAS pastes with only NaOH or Na2CO3. They found that the drying shrinkage of AAS pastes with 5% Na2CO3 was higher than that with 5% NaOH after 231 d. Wittmann and Collins [28,53] showed that the pore size distribution of mesopores and the C-S-H gel features had a significant effect on the drying shrinkage. The radius of the pores where the meniscus formed was the major factor determining the drying shrinkage due to the larger capillary tensile forces. A detailed explanation of the drying shrinkage behavior is discussed with respect to the pore structures in Section 3.6.
3.5. Microstructure and composition SEM/EDS was performed to study the reaction products and study the interfacial transition zone (ITZ) of hardened AAS mortars. Figs. 6 and 7 show the SEM and EDS micrographs of AAS mortars with different Na2CO3-to-NaOH ratios at 28 d, respectively. A series of amorphous particle products was observed for all AAS mortars, which were mainly comprised of Ca, Si, Na, Mg, Al, and O. The main reaction hydrates were C-S-H gels, hydrotalcite, and gaylussite [36,54]. Moreover, the microstructure became gradually denser with increasing Na2CO3-to-NaOH ratio, which was consistent with the compressive strength development shown in Section 3.3. A similar result was found where the hydration products became denser as the Na2CO3-to-NaOH ratios increased in AAS mortars [46]. In addition, the composition and morphology of the reaction products was closely related to the compressive strength. Previous studies [17,36,55] indicated that the CO23 anion produced to form carbonate compounds like Na2Ca(CO3)25H2O and the cross-linked structure of C-(A)-S-H enhanced the compressive strength. The ITZ between the aggregate and paste also plays a key role in the devel-
opment of compressive strength in the mortars [56]. It can be seen that there were some small cracks in the C0N10-6M and C4N6-6M samples, but none in C8N2-6M. However, the ITZ of all AAS mortars showed good adhesion and there was no clear distinction between the aggregates and paste for different Na2CO3-to-NaOH ratios. These microstructural properties can explain the development of the mechanical performance of our AAS mortars.
3.6. Pore-size distribution The classification of the pore structures of the AAS materials according to the standards of the International Union of Pure and Applied Chemistry showed micropores (d < 2.5 nm), mesopores (2.5 nm < d < 50 nm), macropores (50 nm < d < 10000 nm), and voids and microcracks (d > 10000 nm). The calculated pore diameters were in the range of 5 nm–360 mm. The effect of the Na2CO3to-NaOH ratio on the pore structures of AAS mortars is depicted in Fig. 8, where the total pore volumes of the AAS mortars decreased as the Na2CO3-to-NaOH ratio increased from 0/100 to 80/20 (Fig. 8(a)). With increasing Na2CO3-to-NaOH ratio, a higher fraction of carbonates formed a denser pore structure at the later stage. In addition, Fig. 8(b) shows that the two peaks (mesopores and macropores) both had a slightly higher intensity for the lower Na2CO3-to-NaOH ratios; with increasing Na2CO3-to-NaOH ratios, the peaks shifted towards smaller pore sizes. This shift in the mean pore size can be explained by gradual filling of the larger pores with reaction products with increasing Na2CO3-to-NaOH ratio. Fig. 8(c) shows that the porosity decreased from 14.2% to 10.9% as the Na2CO3-to-NaOH ratio increased from 0/100 to 80/20 in AAS mortars with 6% Na2O. The finer and denser pore structures indicated that more slag was hydrated and there was a stronger bonding between the aggregates and paste; this is consistent with the higher compressive strength of these samples (Section 3.3).
Fig. 6. SEM micrographs of the fracture surface of hardened AAS mortars cured for 28 d. (a) C0N10-6 M; (b) C4N6-6 M; and (c) C8N2-6 M.
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Fig. 7. EDS spectra for the regions labeled in Fig. 6. (a) Spectrum 1; and (b) Spectrum 2.
Fig. 8. Pore size distribution of AAS mortars with different Na2CO3-to-NaOH ratios. (a) Cumulative intrusion data as a function of pore size; (b) differential intrusion data as a function of pore size; (c) porosity.
Meanwhile, the mesopore volume decreased with increasing Na2CO3-to-NaOH ratio. Wittmann [53] indicated that there was an important relationship between the pore size distribution and drying shrinkage. The larger mesopores volumes resulted in a higher drying shrinkage due to the higher contracting stress [12,28], which was consistent with the trend in the drying shrinkage behavior (Section 3.4). A similar relationship between the refined pore structure and shrinkage was reported by Gao et al. [57]. Collins et al. [28] stated that a lower porosity and narrower pore size distribution usually results in a higher sample strength. They concluded that the water loss through mesopores and a macropore size distribution had a positive impact on the drying shrinkage. 4. Conclusions The effects of Na2O content and Na2CO3-to-NaOH ratio on the setting time, fluidity, mechanical performance, and drying shrinkage of AAS mortars were investigated. The main conclusions of this study are as follows. (1) Both the Na2O content and Na2CO3-to-NaOH ratio played a significant role in determining the properties of fresh and hardened AAS mortars.
(2) The setting time increased with the increasing Na2CO3-toNaOH ratio and Na2O content. The setting time of the AAS mortars was in the range of 11.3–44.0 min. (3) In the early stage of curing, the compressive strength increased as the Na2CO3-to-NaOH ratio increased from 0/100 to 40/60, and then decreased as it increased from 60/40 to 80/20; however, in the later stage, the higher Na2CO3-to-NaOH ratio resulted in a higher compressive strength. (4) For lower Na2O contents, the higher Na2CO3-to-NaOH ratios resulted in higher drying shrinkage; however, increasing the Na2CO3-to-NaOH ratios effectively reduced the drying shrinkage of AAS mortars with higher Na2O contents. (5) With increasing Na2CO3-to-NaOH ratios, the microstructure of the AAS mortars became denser and the mesopore volume decreased, which were consistent with the higher late compressive strength and lower drying shrinkage.
Conflicts of interest None.
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