Development of high strength one-part geopolymer mortar using sodium metasilicate

Construction and Building Materials 236 (2020) 117611

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Development of high strength one-part geopolymer mortar using sodium metasilicate Minhao Dong, Mohamed Elchalakani ⇑, Ali Karrech School of Civil, Environmental and Mining Engineering, The University of Western Australia, WA 6009, Australia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


High strength geopolymer was

produced with sodium metasilicate.
The solid activator outperformed the

corresponding liquid activator.
The fineness of the metasilicate

particles had a great effect on strength.
Mixes with microsilica were more resistant to efflorescence.

a r t i c l e

i n f o

Article history: Received 27 August 2019 Received in revised form 12 October 2019 Accepted 12 November 2019

Keywords: Sodium metasilicate One-part Geopolymer Efflorescence Alkalinity

a b s t r a c t In this study, the solid activator – the synthetic sodium metasilicate pentahydrate – was compared against water and a hybrid sodium silicate and sodium hydroxide activator solution. It was found that the solid activator outperformed the liquid activator as a part of water remained chemically bound to the undissolved particles, thus reducing the apparent water to binder ratio. Contrary to the liquid activator, the increase in activator content past a certain limit did not correspond to improvement in strength. Instead, the compressive strength reduced and the risk of efflorescence increased significantly. Scanning electron microscopy (SEM) analysis showed that the denser reaction products took up less volume and formed a shell as the remaining metasilicate particles gradually dissolved under moisture ingression. This would eventually develop into a void, and cause strength- and durability-related issues. Additionally, fly ash content, microsilica addition, particle size of the activator, binder content, water to binder content and curing condition also played an important role in the hardening and pore refinement of the mixes. The one-part synthesis method could improve the safety and efficiency in geopolymer production. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Geopolymer is an alternative cementitious material derived from aluminosilicate-rich materials, typically industrial byproducts such as fly ash from coal-fired power plants. The ⇑ Corresponding author. E-mail address: [email protected] (M. Elchalakani). https://doi.org/10.1016/j.conbuildmat.2019.117611 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

aluminosilicate reacts with the activator solution, often an alkaline solution, and produces the geopolymeric aluminosilicate gels. The geopolymeric gels are dense and resistant to the ingression of many deleterious materials, such as sulphate [1], CO2 [2], and chloride [3]. With the addition of ground granulated blast-furnace slag (GGBS) in alkali-activated slag (AAS) systems, the reaction product also comprises calcium-silicate-hydrate (C-S-H) or calcium-alumi nate-silicate-hydrate (C-A-S-H) gels. The C-S-H and C-A-S-H gels

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M. Dong et al. / Construction and Building Materials 236 (2020) 117611

coexist with the geopolymeric gels in a geopolymeric system [4], improving pore refinement and increasing the compressive strength [5]. However, the risk of decalcification increases as the CaO content in the binder increases [6], which corresponds to long-term durability and performance problems. Therefore, the fly ash to GGBS ratio could be optimised to cater for both strength and durability. In addition, supplementary materials such as microsilica or calcium carbonate were also added to the geopolymer to modify the workability and hardening [7,8]. The synthesis of geopolymer does not involve the high temperature calcination process used in ordinary Portland cement (OPC) production, which makes it more attractive in terms of carbon emissions. The alkaline activator solution usually contains sodium- or potassium- based hydroxides [9], silicates [10], carbonates [11] or a combination of those materials [7,12], which makes it highly caustic and non-user-friendly. The transportation, storage and handling of the alkaline solution requires the implementation of additional safety measures, slowing down production and elevating the cost [13]. The use of solid activator has been explored to produce one-part ‘‘just add water” geopolymer. Notable materials used as alkali suppliers include, sodium hydroxide combined with various silica sources such as fly ash [14], rice husk ash [15], or microsilica [15]; calcium hydroxide [14]; different grades of sodium metasilicate [15–18]; and red mud [19]. Conventionally, for the two-part synthesis method, it has been demonstrated that the silicate based activator solution is the most effective for strength development [20]. It was found that, an optimum Ms (Ms = SiO2/M2O, with M being Na or K) of 1–1.5 exists for sodium silicate and hydroxide hybrid activator [10]. The commercially available sodium silicate solutions typically have an Ms of 2–3.2, which is outside the optimum range. This could be mitigated by combining hydroxides, which has an Ms of 0. For the one-part alkali-activated GGBS, the mixes with synthetic anhydrous sodium metasilicate had a tripled compressive strength compared to the hydroxide and silica counterpart [15]. The commercially available sodium metasilicate typically has an Ms of 0.9–1.0, which is close to the optimal range. Therefore, metasilicate could reduce the mixing steps and streamline the production. The low embodied CO2 index and high performance made it an attractive material to use in construction and building industries [18]. In a geopolymer system with an alkaline activator, the alkalis react with the atmospheric CO2 and form a white carbonate salt known as efflorescence. The sodium carbonate is not a precipitate, therefore the efflorescence in geopolymer does not slow down over time, unlike an OPC system, where calcium carbonate gradually covers up the surface. The alkaline pore solution continues to migrate to the surface due to the capillary action, impairing the protection to the embedded reinforcement. Critical efflorescence not only damages the aesthetics of the structures, but also deteriorates long-term stability [21]. In a two-part system, it was recommended that the Na2O content in the activator should be less than approximately 6% by weight of the binder, to reduce excessive the pore alkalis [22,23]. The pore condition of one-part geopolymer differs from two-part geopolymer, due to the differences in the state of the activator. However, there is a lack of information on the leaching and efflorescence on one-part geopolymers. In this study, the effect of sodium metasilicate particles on the curing of various one-part ‘‘just add water” geopolymer and AAS mixes was comprehensively investigated over 90 days. Common factors in the mix design, for instance, activator Na2O to total binder ratio (Na2O%), Ms, total water to water ratio (w/b), binder content by weight (b%), fineness of metasilicate particles, additional silica content provided by microsilica, and curing conditions are studied. The effect of the increasing calcium content as the mix transitioned to AAS mortars was also investigated. Compressive

strength, hardening trends, leaching, efflorescence, and microstructure were assessed for each mix. The aim is to improve the predictability of the easy-to-use one-part geopolymer, which would facilitate its use in various construction projects, especially in the more corrosive environments, such as the coastal zones in Australia.

2. Experimental program 2.1. Materials The binder consisted of fly ash (specific gravity = 2.35) and GGBS (specific gravity = 2.95). The microsilica (specific gravity = 2.25) was added to selected mixes (M6, M11, M18, M20) to provide additional silica content. The oxide composition of the binder and filler is shown in Table 1. The hydration modulus [HM = (CaO + MgO + Al2O3)/SiO2] is a measurement of the hydraulic activity of a material [24]. Typically, higher HM corresponds to faster hardening and a higher compressive strength. The high HM of GGBS allows the mix to be cured in ambient conditions and have a sufficient compressive strength for any practical uses. On the other hand, fly ash improves the workability of fresh mortar with its smooth spherical particles [12] and produces the dense and stable geopolymeric gels upon activation. Dune sand (specific gravity = 2.65) was used as fine aggregates in the geopolymer mortar mixes. The particle size distribution of the materials is shown in Fig. 1. Solid and liquid activators were compared in this study. The solid activator was the sodium metasilicate pentahydrate (Na2SiO35H2O) with a specific gravity of 0.96. The Na2O and SiO2 accounted for 27.0% and 28.0% of the total weight (Ms = 0.97), respectively, while the rest is chemically bound water. 90% of the particles are within 14–30 Mesh, equivalent to 0.595 mm–1.41 mm. The metasilicate solution had a solubility of 400 g/L at 20 °C, pH of 13.4 at saturation. The metasilicate was finely ground in a ring-mill to a fine powder. Due to the nature of metasilicate, the particle sizes of the ground powder could only be estimated using a scanning electron microscope (SEM). The typical particle ranged between 5 and 200 lm (Fig. 2). A commercially available sodium silicate solution (Ms = 2.0, specific gravity = 1.53) was mixed with a 12 M sodium hydroxide solution to achieve the same Ms of 0.97 as the metasilicate. The sodium silicate solution has a pH of 12.7 while the sodium hydroxide has a theoretical pH of 15.1.

2.2. Mix design, mixing and curing Table 2 shows the mix designs based on a target density of 2200 kg/m3 [12]. A total of 20 mortar mixes was synthesised. The binder content (b%) was kept at 35% (770 kg/m3) based on [12], except for M15 with 45% binder (990 kg/m3). The Ms of the alkaline activators was kept consistent at 0.97, which was close to the range (1– 1.5 [10]) for optimal performance, and comparable to the as-received sodium metasilicate pearls. The first two mixes had no or liquid activator. M1 could also be considered as a ‘‘just add water” mix, while M2 has the common liquid silicate/hydroxide hybrid activator. The finely ground and as-received metasilicate were used in the mixes to study the effect of particle fineness on the hardening of geopolymer. Na2O% ranged from 0% to 12% at an increment of 3%. The effect of three different w/b ratios, 0.31, 0.34 and 0.37, were assessed. Note that the total binder referred to the binder and the solid content (excluding chemically bound water [18]) in the activator, and total water referred to the total water in the activator and any additional free water. The chemically bound water in the sodium metasilicate was also considered in the total water, as it would be released upon dissolution. The binder consisted of fly ash and GGBS, with 3 different FA% of 80%, 40% and 0%. It was found in the preliminary tests that at least 20% GGBS was needed to substantially cure the mortar at ambient temperature. Finally, additional microsilica amounting to 10% of weight of the binder (68–73 kg/m3) was added to four mixes, M6, M11, M18 and M20. For the one-part mixes, the dry materials, including fly ash, GGBS, microsilica, sand and metasilicate, were mixed in a planetary mixer for three minutes. Water was then added, and the wet mixing continued for another 3 min before moulding. For two-part M2, the activator was prepared 24 h prior to mixing. The dry mixing was the same as one-part mixes, after which the activator solution was added to the mixture with additional free water. The wet mixing only took 30 s to achieve homogeneity. The fresh mortar was then poured into 50 mm cubic moulds for compression testing. The specimens were placed in an ambient curing room with set temperature and humidity of 23 °C and 95%, respectively. After 24 h of initial curing, the specimens were demoulded. For each mix, one group of specimens was cured in the ambient curing room until the test day, whilst another group was cured in water. Each cube was fully submerged in 250 mL deionised water. This forced the leaching of alkali from the pores and allowed the pH to be measured over time.

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M. Dong et al. / Construction and Building Materials 236 (2020) 117611 Table 1 Oxide analysis of fly ash, GGBS and microsilica. Al2O3 %

CaO %

Fe2O3 %

K2O %

MgO %

MnO %

Na2O %

P2O5 %

SiO2 %

SO3 %

TiO2 %

LOI1 %

Hydration modulus

Fly ash GGBS Microsilica

23.9 13.1 0.1

7.0 43.2 0.3

7.9 0.8 0.0

1.0 0.3 0.6

1.3 5.5 0.7

0.1 0.2 0.0

0.4 0.3 0.4

0.5 0.0 0.0

55.9 31.4 92.8

0.3 4.0 0.0

1.3 0.6 0.0

0.4 0.6 5.1

0.6 2.0 0.0

Loss on ignition.

% Passing

1

Material

100 90 80 70 60 50 40 30 20 10 0 0.1

At 90 days, pieces of the specimens were collected for X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and SEM analysis. The powder samples of the specimens were analysed using an X-ray diffractometer with Cu K radiation (k = 1.5404 Å), at 40 kV and 40 mA. The samples were scanned between 2h = 590° at a rate of 1° per minute. The coated mortar chips were then analysed using a field emission SEM and the attached EDS module.

Sand Fly ash GGBS Microsilica

3. Results and discussion 3.1. Compressive strength

1

10

100

1000

Particle diameter (μm)

The compressive strengths of the 20 mixes at 3, 28 and 90 days are shown in Table 3. For each mix, the results for the specimens cured in the ambient curing room and submerged in water (with the suffix ‘‘W”) are available.

Fig. 1. Particle size distribution curves of the raw materials.

Fig. 2. As-received and ground metasilicate particles.

2.3. Test methods Various tests were performed at 3, 28 and 90 days. The pH of water used for curing was firstly measured using an electronic pH meter. The effect of temperature was automatically measured and adjusted by the meter. All the specimens were then oven dried before the efflorescence measurement. The salt formed on the surface was carefully collected and weighted. The compressive strength of each sample was measured using a 600 kN universal testing machine. The loading rate was set to a constant 20 MPa/min in accordance with AS 1012.9 [25]. After the compression test, the specimen was finely ground into powder. One gram of the powder was dispersed into a beaker containing 40 mL of deionised water. The mixture was stored at 23 °C for three days before the pH measurement.

3.1.1. Type of activator Fig. 3 compares the hardening of four similar mixes with different types of the activator, namely M1, M2, M4 and M7. M1 was a one-part mix activated by water only. The compressive strength developed slowly and reached 48 MPa at 28 days. The cubes cured in water had slighted reduced compressive strengths. Slow increase was observed for the cubes in both curing conditions after 28 days. The activator in M2 was the depolymerised sodium silicate solution with an Ms of 0.97. The compressive strengths of M2 at different ages greatly exceeded those of M1, indicating a more complete polymerisation occurred in the matrix. The strengths between the ambient and water cured specimens were comparable until 28 days, after which the strength of the water cured specimens deteriorated. This was likely due to the excessive leaching of the free alkalis through the pores, which reduced the pore alkalinity and resulted in instability of the NaAO bonds [26]. M4 and M7 were otherwise identical, except for the fineness of metasilicate granules. Both mixes consistently had higher compressive strengths over M1 and M2 over 90 days. This was due to that the metasilicate particles added to the mix were not completely dissolved, resulting in a lower w/b available than the design value. Over time, the undissolved metasilicate particles attracted moisture in the surrounding environment, releasing more sodium silicate into the matrix, thus strengthening the geopolymer. This could be observed from Fig. 3, where the submerged specimens of M4 and M7 surpassed their ambient cured counterpart after 28 days. However, the undissolved or partially dissolved metasilicate particles were much weaker than the geopolymer matrix. A large number of undissolved particles will adversely affect the compressive strength. This is especially evident in M7 with the as-received or coarse metasilicate granules, where the compressive strengths at 28 days were on average 21% lower than those of M4. Overall, the solid metasilicate outperformed its liquid equivalent. The most notable difference occurred at early age, and similar trends were observed after 28 days of curing. Similarly, mix pairs such as M9 and M12, M13 and M16, as well as M18 and M20 also compared the effect of fineness of the solid activator on the hardening of different types of geopolymers. The first two pairs had 80% fly ash in the binder. The effect of the fineness was large at a relatively low Na2O% of 6% and decreased as the Na2O% increased to 9%. When overdosed with coarse metasilicate

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M. Dong et al. / Construction and Building Materials 236 (2020) 117611

Table 2 Mix designs.

1

Designation

Na2O%

Type of activator

w/b

FA%

Fly ash (kg/m3)

GGBS (kg/m3)

Microsilica (kg/m3)

Activator (kg/m3)

Water (kg/m3)

Fine aggregate (kg/m3)

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20

0% 6% 3% 6% 9% 6% 6% 6% 6% 6% 6% 6% 9% 12% 6% 9% 3% 3% 3% 3%

Water Liquid GM1 GM GM GM AM2 GM GM GM GM AM GM GM GM AM GM GM GM AM

0.31 0.31 0.31 0.31 0.34 0.31 0.31 0.34 0.31 0.34 0.31 0.31 0.31 0.37 0.31 0.31 0.31 0.34 0.34 0.34

40% 40% 40% 40% 40% 40% 40% 40% 80% 0% 80% 80% 80% 40% 40% 80% 80% 0% 0% 0%

308 269 290 272 254 272 272 272 545 0 545 545 508 236 349 509 581 0 0 0

462 404 436 408 381 408 408 408 136 680 136 136 127 354 524 127 145 726 726 726

0 0 0 0 0 68 0 0 0 0 68 0 0 0 0 0 0 73 0 73

0 239 77 156 236 157 157 156 156 157 156 156 235 315 204 235 77 78 78 78

238 97 206 172 161 172 172 195 172 194 172 172 138 150 220 138 206 228 228 228

1191 1191 1191 1191 1168 1124 1191 1168 1191 1168 1123 1191 1191 1145 903 1191 1191 1096 1169 1096

Ground sodium metasilicate;

2

as-received sodium metasilicate.

particles, M16 had a severe reduction in strength between 28 and 90 days. This was not observed in M13, which continued to harden until 90 days. The large voids left by the metasilicate particles when they gradually dissolved into the infused water were especially damaging, especially for mixes with a high fly ash content. Another reason for the reduction of performance of the submerged specimens was due to the low stability of sodium-based geopolymeric gels in water [27]. The additional alkalis compensated for the loss of pore refinement due to the low calcium content, however increased the number of voids. When comparing M18 and M20 with 100% GGBS, it could be seen that even at 3% Na2O, the mixes achieved comparable strengths to the above mixes. The effect of large voids was less evident. The compressive strength of M20 kept increasing until 90 days, and only when subjected to forced leaching did the compressive strength of M20W decreased. The difference between M18 and M20 started to appear after 28 days of curing, which showed the early age pore refinement from GGBS. Overall, the as-received metasilicate had a comparable performance to the liquid activator, which meant that it could be easily and effectively incorporated in the synthesis of geopolymer. Fig. 4 showcased the two cubes from M6 and M12 before and after testing at 28 days. It could be seen that the fine metasilicate particles had a less impact on the texture of the surface and the matrix, whereas the voids left by the coarse metasilicate grains were clearly visible on the surface and within the mortar. By finely grinding the metasilicate particles, the compressive strength could be improved by approximately 20%30%, which may justify the carbon footprint and cost of grinding. The benefit of using solid metasilicate over the liquid hybrid activator was the gradual release of silicate ions into the matrix. This reduced the likelihood of flash setting by limiting the amount of available activator in the fresh mortar. The water was held structurally in the metasilicate crystals, which reduced the generation of pores in the geopolymer, leading to stronger performances over the liquid activator. The one-part geopolymer without water still had acceptable strengths at 28 days, which could be adopted for projects such as foundations. 3.1.2. Effect of Na2O% Fig. 5 shows four similar mixes (M3, M4, M5 and M14) with the main difference being the Na2O%. The Na2O% represented the

amount of the activating material in the mix when Ms remained the same. It was 3%, 6%, 9% and 12%, respectively. Note that to accommodate different quantities of the metasilicate, the w/b varied slightly among the four mixes. It could be seen from the figure that the compressive strengths increased as the Na2O% increased from 3% to 6% then decreased as the Na2O% continued to increase. For liquid activators, it was found that marginal improvement from increasing the activator content diminished after a certain point [28]. Additionally, the additional free water needed to ensure a workable mixture for M5 and M14 resulted in the reduced strength. It could also be seen that M3W and M4W had greater compressive strengths than M3 and M4 at 90 days, whereas M5W and M14W had lower strengths. This was likely due to the much greater extent of leaching due to the large pores left by the metasilicate particles. Based on the results, it was recommended to limit the Na2O% content to less than 6% for the optimal costeffectiveness. When fly ash accounted for 80% of the binder, the effect was Na2O% was different. By comparing M9, M13 and M17, it could be found that the compressive strength kept increasing as the Na2O content increased. This was attributed to that the improvement from the additional activator was significant when the mixes were comparably weaker than the previous group. The adverse effect of the excess metasilicate was evident between M12 and M16. M16 with more activator had higher compressive strengths throughout the testing period, however suffered from severe strength degradation after 28 days. 3.1.3. Fly ash to GGBS ratio Fig. 6 compares the effect of different FA% on the hardening of one-part geopolymers. The compressive strengths of M9 were the lowest with 80% fly ash in the binder. The strengths increased when fly ash content reduced to 40%. However, contrary to the findings that higher calcium content corresponding to better pore refinement and additional C-S-H gels [29,30], M10 with 100% GGBS had lower strengths than M4. This was attributed to that at a low w/b ratio of 0.31, the mixture was viscous therefore contained a large number of entrapped air bubbles. Therefore, it was important to incorporate fly ash to reduce the w/b ratio, which could offset the drawbacks of a lower HM. By comparing M3, M17 and M19 with 3% Na2O, it could be seen that a similar trend was obtained. The mix with 80% fly ash only

Table 3 The mechanical and chemical test results of the 20 mixes. Designation

1 2

Efflorescence (g) 3 days 28 days

90 days

pH of ground sample 3 days 28 days 90 days

pH of curing water 3 days 28 days

90 days

Activator conversion rate (%)1 3 days 28 days 90 days

Activator conversion rate (%)2 3 days 28 days 90 days

19 21 40 41 25 32 65 62 30 33 45 53 46 40 45 43 20 17 36 46 18 16 5 6 21 14 31 33 43 43 22 21 0 1 28 30 21 22 27 30

0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 0 –

0.01 – 0.28 – 0.01 – 0.25 – 0.35 – 0.11 – 0.13 – 0.31 – 0.06 – 0.03 – 0.03 – 0.06 – 0.19 – 0.7 – 0.36 – 0.14 – 0.03 – 0.02 – 0.02 – 0.03 –

10.3 10.2 11.6 11.5 11.3 11.4 11.8 11.8 11.5 11.4 11.5 11.6 11.6 11.7 11.7 11.7 11.0 11.0 11.5 11.5 11.5 11.1 11.5 11.3 11.4 11.2 11.4 11.4 11.6 11.5 11.7 11.5 10.9 10.7 11.0 11.0 11.2 11.2 11.1 11.0

– 11.5 – 12.7 – 12.2 – 12.7 – 12.9 – 12.6 – 12.7 – 12.7 – 12.9 – 12.6 – 12.9 – 12.9 – 13.0 – 12.9 – 12.8 – 12.9 – 12.8 – 12.1 – 12.1 – 12.1

– 11.8 – 12.8 – 12.3 – 12.7 – 12.9 – 12.3 – 12.7 – 12.7 – 12.7 – 12.7 – 12.6 – 12.5 – 12.9 – 12.7 – 12.1 – 12.7 – 12.5 – 11.6 – 12.0 – 12.3

77.9 78.1 75.3 75.7 76.2 70.4 33.2 18.3 61.4 50.0 62.3 45.7 51.6 28.8 45.3 36.4 89.4 69.1 70.6 61.3 64.8 67.1 67.3 55.9 69.6 51.7 69.3 50.3 61.4 59.5 40.7 37.7 92.1 78.4 88.2 86.0 82.9 79.4 85.4 85.7

77.9 78.1 75.3 75.7 76.2 70.4 66.6 59.2 87.0 83.2 81.3 73.2 76.0 64.6 69.6 64.6 94.7 84.5 83.7 78.6 82.5 83.6 83.6 77.9 91.0 85.7 92.2 87.3 81.0 80.1 82.4 81.5 92.1 78.4 88.2 86.0 82.9 79.4 85.4 85.7

48 45 62 65 48 39 105 97 63 54 80 89 84 76 62 63 41 37 78 61 33 32 13 12 53 37 51 60 68 63 44 40 2 4 43 46 29 33 43 43

55 50 76 61 46 38 99 108 70 57 101 92 80 82 78 69 45 44 64 59 36 35 17 15 63 50 69 50 93 92 41 29 3 3 66 64 40 37 52 36

0.01 – 0.18 – 0 – 0.12 – 0.32 – 0.09 – 0.09 – 0.19 – 0.05 – 0.02 – 0.02 – 0.02 – 0.09 – 0.42 – 0.16 – 0.04 – 0.03 – 0.02 – 0.01 – 0.02 –

10.5 10.6 11.6 11.5 11.3 11.2 11.5 11.4 10.8 10.2 11.0 11.1 11.5 11.5 11.3 11.5 10.3 10.2 10.7 10.5 10.2 10.2 10.1 10.1 10.7 10.0 11.1 11.2 11.2 11.2 10.5 10.2 9.8 9.4 10.4 10.5 10.5 10.5 10.4 10.4

10.4 10.5 11.0 10.9 10.6 10.6 10.9 10.8 10.5 10.2 10.8 10.0 10.9 11.0 11.0 10.4 10.0 9.8 10.0 9.9 10.1 10.0 9.9 9.9 10.5 9.9 10.5 10.4 10.6 10.4 10.2 10.0 9.2 8.9 10.0 10.0 10.1 10.2 10.0 9.9

– 11.8 – 12.8 – 12.5 – 12.9 – 12.9 – 12.7 – 12.8 – 12.9 – 12.8 – 12.7 – 12.9 – 13.1 – 13.0 – 12.9 – 12.9 – 13.1 – 12.8 – 12.4 – 12.5 – 12.4

68.0 42.4 79.9 74.8 79.8 75.1 61.6 51.0 93.1 78.2 88.9 72.6 68.0 51.5 78.7 54.0 97.9 83.2 95.3 84.8 98.2 77.2 98.5 68.4 93.4 72.0 87.2 63.1 85.6 72.7 96.2 58.4 99.4 85.5 97.2 89.9 96.5 90.1 97.2 91.7

73.4 57.2 94.1 88.6 95.9 90.7 91.6 80.5 96.2 78.5 92.8 93.9 91.4 76.1 90.3 85.2 98.9 87.8 99.0 89.3 98.6 87.9 99.0 91.2 96.1 76.7 96.1 85.7 96.5 95.2 98.0 85.8 99.8 92.2 98.9 97.9 98.7 96.3 99.0 94.5

68.0 42.4 79.9 74.8 79.8 75.1 80.8 75.5 97.7 92.7 94.5 86.5 84.1 75.9 88.2 74.4 99.0 91.6 97.4 91.6 99.1 88.6 99.3 84.2 98.0 91.7 96.7 90.6 92.9 86.6 98.9 87.6 99.4 85.5 97.2 89.9 96.5 90.1 97.2 91.7

73.4 57.2 94.1 88.6 95.9 90.7 95.8 90.2 98.7 92.8 96.4 97.0 95.7 88.1 94.6 91.8 99.4 93.9 99.4 94.1 99.3 94.0 99.5 95.6 98.8 93.1 99.0 96.4 98.3 97.7 99.4 95.8 99.8 92.2 98.9 97.9 98.7 96.3 99.0 94.5

M. Dong et al. / Construction and Building Materials 236 (2020) 117611

M1 M1W M2 M2W M3 M3W M4 M4W M5 M5W M6 M6W M7 M7W M8 M8W M9 M9W M10 M10W M11 M11W M12 M12W M13 M13W M14 M14W M15 M15W M16 M16W M17 M17W M18 M18W M19 M19W M20 M20W

Compressive strength (MPa) 3 days 28 days 90 days

Considering the dissolved activator at saturation in the mixing water. Considering the total amount of activator.

5

6

M. Dong et al. / Construction and Building Materials 236 (2020) 117611

Fig. 3. Comparison of mixes with different types of activators.

Fig. 6. Comparison of mixes with different fly ash content.

achieved 3 MPa at 90 days, while the mixes with 40% and 0% fly ash achieve approximately 40 MPa at 90 days. M19 with 100% GGBS was less susceptible to leaching as seen from its continued increase in compressive strength. In addition, M7 and M12 had 40% and 80% fly ash, respectively, paired with the as-received metasilicate. It was evident that M12 suffered significantly from the voids as a result of the voids. The difference between 40% and 80% fly ash reduced considerably when the fine metasilicate equivalent to 9% Na2O was used.

Fig. 4. M6 and M12 specimens before and after testing at 28 days.

Fig. 5. Comparison of mixes with different Na2O%.

3.1.4. Microsilica Fig. 7 shows two pairs of similar mixes (M4 and M6; M18 and M19) with or without microsilica. M18 and M19 had relatively low compressive strengths of around 40 MPa at 28 days. It could be seen that M18 with the additional microsilica outperformed M19 without any microsilica. The difference between the two mixes gradually increased and reached 40% at 90 days. The contrast in the behaviour was attributed to the fly ash content. Lee et al. [31] reported that the addition of microsilica resulted in a decrease in reactivity of the fly ash but an increase in reactivity of GGBS. The alkali activator tended to react with microsilica particles before fly ash, reducing the strength of mixes with heavy fly ash loading. However, the presence of microsilica increased the rate of formation of C-A-S-H gels in a system with mainly GGBS as the binder. Therefore, with 100% GGBS, the addition of microsilica increased the strength by 67.7% on average when comparing M18 and M19. This increment reduced to 6.5% when fly ash content increased to 40% (M4 and M6). Finally, at 80% fly ash, the compressive strength of M11 with microsilica was 19.5% lower than its counterpart (M9). Additionally, microsilica also acted as a filler that improved the microstructure of mixes. However, the effect

Fig. 7. Comparison of mixes with or without microsilica.

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was not as profound as the effect on the reactivity of the main binders. 3.1.5. Binder content Fig. 8 shows the hardening curves of M4 and M15 with 35% (770 kg/m3) and 45% binder (990 kg/m3), respectively. M4 with lower binder content had consistently higher compressive strengths than M15. The difference reduced from 53% at 28 days to 12% at 90 days. Initially, the excess binder in M15 resulted in a large number of unreacted particles in the matrix, which negatively affected the hardening of the geopolymer. Additional metasilicate particles progressively dissolved into the matrix and reacted with the unreacted binder, resulting in a relatively high rate of cuing even past 28 days. Another reason could be the excessive shrinkage cracking from the large binder content [7]. This affects the integrity of the microstructure, resulting in the reduced efficiency compared to mixes with lower binder content. 3.1.6. Water to binder ratio Fig. 9 shows the M4 and M8 with w/b ratios of 0.31 and 0.34, respectively. It was expected that with a higher w/b ratio, the compressive strength was lower due to the higher porosity. However, for one-part metatsilicate-based geopolymers, additional water content meant that more metasilicate could be dissolved at the mixing stage, thus more binder could react and form gels. It was found that with enough moisture content in the surrounding environment, the extra dissolved ions could not offset the impact of a high porosity. The metasilicate particles could absorb water from the environment after pouring, and potentially fill the pores with the reaction products. Therefore, a gradual release of the activator ions in the matrix was more desirable in this one-part system.

Fig. 8. Comparison between M4 and M15 with different binder content.

7

3.2. Efflorescence The efflorescence measurements were taken at 3, 28 and 90 days for each cube as a qualitative representation of the leaching. Note that the efflorescence measurements were only conducted on the cubes cured in the ambient curing room, as it was assumed that the fully submerged cubes were not being subjected to the CO2 ingress. The results are presented in Table 3. The efflorescence results varied greatly among the 20 mixes. Expectedly, the efflorescence of all the mixes increased as the curing continued. Additionally, no noticeable efflorescence occurred in any of the mixes at 3 days. In general, it could be seen that an approximate threshold could be set for mixes that were more prone to efflorescence and those that were resistant to it. This threshold was 0.1 g at 90 days, which divided the 20 mixes into two even groups of 10 mixes (low and high efflorescence). 3.2.1. Type of activator The efflorescence measurements for M1 and M2 were 0.01 and 0.28 g, respectively. M1 with no activator had effectively no efflorescence due to the absence of free sodium ions. M2 had severe efflorescence at both 28 and 90 days, showing that the recommendation by Gao et al. [23] may not be universally applied to all mix designs. For the equivalent mixes with the sodium metasilicate, M4 and M7 also had severe efflorescence after 90 days of curing. However, the amount of efflorescence of M7 with the coarse metasilicate was significantly lower than that of M4. This was due to that the coarse metasilicate particles had lower surface area per unit volume, which meant that a greater proportion of the metasilicate particle will remain dry. Lower amounts of efflorescence were observed in both mixes compared to M2, indicating a denser microstructure due to the chemically bound water. Similar trends could be discovered in pairs such as M9 and M12, M13 and M16, M18 and M20. 3.2.2. Effect of Na2O% The effect of Na2O% on efflorescence could be seen from M3, M4, M5 and M14, with Na2O% ranging from 3% to 12%. It was clearly seen that as the Na2O% increased, the amount of sodium carbonate increased significantly. M3 with 3% Na2O had virtually no efflorescence at 90 days, while M14 with 12% Na2O had 0.7 g, 7 times the threshold amount. M4 had 0.25 g efflorescence at 90 days, however the compressive strength more than doubled compared to M3. It was expected that mixes with 40% fly ash in the binder could be designed for effective efflorescence control (Na2O% between 3% and 6%), while having a high compressive strength. Similar trends could be found between M12 and M16, M10 and M19. The excess Na2O would almost always result in a higher efflorescence production. 3.2.3. Fly ash to GGBS ratio The effect of fly ash content on efflorescence could be assessed by comparing M4, M9 and M10. Interestingly, M9 and M10 with more (80%) and less (0%) fly ash had significantly lower amounts of efflorescence than M4 (40% fly ash). This was likely due to that GGBS increased the susceptibility to efflorescence, however also contributed to the pore refinement, which would limit the leaching and CO2 ingress [32]. With 60% GGBS, the pore was not highly refined to effectively prevent leaching, therefore M4 had higher efflorescence compared to M9 and M10. This was also seen between M5 and M13 with 9% Na2O. M13 with 80% fly ash still had a lower efflorescence than M5 with 40% fly ash.

Fig. 9. Comparison between M4 and M8 with different w/b ratios.

3.2.4. Microsilica, w/b ratio and binder content By comparing M4 and M6, M9 and M11, it could be seen that microsilica could reduce the efflorescence. The fine microsilica

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particles (d50 < 2 mm) acted as a micro-filler that reduced the movement of the free alkalis [31]. The effect of w/b ratio was assessed by comparing M4 and M8. M8 with a higher w/b ratio had more efflorescence at 28 and 90 days. This was attributed to the pore sizes and distribution in the matrix. The additional water created additional pores, which reduced the compressive strength by creating local stress concentrations and increased the permeability hence the leaching of pore alkalis. The effect of binder content was analysed by comparing M4 and M15. By maintaining a similar Na2O% while having an almost 30% higher b%, M15 had significantly more excess Na2O than M4. This resulted in higher efflorescence measurement at both 28 and 90 days. Another reason was the more severe shrinkage cracking due to the higher binder content [7]. This increased the permeability and promoted leaching. Overall, it was seen that the excess Na2O, calcium content and permeability were the major factors contributing to efflorescence. The difference between solid and liquid activator was small, therefore the known efflorescence control methods could be adopted for the one-part geopolymer and AAS mixes. The coarse metasilicate particles could be better preserved in the pores due to their low unit surface areas, however this came with the trade-off of low compressive strengths. 3.3. Alkalinity The alkalinity of the geopolymer matrix and the curing water was monitored at 3, 28 and 90 days. The results are reported in Table 3. In general, the two-part (M2) and one-part geopolymer mixes (M3–M29) with metasilicate had a decreasing trend throughout the curing, with the only exception of M5W, where the pH increased slightly after 28 days. This could be due to the variation of the testing conditions and slight calibration error of the testing apparatus. On the contrary, the pH of M1 increased between 3 and 28 days then decreased until 90 days. This could be due to the differences in the dissolution of the activator ions. For M1, there was no free alkalis from the beginning, therefore the pH relies on the generation of [OH] from the hydration of GGBS and from the release of [OH] when the monomers began to interlink [33]. After this, the gradual depolymerisation reaction between the [OH] and Ca2+ took place, reducing the alkalinity of the matrix. Additionally, the leaching and CO2 ingress constantly reduced the pore alkalinity since the demoulding at 1 day. This process also happened in the other mixes, however this was much lower than the amount of free alkalis from the activators. For M2, the full 6% Na2O was available, resulting in rapid hardening and a high pore alkalinity in the beginning. For the one-part mixes, a large number of metasilicate particles remained undissolved at 3 days due to the slow moisture ingress, which resulted in the high pH value of the matrix. As the metasilicate got dissolved, the free alkalis were consumed by the unreacted binder particles, reducing the matrix pH. It was also seen that the pH of the cube samples cured in water were generally lower than their ambient cured counterpart. The concentration gradient existed between the pores and the water, which caused the diffusion of alkalis into the curing water. This process was faster than the capillary action in the ambient cured samples, therefore the leaching was more severe in this curing condition. Fig. 10 shows the relationship between the pH of the ambient cured specimens and the compressive strength over time. The different trends between the two groups of specimens (M1; M2–M20) could be clearly seen. M1 showed an increase in pH then a drop after 28 days, while having an increase in the compressive strength over 90 days of curing. For the mixes with activators (M2–M20), the gradual decrease in the pH could be seen. In the meantime, the strength increased and the regression lines shifted left and

Fig. 10. The pH measurements of the mixes over time.

upwards. The gap between the 3-day line and the 28-day line was bigger than that between 28 and 90 days, demonstrating a slow-down in the hardening after 28 days. The conversion rate of the activator was quantified by comparing the total amount of [OH] in the raw materials and the matrix at different curing ages. The method was summarised in Eq. (1) [12].

Activ atorconv ersionrateð%Þ ¼

originaltotalalkalinity originaltotalalkalinity  residualtotalalkalinity

ð1Þ

As the chemical reactions involved were complex in nature, the following assumptions were made to simplify the calculations: (1) Each Na2O molecule corresponded to one [OH]. The Na2O available in the one-part metasilicate mixes was assumed to be the amount of dissolved metasilicate at the mixing stage. It was assumed that the mixing water reached saturation at the solubility specified by the manufacturer. The conventional method for liquid activators where the conversion rates accounted for the total amount of metasilicate was also provided. Both methods produced the same trends. However, the first method was more applicable to the early age, which was crucial for the hardening of the mix, therefore was used in the comparisons. (2) The pH of the sample is representative of the entire cube. Measures such as taking broken pieces from all depths for grinding and taking the sample from a relatively large pool of ground powder were adopted in the test. (3) The formation of [OH] during curing is ignored. This would not greatly affect the comparison between the mixes. The original total alkalinity was based on the amount of Na2O in the mix and the residual total alkalinity was obtained from the pH measurements. Two values were available for each mix at different curing ages, specifically, one for ambient cured specimens and another for water cured specimens. For the submerged specimens, the [OH] calculated from the two pH measurements (ground sample and water) was used. The results are shown in Table 3. It could be immediately observed that the conversion rate of the submerged cubes was generally lower than those cured in the ambient room. This could be due to that the rate of dissolution of CO2 was much higher for the cubes in contact with air. This resulted in the decrease in pore alkalinity and the production of efflorescence on the surface, which was not seen in the water cured specimens. The amount of CO2 ingress could not be easily

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Fig. 11. XRD analysis of (a) the raw materials, and (b) mortar samples. Q: quartz; S: silicon carbide; M: mullite; H: hatrurite; G: magnetite; C: gypsum; B: basanite; CSH/CC: C-S-H gels or calcium carbonate.

determined; however, it could be estimated by comparing the ambient- and water-cured specimens. The conversion rates of M1 decreased after 3 days then increased after 28 days whereas those of M2 to M20 kept increasing until 90 days. This distinction in behaviour at the early age for M1 was likely due to the gradual generation of [OH] from the calcium hydration and polymerisation reactions instead of relying on the free alkalis in the activator. The trends of the mixes with activators were consistent with those reported by Dong et al. [12]. The increase in the activator conversion rate of all one-part metasilicate mixes over 90 days demonstrated the constant dissolution of the remaining metasilicate particles into the pores. The delayed dissolution was beneficial for controlling the rate of reaction at the

mixing stage. The undissolved metasilicate particle also acted as a source of silica for the unreacted fly ash and GGBS particles, which continuously improved the pore structure. 3.4. Microanalysis 3.4.1. XRD analysis Fig. 11 shows the XRD spectra of the raw materials and the samples taken from various mixes between 2h = 10°–70°. From Fig. 11 (a), it could be seen that the precursors were mostly amorphous with small crystalline peaks, while the sodium metasilicate pentahydrate used in this study was highly crystalline. The fly ash mainly consisted of mullite and quartz, and gypsum was the main

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source of sulphur found in the GGBS. In Fig. 11(b), the spectra of 7 representative mixes were compared. The spectrum of M2 synthesised with the liquid activator was mostly amorphous, expect for the quartz peaks from fly ash and sand. The intensity of peaks found in the raw materials reduced significantly, which means that a moderate degree of reaction was achieved. On the contrary, the peaks from the raw materials of other six mixes with the solid activator were generally more noticeable. For example, the gypsum peak at 11° 2h and the mullite peaks at 26° 2h. This showed that metasilicate-based mixes had more unreacted binder particles. Additionally, small peaks throughout the range of the scan could be observed, resulting in a less smooth curve compared to that of M2. This was likely attributed to the undissolved metasilicate particles. Among these six mixes, the curve of M4 resembled that of M2 the most, meaning it has less unreacted particles. Therefore,

combined with the lower initial water content, M4 and M4W had the highest compressive strength of 104 MPa on average. The peak at 29.5° 2h normally corresponded to C-S-H gels and calcium carbonate [15]. In addition, several peaks from the metasilicate particles may also be at this angle. The intensity of the peaks of M5, M12, M14 and M20 at this angle was significantly higher than others. It was expected that the high metasilcate content in M5 and M14 as well as the coarse metasilicate grains in M12 and M20 led to a large quantity of undissolved metasilicate particles, which firstly had overlapping peaks with the C-S-H gels, and secondly absorbed CO2 from the atmosphere to form calcium carbonate with GGBS. The compressive strengths of those four mixes were all lower than that of M4 with either comparably finer or less metasilicate. Therefore, it was recommended to limit the amount of metasilicate to avoid wastage and finely grind the granules to improve dissolution.

Fig. 12. SEM images of various mixes cured in the ambient curing room and water.

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11

Fig. 12 (continued)

3.4.2. SEM analysis Fig. 12 shows the SEM images of various mixes cured in both ambient conditions and in water for 90 days. M4 was the strongest among all 20 mixes. Its microstructure was dense and virtually free of micro-cracking, however cracks were found at the interface transition zone (ITZ), which likely caused the slight reduction in strength at 90 days. A large number of fly ash and GGBS particles remained unreacted. The cubes cured in water also had a dense microstructure and no noticeable microcracks. The unreacted particles had mostly transformed into partially developed matrix, which caused its continued increase in strength. M5 as compared with M4 had a large amount of Na2O, which resulted in the formation of efflorescence on the surface and within the pores. The M5W specimens were free of efflorescence due to the absence of CO2, however the high water and activator content resulted in a rough microstructure and shrinkage cracking. With the addition of

microsilica, the matrix of both M6 and M6W appeared dense and smooth. In some regions of the submerged specimens, the gel was not fully developed, which was likely due to the loss of alkalinity from the leaching. M7 was similar to M4, except for having the coarse metasilicate as the activator. Regions in the matrix was filled with partially developed C-A-S-H gels, therefore appeared less smooth than M4. This was likely due to the variable availability of silica in the matrix due to the uneven distribution of the metasilicate grains. The region away from any metasilicate particles was likely to have reduced growth rate of the gels. Its greater particle size meant that for a given volume, the number of coarse particles was lower than the fine particles, thus the interparticular distance was greater. This, combined with the larger voids created by the coarser grains, had an adverse effect on the compressive strength. As seen from the SEM image of M7W, the gel structure appeared slightly smoother. This was likely attributed

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to the ingress of water. The water assisted in the distribution of silica through pores, improving the overall microstructure. Therefore, a small improvement in the compressive strength was observed in M7W over M7. M12 was similar to M7 but with 80% fly ash instead of 40%. The figure shown demonstrated the gradual dissolution of the metasilicate core left after the mixing stage. The reaction product formed a shell around the metasilicate particle, and a huge gap was found in between. Each undissolved metasilicate particle was essentially equivalent to an air void, hence an excessive amount would critically reduce the compressive strength. M12W had a rough microstructure filled with unreacted fly ash particles. The matrix could not fully cover the unreacted particles, resulting in a less dense structure. Therefore, the compressive strength was very low at less than 20 MPa at 90 days. M15 had 45% binder instead of 35%. The microstructure suffered significantly from shrinkage cracking, reducing the compressive strength. As a result, M15 had a 6% lower compressive strength than M4 despite of the large binder content. M15W also had a great extent of shrinkagecracking, despite of being submerged in water. The cracks appeared finer, however a large number of unreacted particles was observed. This was likely due to the suppression of the sodium-based gels when cured in water [27]. M16 and M16W had a very rough microstructure. However, the bonding between the unreacted fly ash particles and the matrix was improved. The matrix in some areas was not fully developed despite of the high activator content. This could be attributed to the uneven distribution of the alkalis due to the coarse metasilicate particles. In both M20 and M20W with 100% GGBS and 3% Na2O, a large amount of ettringite was observed. In M20, it was most commonly found in the voids left by air, metasilicate grains or unreacted GGBS particles. However, clusters of ettringite were found throughout the mortar matrix. The appearance of ettringite was attributed to the high sulphate content in the GGBS. The low activator content was not enough to sufficiently promote the formation of C-S-H and geopolymeric gels over ettringite. In addition, the matrix of M20W was less developed, whereas M20 had a relatively smooth and ettringite-free matrix. Therefore, the compressive strength of M20W was more affected after 28 days.

in the reactions, but rather increased the absorption of CO2 from the atmosphere. The metasilicate particles, especially the asreceived coarse granules, left undissolved at the mixing stage created air voids, which reduced the compressive strength of the matrix. It was expected that, as the fineness further decreased, the metasilicate could promote the delayed polymerisation reaction of the unreacted binder without creating damaging air voids. Future studies may be carried out to quantify the porosity in one-part geopolymer composites over the curing period. 3. The addition of microsilica increased the reactivity of GGBS but reduced that of fly ash. However, due to its filler effect as shown from the dense microstructure in the SEM images, it greatly improved the efflorescence resistance. Higher fly ash content resulted in a reduction in compressive strength due to its lower hydraulic activity compared with GGBS. However, higher GGBS content increased water demand and thus decreased the compressive strength. 4. Despite that more metasilicate particles could be dissolved at a higher water to binder ratio and participate in the reactions, the compressive strength still noticeably decreased. By having 45% binder instead of 35%, severe shrinkage cracking was observed in the SEM analysis, greatly reducing the compressive strength. 5. The specimens cured in water generally had a comparable to lower compressive strength to their ambient cured counterparts. This was shown from their lower pH measurements due to the forced leaching through diffusion. However, the submerged specimens were free of efflorescence due to the lack of atmospheric CO2 and had reduced shrinkage cracking as shown from the SEM analysis. Overall, the resulted highlighted that the solid sodium metasilicate pentahydrate could be used to synthesise high strength geopolymer composites. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

4. Conclusions Acknowledgements This paper experimentally studied 18 one-part geopolymer and AAS mixes with sodium metasilicate pentahydrates and compared them against two control mixes using water and liquid activators. The following conclusions could be drawn: 1. The solid activator using sodium silicate pentahydrate outperformed the often-used liquid activator in terms of compressive strength of the mortar. This was attributed to that a part of the water was chemically bound to the undissolved metasilicate particles at the mixing stage, lowering the water to binder ratio thus improving the compressive strengths. As a result, the efflorescence was slightly lower than the comparable two-part mix. The undissolved metasilicate particles were prone to leaching, although the leaching rate was impeded due to the denser matrix, as seen by the comparable pH of the curing water. The activator conversion rate continued to increase until 90 days, demonstrating the continuous dissolution of the remaining metasilicate particles. 2. The compressive strength decreased and efflorescence increased significantly once the metasilicate content past Na2O% = 6%. As seen from the XRD analysis, larger calcium carbonate peaks were found for mixes with 9% and 12% Na2O at 29.5° 2h, indicating that the extra activator did not participate

This research was supported by an Australian Government Research Training Program (RTP) Scholarship. The authors acknowledge the facilities, and the scientific and technical assistance of Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Thanks are also given to former students Mr Christopher Unsworth, Mr Wenjie Zhao and the laboratory technicians Mr Jim Waters, Mr Brad Rose and Mr Matt Arpin for the help with the many practical aspects of this project. Thanks are given to the generous support from Sika in Western Australia. References [1] H. Djwantoro, Steenie E. Wallah, M.J. Dody Sumajouw, B. Vijaya Rangan, On the development of fly ash-based geopolymer concrete, Aci. Mater. J. (2004) 467– 471. [2] S.A. Bernal, J.L. Provis, B. Walkley, R. San Nicolas, J.D. Gehman, D.G. Brice, et al., Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation, Cem. Concr. Res. 53 (2013) 127–144, https://doi.org/10.1016/j.cemconres.2013.06.007. [3] I. Ismail, S.A. Bernal, J.L. Provis, R. San Nicolas, D.G. Brice, A.R. Kilcullen, et al., Influence of fly ash on the water and chloride permeability of alkali-activated

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