1. bookVolume 39 (2021): Issue 4 (December 2021)
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2083-134X
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16 Apr 2011
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access type Open Access

Delayed setting time for alkali-activated slag composites using activator containing SiO2 and Na2O

Published Online: 26 Apr 2022
Volume & Issue: Volume 39 (2021) - Issue 4 (December 2021)
Page range: 570 - 579
Received: 07 Jan 2022
Accepted: 06 Mar 2022
Journal Details
License
Format
Journal
eISSN
2083-134X
First Published
16 Apr 2011
Publication timeframe
4 times per year
Languages
English
Abstract

This study examines the influence of SiO2 and Na2O mix proportions on the properties of alkali-activated slag (AAS) pastes. In order to solve the rapid setting problem associated with AAS, phosphoric or silicic acid is commonly added to the alkaline activator. The retarding effect of these additions suggests a close relationship between pH and setting time. In this study, AAS pastes with various SiO2 and Na2O dosages were analyzed. Regression analysis was carried out using pH value and setting time to identify the main parameters affecting the setting time. Results showed that the alkali modulus (i.e., the weight ratio of SiO2 over Na2O) is correlated with the pH value. Specifically, the initial and final setting times were significantly reduced under lower SiO2 and Na2O dosages. Therefore, a higher alkali modulus prolonged the setting time.

Keywords

Introduction

The first step in reducing global warming is to reduce CO2 emissions. Green concrete makes use of industrial by-products to reduce the amount of aggregate as well as CO2 emissions attributable to cement production [1]. Alkali-activated slag (AAS) is an innovative development in cement production aimed at achieving this goal [2]. AAS is a novel binder based entirely on slag with an activator [3], which leads to the production of a three-dimensional network of aluminosilicate materials [4], providing excellent mechanical properties [5] and durability [6]. In the reactive components of most AA binders, the availability of calcium for the production of a zeolite-like network structure is very low. Low-calcium fly ash and metakaolins are the most popular raw materials in the synthesis of AA binders [7]. Furthermore, the raw materials can be used without thermal treatment or calcination, which means that the energy requirements for manufacturing are far lower than those of Portland cement. In many cases, AAS can be used as a replacement for Portland cement to reduce CO2 emissions [8].

Two extensive reviews have discussed the widespread application of AAS in Eastern Europe [9], Scandinavia, and China [10]. AAS composites (AASC) materials have recently been shown to outperform ordinary concrete based on Portland cement in terms of the former's low hydration heat [10], high early strength (>60 MPa) [11], and excellent durability in aggressive environments [12].

Nonetheless, one previous study has reported that this binder system is subject to a prohibitively short setting time (as short as 10 min) [13], under the accelerating effects of highly alkaline activators. AAS-based mixtures are also prone to cracking and premature failure as a result of excessive shrinkage under the effects of moisture loss [14], which can occur internally via hydration (autogenous shrinkage) [15] or externally via evaporation (drying shrinkage) [16]. The addition of suitable chemical additives [17] and alkaline activators to AAS-based mixtures has been reported to help improve the drying, setting time, and mechanical properties [18]. It has been suggested that increasing the concentration of Na2O can shorten the final setting time of AASC [19]. Shi et al. [20] have shown that the alkali dosage and silicate modulus affected the alkali-silica reaction of AAS mortars, resulting in volumetric instability. AAS mortar also demonstrates excellent resistance to sulfate attack [21].

The strength development of AASC depends on the type and concentration of the activator [22]. Sodium silicate-based activators (i.e., sodium silicate plus sodium hydroxide) provide the best strength development performance; however, they are also associated with rapid setting times. This necessitates a stable high-pH environment [23] to prolong the reaction between raw material [24] and activator [25]. Phosphate [26] and malic acid [27] are often used to create this high-pH environment.

However, previous studies have found that pH may be one of the factors [10] affecting the setting time of AAS [27]. This study found that pH may not be the main factor affecting the setting time of AAS and that the amount and proportion of mixed alkaline activators have a significant effect on the setting time. Therefore, it has been deemed worthwhile to investigate the effect of SiO2 and Na2O dosages on pH value and AAS setting time. This study investigated the alkali modulus (SiO2/Na2O) in binary alkaline activators containing sodium hydroxide and sodium silicate, as well as its effect on pH value, setting time, and microstructure, for use as a reference in developing AAS-based mix designs.

Experimental
Alkaline activator preparation

Alkaline activators are made up of water, sodium silicate, and sodium hydroxide. The content of SiO2 and Na2O in a given alkaline activator mix design depends on the volume of alkaline activator. To determine the SiO2: Na2O ratio, the sodium silicate solution was first weighed to determine the weight of Na2O and H2O. These values were then used to calculate the amount of NaOH and Na2O required to equal the Na2O content in the alkaline activator mix. Finally, the weight of water (distillate water) consumed in the preparation of alkaline activator is equal to the weight of total water minus the weight of water in the sodium silicate solution. In cases where phosphoric acid is added, we deduct the weight of phosphoric acid from the weight of water consumed in the preparation of the alkaline activator.

The process of mixing water, sodium silicate, sodium hydroxide, and phosphoric acid must be implemented in a systematic manner. This process begins with the mixing of sodium silicate with water, to which is added sodium hydroxide under stirring until it dissolves entirely (including sediment). A reaction between sodium silicate and sodium hydroxide increases the temperature of the resulting solution. In the event that phosphoric acid is added to the solution, it is important to maintain stirring until all of the sediment (i.e., sodium ions and phosphoric acid) disappears. When the alkaline activator reaches room temperature, it is ready for use in the production of AAS.

Materials, mix design, and test methods

Solid sodium hydroxide (in the form of pellets), liquid sodium silicate, and pure water were used in the preparation of alkaline activators with various SiO2 and Na2O ratios. The slag in the current study was ground granulated blast-furnace slag, the composition of which is listed in Table 1. The fineness of slag was 4,000 cm2/g, and the grain size distribution of the slag is summarized in Table 2. The sodium hydroxide used in the experiments was of reagent grade, the chemical composition of which is shown in Table 3. The sodium silicate was extrapure reagent grade, the chemical composition of which is provided in Table 4.

Chemical and physical properties of slag

Main chemical composition of slag (by weight %) SiO2 33.87
Al2O3 14.42
Fe2O3 0.69
CaO 39.54
MgO 5.35
SO3 2.47

Basicity coefficient Kb (CaO+MgO)/(SiO2+Al2O3) 0.93

Physical properties Specific weight 2.90
Ignition loss (%) 0.28

Note: Results of the author's analysis using X-ray fluorescence.

Grain size distribution of the slag

Sieve size No. 50 No. 100 No. 200 No. 325 Bottom
Cumulative retained percentage (%) 0.20 0.40 32.50 77.23 100.00

Chemical composition of sodium hydroxide used in experiments

Chemical composition Content (wt. %, maximum)
Chloride (Cl) 0.005
Sulfate (SO4) 0.003
Silicate (SiO2) 0.01
Phosphate (PO4) 0.001
Heavy metal (As, Pb) 0.001
Iron (Fe) 0.0007
Aluminum (Al) 0.003
Calcium (Ca) 0.001
Magnesium (Mg) 0.0005
Potassium (K) 0.1
Total nitrogen (N) 0.001
Arsenic (As) 0.0002
Sodium carbonate (Na2CO3) 2.0
Assay (NaOH) 95.0

Note: Results were obtained from the supplier.

Chemical composition of sodium silicate used in the experiments

Items Results
Water-insolubles (%) 0.01 (max)
SiO2 (%) 37.0
Na2O (%) 17.7
Mole ratio 2.16
Fe (%) 0.02 (max)

Note: Results were obtained from the supplier.

In all AASC designs in this study, the liquid/binder ratio (i.e., the ratio of alkaline activator to slag by weight) was fixed at 0.5. The mix designs used in this study are listed in Table 5. In each series of experiments, the pH of the powder samples ground from the concrete specimens was measured in accordance with the specifications of the American Society for Testing and Materials (ASTM)-E70. Initial and final setting times were established using the Vicat test in accordance with ASTM C191 specifications. Scanning electron microscopy (SEM) analysis was used to characterize representative samples measuring 1 mm × 1 mm × 1 mm, in accordance with ASTM C1723 specifications. The specimen was mixed and placed in a steel mold and left at room temperature for 24 h before unmolding. The AASC specimens were then cured in saturated lime water for 3 days and subjected to SEM. The specimens for SEM were cleaned and placed in an oven (105°C, 24 h), dried, and then removed to room temperature (approx. 5 h); next, they were placed in a vapor-plating apparatus for gold plating. The specimens were then vacuum-dried (120 s) to complete the preparation process.

Mix designs

Activator (g/l) Weight %

SiO2 Na2O NaOH Na2SiO3 H2O Slag
60 60 0.013 0.054 0.266 0.667
60 80 0.022 0.054 0.257 0.667
60 100 0.031 0.054 0.249 0.667
60 120 0.039 0.054 0.240 0.667
80 60 0.009 0.072 0.252 0.667
80 80 0.018 0.072 0.243 0.667
80 100 0.027 0.072 0.235 0.667
80 120 0.035 0.072 0.226 0.667
100 60 0.005 0.090 0.238 0.667
100 80 0.014 0.090 0.229 0.667
100 100 0.022 0.090 0.221 0.667
100 120 0.031 0.090 0.212 0.667
120 60 0.001 0.108 0.224 0.667
120 80 0.010 0.108 0.216 0.667
120 100 0.018 0.108 0.207 0.667
120 120 0.027 0.108 0.198 0.667
Results and discussion
Properties of alkali-activated liquid

NaOH, Na2SiO3, and H2O were combined in various proportions to form the various activators. Note that using aqueous NaOH alone would likely result in excessively rapid setting. Hence, sodium silicate was added to the aqueous NaOH solution to retard this effect. Note that the addition of sodium silicate to highly alkaline NaOH lowered the pH. The silicate-based activator is a mixture of sodium hydroxide and sodium silicate; therefore, the production of SiO2 can be traced back to Na2SiO3 (aq.), whereas the production of Na2O can be attributed mainly to NaOH (s.) with a small amount derived from Na2SiO3 (aq.). As shown in Figure 1, increasing the concentration of SiO2 with a fixed concentration of Na2O reduced the pH value of the activator. Increasing the concentration of Na2O with a fixed concentration of SiO2 increased the pH of the activator. This can be attributed to the fact that a higher concentration of SiO2 resulted in the production of larger quantities of SiO32 {\rm{SiO}}_3^{2 - } capable of reacting with H+ ions from the activator. Conversely, a higher concentration of Na2O made available a larger number of Na+ ions to react with OH ions from the activator.

Fig. 1

pH value of activator as a function of SiO2 concentration under various Na2O concentrations

The composition of alkaline activators can be characterized in terms of alkali modulus and total alkali content. Alkali modulus refers to the weight per unit volume of SiO2 divided by the weight per unit volume of Na2O (SiO2/Na2O). Total alkali content refers to the total weight per unit volume of SiO2 plus the total weight per unit volume of Na2O (SiO2+Na2O). As shown in Figure 2, increasing the dosage of SiO2+Na2O produced a corresponding increase in the pH value when the alkali modulus (SiO2/Na2O) was fixed at 1.0. Increasing the dosage of SiO2+Na2O increased the availability of both SiO2 and Na2O; however, the quantity of OH (from Na2O) exceeded the quantity of H+ (from SiO2), which caused the increase in pH. As shown in Figure 3, when the dosage of alkaline activator was fixed at 160 g/l or 200 g/l, the higher pH values were due to a lower relative proportion of SiO2 or higher proportion of Na2O. In other words, with the total quantity of SiO2 and Na2O fixed, the higher pH values can be attributed to a lower alkali modulus.

Fig. 2

pH values of activators with fixed alkali modulus of SiO2/Na2O = 1

Fig. 3

pH values of activators with fixed total alkali content of SiO2+Na2O = 160 g/l or 200 g/l

As shown in Figure 4, increasing the alkali modulus led to a corresponding decrease in the pH value of the activator, i.e., pH values varied only with a change in the alkali modulus. Figure 5 illustrates the relationship between pH and SiO2+Na2O. The analysis of variations in pH under a fixed SiO2+Na2O dosage indicates that the SiO2+Na2O dosage had no effect on pH. The alkali modulus (SiO2/Na2O) can be used to indicate an increase in SiO2 content, resulting in a lower pH. This means that adding sodium silicate to a silicate-based activator (to retard the setting process) increases the alkali modulus (SiO2/Na2O) to above that of aqueous NaOH solution, while reducing the pH to below that of aqueous NaOH. Zhang's [28] study indicated that excessive SiO2 content could hamper the alkali activation reaction and the excessive SiO2 content decreases the overall pH, leading to a slower rate of reaction. The conclusion of the cited study is consistent with that of our study.

Fig. 4

Relationship between SiO2/Na2O and pH value of activator

Fig. 5

Relationship between SiO2+Na2O and the pH value of activator

Setting time of AASC

The effects of SiO2 and Na2O on the setting time were examined by varying their respective concentrations. As shown in Figure 6, increasing the concentration of SiO2 under a fixed Na2O concentration reduced the setting time. It was due to the rapid production of the initial calcium silicate hydrate (C–S–H) gel as a result of the bonding of the calcium ions present in the slag to the silicate ions in the pore solution, which can harden the AASC paste. It is also consistent with the conclusions of the studies by Chang [29] study and Tong et al's [30]. At SiO2 concentrations of 100 g/l or 120 g/l, increasing the concentration of Na2O decreased the initial and final setting times. However; the effect was insignificant at other SiO2 concentrations.

Fig. 6

Setting time (Na2O = 60–120 g/l): (A) initial time; (B) final time

Previous studies have demonstrated a correlation between the setting time of AAS pastes [29] and the concentration of SiO2 and Na2O in the alkaline activator [30, 31]. It was then sought to determine whether the observed effect was the result of decreases in pH. Therefore, experiments were performed comparing the influence of alkali modulus (SiO2/Na2O) and total alkali content (SiO2+Na2O) on the setting time. Under a constant alkali modulus, increasing the SiO2+Na2O dosage decreased the setting time (see Figure 7), while increasing the content of both sodium silicate and NaOH. It was observed that higher reactant concentrations produced higher reaction rates. Increasing the content of aqueous silica led to a higher SiO32 {\rm{SiO}}_3^{2 - } concentration, with a corresponding increase in the reaction rate. Increasing the NaOH concentration was also shown to increase the reaction rate, due to an increase in the [OH] concentration, which facilitated the dissolution of Ca2+ from the slag.

Fig. 7

Setting time under SiO2/Na2O = 1: (A) initial time; (B) final time

As shown in Figure 8, increasing the concentration of SiO2 decreased the setting time, regardless of the SiO2+Na2O dosage. In other words, a higher alkali modulus (indicating a larger quantity of Na2O dissolved from the sodium silicate) could be used as an index indicating a shorter setting time. The results confirmed that NaOH facilitated the dissolution of Ca2+ from slag particles; however, the content of aqueous silica also played a key role in the early hydration of AAS. From the results, it can be seen that the alkali content of SiO2 had a significantly higher effect on the setting time than Na2O. For a fixed total alkali content (SiO2+Na2O), the higher the SiO2 content, the shorter was the setting time, which is in line with previous studies [29].

Fig. 8

Setting times under SiO2+Na2O = 160 g/l or 200 g/l: (A) initial time; (B) final time

Regression analysis was used to compare the effects of alkali modulus (SiO2/Na2O) and total alkali content (SiO2+Na2O) on the setting time. As shown in Figure 9, there was a relationship between alkali modulus and setting time; however, a clear link was observed between alkali modulus and pH. From this, we can conclude that pH did not affect the initial or final setting time.

Fig. 9

Relationship between setting time and SiO2/Na2O: (A) initial time; (B) final time

As shown in Figure 10, it was confirmed that SiO2+Na2O dosage (i.e., the total concentrations of SiO2 and Na2O) had a pronounced effect on setting time, in a dose-dependent manner. This suggests that retardation of setting time cannot necessarily be attributed to a reduction in pH. The tests revealed that at SiO2 = 60 g/l, the setting time retardation effects became evident under a high alkali modulus and became far more pronounced under a low alkali modulus. The effect of total alkali content on the setting time presented a clearer correlation.

Fig. 10

Relationship between setting time and SiO2+Na2O: (A) initial time; (B) final time

SEM analysis

SEM was used to examine the effects of SiO2+Na2O dosage on the microstructure of AASC specimens. As shown in Figure 11, two specimens of the same alkali modulus presented a fine pore structure. Although the difference in pH values between the two specimens was not significant, the difference in SiO2+Na2O dosage resulted in different microstructures. The differences in microstructure were mainly influenced by the setting time. SEM images obtained at 3 days revealed a large number of microfractures on the surface of the samples prepared using a paste containing 120 g/l of SiO2 and 120 g/l of Na2O. This can be attributed to the production of AAS amorphous reaction products under the effects of rapid hydration and condensation. These results support those of previous studies by Collins and Sanjayan [32], Yang et al. [33], and Opiso et al. [34]. By comparison, specimens prepared using paste containing 80 g/l of SiO2 and 80 g/l of Na2O revealed totally and partially hydrated slag grains (due to a slow hydration reaction), as well as fewer microcracks. The main reaction products were fibrous structures corresponding to C–S–H gel, dense gel phase, partially reacted cenosphere, and a fully reacted matrix. A small quantity of calcium aluminosilicate hydrate gel (C–A–S–H gel) was also observed.

Fig. 11

SEM photos (1,000× magnification): (A) SiO2 = 120 g/l and Na2O = 120 g/l for 3 days; (B) SiO2 = 80 g/l and Na2O = 80 g/l for 3 days. SEM, scanning electron microscopy

The partially hydrated particles would continue to increase the strength of the paste as it was cured over time. Under the activator composition of SiO2/Na2O = 1, the pastes set rapidly, but no visible cracks were observed on the microstructure surface. The faster setting time resulted in more partially hydrated slag grains on the surface of the specimens. The primary benefit of slowing the setting process through the addition of silicate-based activator is enhanced workability and the production of a complex microstructure with fewer cracks and more stable strength development. The results are consistent with previous studies by Atiş et al. [35] and Mastali et al. [36]. AAS has a more compact and dense crystal structure than the structure of C–S–H, formed during the hydration of ordinary Portland cement-based composites proposed by Alharbi et al. [37] and Paradiso et al. [38].

Conclusion

This study performed a systematic analysis of AAS paste and the effects of adding silicate-based activator. The results confirmed that the effect of silicic acid on setting time could be attributed to a decrease in the pH of the alkaline activator. Nonetheless, SiO2+Na2O dosage had a more pronounced effect than the pH value. The results confirmed that the pH of the activator could be reduced by increasing the concentration of SiO2 or Na2O; however, the main factor affecting pH was actually the alkali modulus. Under a fixed liquid/slag ratio, the SiO2+Na2O dosage and alkali modulus were both shown to affect the setting time. Under similar pH conditions, the pH value for SiO2 = 120 g/l and Na2O = 120 g/l was 13.7 and the final setting time was 35 min; the pH value for SiO2 = 80 g/l and Na2O = 80 g/l was 13.6 and the final setting time was 122 min. This showed that SiO2+Na2O dosage was strongly correlated with setting time, whereas the alkali modulus was not.

Fig. 1

pH value of activator as a function of SiO2 concentration under various Na2O concentrations
pH value of activator as a function of SiO2 concentration under various Na2O concentrations

Fig. 2

pH values of activators with fixed alkali modulus of SiO2/Na2O = 1
pH values of activators with fixed alkali modulus of SiO2/Na2O = 1

Fig. 3

pH values of activators with fixed total alkali content of SiO2+Na2O = 160 g/l or 200 g/l
pH values of activators with fixed total alkali content of SiO2+Na2O = 160 g/l or 200 g/l

Fig. 4

Relationship between SiO2/Na2O and pH value of activator
Relationship between SiO2/Na2O and pH value of activator

Fig. 5

Relationship between SiO2+Na2O and the pH value of activator
Relationship between SiO2+Na2O and the pH value of activator

Fig. 6

Setting time (Na2O = 60–120 g/l): (A) initial time; (B) final time
Setting time (Na2O = 60–120 g/l): (A) initial time; (B) final time

Fig. 7

Setting time under SiO2/Na2O = 1: (A) initial time; (B) final time
Setting time under SiO2/Na2O = 1: (A) initial time; (B) final time

Fig. 8

Setting times under SiO2+Na2O = 160 g/l or 200 g/l: (A) initial time; (B) final time
Setting times under SiO2+Na2O = 160 g/l or 200 g/l: (A) initial time; (B) final time

Fig. 9

Relationship between setting time and SiO2/Na2O: (A) initial time; (B) final time
Relationship between setting time and SiO2/Na2O: (A) initial time; (B) final time

Fig. 10

Relationship between setting time and SiO2+Na2O: (A) initial time; (B) final time
Relationship between setting time and SiO2+Na2O: (A) initial time; (B) final time

Fig. 11

SEM photos (1,000× magnification): (A) SiO2 = 120 g/l and Na2O = 120 g/l for 3 days; (B) SiO2 = 80 g/l and Na2O = 80 g/l for 3 days. SEM, scanning electron microscopy
SEM photos (1,000× magnification): (A) SiO2 = 120 g/l and Na2O = 120 g/l for 3 days; (B) SiO2 = 80 g/l and Na2O = 80 g/l for 3 days. SEM, scanning electron microscopy

Mix designs

Activator (g/l) Weight %

SiO2 Na2O NaOH Na2SiO3 H2O Slag
60 60 0.013 0.054 0.266 0.667
60 80 0.022 0.054 0.257 0.667
60 100 0.031 0.054 0.249 0.667
60 120 0.039 0.054 0.240 0.667
80 60 0.009 0.072 0.252 0.667
80 80 0.018 0.072 0.243 0.667
80 100 0.027 0.072 0.235 0.667
80 120 0.035 0.072 0.226 0.667
100 60 0.005 0.090 0.238 0.667
100 80 0.014 0.090 0.229 0.667
100 100 0.022 0.090 0.221 0.667
100 120 0.031 0.090 0.212 0.667
120 60 0.001 0.108 0.224 0.667
120 80 0.010 0.108 0.216 0.667
120 100 0.018 0.108 0.207 0.667
120 120 0.027 0.108 0.198 0.667

Grain size distribution of the slag

Sieve size No. 50 No. 100 No. 200 No. 325 Bottom
Cumulative retained percentage (%) 0.20 0.40 32.50 77.23 100.00

Chemical composition of sodium silicate used in the experiments

Items Results
Water-insolubles (%) 0.01 (max)
SiO2 (%) 37.0
Na2O (%) 17.7
Mole ratio 2.16
Fe (%) 0.02 (max)

Chemical composition of sodium hydroxide used in experiments

Chemical composition Content (wt. %, maximum)
Chloride (Cl) 0.005
Sulfate (SO4) 0.003
Silicate (SiO2) 0.01
Phosphate (PO4) 0.001
Heavy metal (As, Pb) 0.001
Iron (Fe) 0.0007
Aluminum (Al) 0.003
Calcium (Ca) 0.001
Magnesium (Mg) 0.0005
Potassium (K) 0.1
Total nitrogen (N) 0.001
Arsenic (As) 0.0002
Sodium carbonate (Na2CO3) 2.0
Assay (NaOH) 95.0

Chemical and physical properties of slag

Main chemical composition of slag (by weight %) SiO2 33.87
Al2O3 14.42
Fe2O3 0.69
CaO 39.54
MgO 5.35
SO3 2.47

Basicity coefficient Kb (CaO+MgO)/(SiO2+Al2O3) 0.93

Physical properties Specific weight 2.90
Ignition loss (%) 0.28

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