# Effect of Dosage of Alkaline Activator on the Properties of Alkali-Activated Slag Pastes

Authors: Zhenzhen Jiao; Ying Wang; Wenzhong Zheng; Wenxuan Huang
Journal: Advances in Materials Science and Engineering
 (2018)

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## Abstract

This study focused on the engineering properties of alkali-activated slag (AAS) pastes prepared by mixing an activator consisting of sodium silicate and sodium hydroxide at room temperature. The water-to-slag ratio of AAS paste was kept constant at 0.35 by mass. AAS pastes were prepared using the activator with five different silicate moduli of 1, 1.2, 1.4, 1.6, and 1.8 and three different Na2O contents of 6%, 8%, and 10%. The results showed that both the silicate moduli and Na2O contents had significant effects on the engineering properties of AAS pastes. All the AAS pastes exhibited properties such as fast setting, good workability, and high early compressive strength. The final setting time varied from 9 to 36 min, and the fluidity was in the range of 147–226 mm. The 1 d compressive strength of all the AAS pastes, which could be easily achieved, had values above 55 MPa, whereas the highest strength obtained was 102 MPa with the silicate modulus of 1 and Na2O content of 8% at room temperature. The drying shrinkage increased as the silicate modulus increased. Furthermore, the hydration products and microstructures of AAS pastes were explained according to the microanalysis methods.


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## Body

## 1. Introduction

Due to the fast development of the construction industry all over the world, the demand for building materials has grown strong, especially in the developing countries. It is estimated that 85% of the total CO2 emissions originate from the cement industry [1]. Therefore, as the demand and subsequent production of cement increase, the CO2 emission increases, which can cause the greenhouse effect [2]. Blast furnace slag is an industrial by-product, produced during the process of making pig iron, and consists of calcium-rich and silicon-rich glass having latent hydraulic activity [3, 4]. In addition, large amounts of slag are generated worldwide. For example, China produces a total of more than 2.4 million tonnes of slag annually, out of which, only a small portion (20–30%) can be used efficiently, whereas a large portion (70–80%) is deemed as the solid industrial waste [5]. Thus, in order to protect the environment and save resources, new environment friendly building materials must be explored. AAS was prepared by an alkaline activator and aluminosilicate materials. AAS cements are environment friendly cementitious materials, through which the industrial waste can be recycled in an effective way. In recent years, AAS cements have received significantly more attention as potential sustainable alternatives to Portland cement [6], which can convert industrial waste into green building materials.Alkali-activated cementitious materials are greatly affected by the chemical composition of the alkaline activator [7–9]. In this regard, factors such as type and contents of the activator [6, 8, 10–12], curing conditions [13–16], and mineral admixtures [17–19] in the alkali activation process were considered important. Živica [20] reported that AAS used by the NaOH solution, Na2CO3 solution, and water glass of three different types of activators showed an important acceleration for setting as compared to the ordinary Portland cement, and the setting time of slag activated by sodium silicate was the shortest. Jiao et al. [21] reported that the addition of Na2CO3 in the NaOH solution can prolong the setting time, increase the later compressive strength, and reduce the drying shrinkage of AAS paste compared with the alkaline activator of only NaOH. Chang [22] studied the effects of concentrations of SiO2 and Na2O on the setting time and concluded that phosphoric acid can be used as an effective retarder. Wu et al. [23] reported that the activators with higher pH values can efficiently enhance the hydration process of AAS paste with more silicon and aluminum ions dissolving due to the weaker strengths of Si-O and Al-O bonds in the alkaline environment. Ravikumar and Neithalath [24] reported the effect of alkalinity of an alkali activator on the mechanical performance and microstructures of AAS materials. The results suggested that a higher alkalinity was helpful in improving the compressive strength, whereas the major hydration product was calcium silicate hydrate. Ahmari et al. [25] studied the development of compressive strength of alkali-activated binder based on copper mine tailings at 60–120°C of curing temperature. They showed that the strength first increased with the increase in initial temperature and then decreased when the temperature crossed a certain value. Lee and Lee [26] studied the reaction mechanism of a material produced by blending together sodium silicate and sodium hydroxide, which showed that a higher amount of slag resulted in a higher quantity of C-S-H gel and less aluminosilicate gel, which were produced as the hydration products. Many researchers [27–29] reported that the Ca/Si ratio of calcium silicate hydrate in AAS was lower than that in ordinary Portland cement.It is well known that AAS has excellent mechanical performance, good durability, and low porosity as compared to Portland cement. Gebregziabiher et al. [30] reported that the compressive strength of AAS with sodium silicate was 25 and 70 MPa at 1 and 28 days, respectively, and that there was almost no increase thereafter until 56 days. Chi [31] showed that the resistance of AAS to sulfate attack was better than that of Portland cement, as shown by a lower weight loss and reduction in compressive strength of AAS concrete. Shi [32] found that the porosity of AAS was lower than that of the Portland cement, whereas the pore structure was also finer. However, there are also some disadvantages of rapid setting and high drying shrinkage of AAS pastes. Gu et al. [33] reported that the setting time of AAS was about 50 min at room temperature and tried to prolong the setting time by lowering the temperature. They also concluded that the drying shrinkage of AAS was 2-3 times that of the ordinary Portland cement.Although many researchers focused on the properties of AAS, there is still a significant need for further investigations. The fast setting time and high early compressive strength of AAS were used for rapid repair construction and strengthening of concrete structures. The drying shrinkage is an important aspect for the durability of AAS pastes. In order to be better used for the construction, the more appropriate setting time, higher early compressive strength, and less drying shrinkage of AAS were needed to be studied. The composition of slag had a key influence on the properties and reaction mechanism of AAS [10]. The main alkaline activator was the mixture of sodium silicate and sodium hydroxide, which was controlled by the silicate modulus and Na2O content. Therefore, it is essential to investigate the effects of an optimum activator on the engineering properties of AAS pastes. The objective of this paper is to investigate the effects of silicate moduli and Na2O contents on fresh properties (setting time and fluidity) and hardened performance (compressive strength, drying shrinkage, and mass loss) of AAS pastes. The reaction mechanism of AAS was revealed using techniques of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and mercury intrusion porosimetry (MIP).


## 2. Materials and Methods

### 2.1. Materials

Blast furnace slag was procured from the Tangshan steel plant, China, and its chemical composition is presented in Table1. The specific surface area of slag was found to be 424 m2/kg, whereas its specific gravity was determined to be 2.91 [21]. The specific surface area was tested by an electric Brinell permeability surface area analyzer (DBT-127; Wuxi Jianyi Instrument and Machinery Co. Ltd., China), according to the Chinese standard GB/T 8074-2008 [34]. The same sodium hydroxide was used in the previous study [21]. Sodium silicate solution consisting of 10.3% Na2O, 32.1% SiO2, and 57.6% H2O was supplied by Julide Chemical Co. Ltd., Langfang, China. The alkaline activator of AAS pastes was prepared using sodium silicate and NaOH solutions.Table 1
Chemical composition of slag.
Oxide
SiO2
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
Others
Percentage (%)
32.83
17.19
36.69
0.38
0.37
8.20
0.65
1.94
1.75


### 2.2. Mixing Proportions

The AAS pastes were prepared from the slag in alkaline activators, whereas the mixing proportions are listed in Table2. The water-to-slag ratio was kept constant at a value of 0.35 (by weight). To prepare AAS specimens, the activator was introduced into the slag powder in a bowl of cement paste mixer, in which the mixture was mixed at both low and high speeds for around 1.5 min. There are five silicate moduli and three different Na2O contents for activators. The silicate moduli (Ms) consisted of 1, 1.2, 1.4, 1.6, and 1.8, whereas the Na2O contents were 6%, 8%, and 10% (by weight of the slag). The codes “M1N6,” “M1N8,” and “M1N10” represented the silicate moduli of 1 and Na2O contents of 6%, 8%, and 10%, respectively, and so on.Table 2
The mixing proportions used for this study.
Code
Slag (g)
Sodium silicate (g)
NaOH (g)
H2O (g)
M1N6
1000.0
181.2
53.2
233.7
M1N8
1000.0
241.6
71.0
194.9
M1N10
1000.0
301.9
88.7
156.1
M1.2N6
1000.0
217.4
48.4
213.9
M1.2N8
1000.0
289.9
64.5
168.5
M1.2N10
1000.0
362.3
80.6
123.2
M1.4N6
1000.0
253.6
43.5
194.1
M1.4N8
1000.0
338.2
58.1
142.1
M1.4N10
1000.0
422.7
72.6
90.2
M1.6N6
1000.0
289.9
38.7
174.3
M1.6N8
1000.0
386.5
51.6
115.8
M1.6N10
1000.0
483.1
64.5
57.2
M1.8N6
1000.0
326.1
33.9
154.5
M1.8N8
1000.0
434.8
45.2
89.4
M1.8N10
1000.0
543.5
56.5
24.2


### 2.3. Methods

The effects of silicate modulus and Na2O content on the setting times, fluidity, compressive strength, drying shrinkage, and microstructures of AAS pastes were investigated in this paper. The following methods have also been used by the previous study [21].


#### 2.3.1. Test for the Setting Times

The setting times of alkali-activated slag/fly ash were measured by Vicat in the previous studies [22, 35–37]. The fresh pastes were immediately poured into a truncated cone mold (h = 40 mm, dtop = 65 mm, dbottom = 70 mm) to measure the setting time at room temperature. In accordance with the GB/T 1346-2011 standard [38], the setting times of AAS pastes were tested using a Vicat apparatus. The initial setting time was defined as the time between the initial contact of slag with the activating solution and that when the penetration value reached 25 mm (based upon the ASTM C191 standard). The final setting time corresponded to the point when the cyclic annular needle could not display a complete circular print on the AAS paste surface.


#### 2.3.2. Fluidity Test

Based on the GB/T 8077-2012 standard [39], the fluidity of designed mixtures was measured using a truncated cone (h = 60 mm, dtop = 36 mm, dbottom = 60 mm) at room temperature. Firstly, the fresh pastes were filled into a mold. After about 15 s, the mold was lifted vertically. After another 30 s, the diameters of the paste spread were measured along two perpendicular directions. The average diameter was interpreted as the fluidity of the pastes.


#### 2.3.3. Compressive Strength Test

The paste samples were poured into prismatic molds of 40 mm × 40 mm × 160 mm dimensions, followed by shaking them manually. Afterwards, plastic foils were used to seal the surfaces of specimens to avoid any moisture losses. Then, the specimens were put in a standard room (20 ± 2°C and RH > 95%) for 1 day. After demolding them, the specimens were kept in the standard room until tests for the age of strength were conducted. Compressive strength values were tested at 1, 3, 7, 14, and 28 days by means of a YAW-300 compressive strength analysis apparatus at a loading rate of 1.0 kN/s. The mean values of six samples were interpreted as the compressive strength of AAS pastes for each age.


#### 2.3.4. Drying Shrinkage and Mass Loss Tests

According to the Chinese standard JCT 603-2004 [40], the fresh pastes were poured into prismatic molds of 25 mm × 25 mm × 280 mm dimensions. The further procedure was the same as described in Section 2.3.3 until the demolding of specimens. The lengths of specimens were first determined, and then they were put in isothermal chambers (20 ± 2°C and RH = 50 ± 5%). The drying shrinkage and mass of specimens were tested every day until 28 days. The drying shrinkages were monitored by using a comparator with a measuring accuracy of 0.001 mm. Meanwhile, the mass values were measured by means of an electronic balance having an accuracy of 0.1 g.


#### 2.3.5. Microstructure Analytical Methods

After compressive strength tests at the corresponding curing time, the samples from the crushed prisms were kept in a mixture of alcohol and acetone (1 : 1 v/v, respectively) for one week to stop the hydration process. Afterwards, the samples were taken out of the solution and dried in an oven at 60°C under vacuum. Some of the samples grounded into powder were passed through an 80µm sieve and, then, were used for XRD and FTIR spectroscopy analyses. Rest of the samples were used for SEM/EDS and MIP tests. In order to determine the hydration products and reaction mechanism of AAS pastes, the phase and characteristics of the bonds of raw material and products were determined using XRD (D8 ADVANCE manufactured by AXS, Germany) and FTIR spectroscopy (FTIR-650 spectrometer from China) analyses, respectively. The morphology and elemental analysis of the hydration products were determined using the SEM/EDS (ZEISS MERLIN Compact FE-SEM, Germany) technique. The pore structure of AAS was analyzed using MIP (AutoPore IV 9500, Micromeritics Instrument Ltd., USA).


## 2.1. Materials

Blast furnace slag was procured from the Tangshan steel plant, China, and its chemical composition is presented in Table1. The specific surface area of slag was found to be 424 m2/kg, whereas its specific gravity was determined to be 2.91 [21]. The specific surface area was tested by an electric Brinell permeability surface area analyzer (DBT-127; Wuxi Jianyi Instrument and Machinery Co. Ltd., China), according to the Chinese standard GB/T 8074-2008 [34]. The same sodium hydroxide was used in the previous study [21]. Sodium silicate solution consisting of 10.3% Na2O, 32.1% SiO2, and 57.6% H2O was supplied by Julide Chemical Co. Ltd., Langfang, China. The alkaline activator of AAS pastes was prepared using sodium silicate and NaOH solutions.Table 1
Chemical composition of slag.
Oxide
SiO2
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
Others
Percentage (%)
32.83
17.19
36.69
0.38
0.37
8.20
0.65
1.94
1.75


## 2.2. Mixing Proportions

The AAS pastes were prepared from the slag in alkaline activators, whereas the mixing proportions are listed in Table2. The water-to-slag ratio was kept constant at a value of 0.35 (by weight). To prepare AAS specimens, the activator was introduced into the slag powder in a bowl of cement paste mixer, in which the mixture was mixed at both low and high speeds for around 1.5 min. There are five silicate moduli and three different Na2O contents for activators. The silicate moduli (Ms) consisted of 1, 1.2, 1.4, 1.6, and 1.8, whereas the Na2O contents were 6%, 8%, and 10% (by weight of the slag). The codes “M1N6,” “M1N8,” and “M1N10” represented the silicate moduli of 1 and Na2O contents of 6%, 8%, and 10%, respectively, and so on.Table 2
The mixing proportions used for this study.
Code
Slag (g)
Sodium silicate (g)
NaOH (g)
H2O (g)
M1N6
1000.0
181.2
53.2
233.7
M1N8
1000.0
241.6
71.0
194.9
M1N10
1000.0
301.9
88.7
156.1
M1.2N6
1000.0
217.4
48.4
213.9
M1.2N8
1000.0
289.9
64.5
168.5
M1.2N10
1000.0
362.3
80.6
123.2
M1.4N6
1000.0
253.6
43.5
194.1
M1.4N8
1000.0
338.2
58.1
142.1
M1.4N10
1000.0
422.7
72.6
90.2
M1.6N6
1000.0
289.9
38.7
174.3
M1.6N8
1000.0
386.5
51.6
115.8
M1.6N10
1000.0
483.1
64.5
57.2
M1.8N6
1000.0
326.1
33.9
154.5
M1.8N8
1000.0
434.8
45.2
89.4
M1.8N10
1000.0
543.5
56.5
24.2


## 2.3. Methods

The effects of silicate modulus and Na2O content on the setting times, fluidity, compressive strength, drying shrinkage, and microstructures of AAS pastes were investigated in this paper. The following methods have also been used by the previous study [21].


### 2.3.1. Test for the Setting Times

The setting times of alkali-activated slag/fly ash were measured by Vicat in the previous studies [22, 35–37]. The fresh pastes were immediately poured into a truncated cone mold (h = 40 mm, dtop = 65 mm, dbottom = 70 mm) to measure the setting time at room temperature. In accordance with the GB/T 1346-2011 standard [38], the setting times of AAS pastes were tested using a Vicat apparatus. The initial setting time was defined as the time between the initial contact of slag with the activating solution and that when the penetration value reached 25 mm (based upon the ASTM C191 standard). The final setting time corresponded to the point when the cyclic annular needle could not display a complete circular print on the AAS paste surface.


### 2.3.2. Fluidity Test

Based on the GB/T 8077-2012 standard [39], the fluidity of designed mixtures was measured using a truncated cone (h = 60 mm, dtop = 36 mm, dbottom = 60 mm) at room temperature. Firstly, the fresh pastes were filled into a mold. After about 15 s, the mold was lifted vertically. After another 30 s, the diameters of the paste spread were measured along two perpendicular directions. The average diameter was interpreted as the fluidity of the pastes.


### 2.3.3. Compressive Strength Test

The paste samples were poured into prismatic molds of 40 mm × 40 mm × 160 mm dimensions, followed by shaking them manually. Afterwards, plastic foils were used to seal the surfaces of specimens to avoid any moisture losses. Then, the specimens were put in a standard room (20 ± 2°C and RH > 95%) for 1 day. After demolding them, the specimens were kept in the standard room until tests for the age of strength were conducted. Compressive strength values were tested at 1, 3, 7, 14, and 28 days by means of a YAW-300 compressive strength analysis apparatus at a loading rate of 1.0 kN/s. The mean values of six samples were interpreted as the compressive strength of AAS pastes for each age.


### 2.3.4. Drying Shrinkage and Mass Loss Tests

According to the Chinese standard JCT 603-2004 [40], the fresh pastes were poured into prismatic molds of 25 mm × 25 mm × 280 mm dimensions. The further procedure was the same as described in Section 2.3.3 until the demolding of specimens. The lengths of specimens were first determined, and then they were put in isothermal chambers (20 ± 2°C and RH = 50 ± 5%). The drying shrinkage and mass of specimens were tested every day until 28 days. The drying shrinkages were monitored by using a comparator with a measuring accuracy of 0.001 mm. Meanwhile, the mass values were measured by means of an electronic balance having an accuracy of 0.1 g.


### 2.3.5. Microstructure Analytical Methods

After compressive strength tests at the corresponding curing time, the samples from the crushed prisms were kept in a mixture of alcohol and acetone (1 : 1 v/v, respectively) for one week to stop the hydration process. Afterwards, the samples were taken out of the solution and dried in an oven at 60°C under vacuum. Some of the samples grounded into powder were passed through an 80µm sieve and, then, were used for XRD and FTIR spectroscopy analyses. Rest of the samples were used for SEM/EDS and MIP tests. In order to determine the hydration products and reaction mechanism of AAS pastes, the phase and characteristics of the bonds of raw material and products were determined using XRD (D8 ADVANCE manufactured by AXS, Germany) and FTIR spectroscopy (FTIR-650 spectrometer from China) analyses, respectively. The morphology and elemental analysis of the hydration products were determined using the SEM/EDS (ZEISS MERLIN Compact FE-SEM, Germany) technique. The pore structure of AAS was analyzed using MIP (AutoPore IV 9500, Micromeritics Instrument Ltd., USA).


## 2.3.1. Test for the Setting Times

The setting times of alkali-activated slag/fly ash were measured by Vicat in the previous studies [22, 35–37]. The fresh pastes were immediately poured into a truncated cone mold (h = 40 mm, dtop = 65 mm, dbottom = 70 mm) to measure the setting time at room temperature. In accordance with the GB/T 1346-2011 standard [38], the setting times of AAS pastes were tested using a Vicat apparatus. The initial setting time was defined as the time between the initial contact of slag with the activating solution and that when the penetration value reached 25 mm (based upon the ASTM C191 standard). The final setting time corresponded to the point when the cyclic annular needle could not display a complete circular print on the AAS paste surface.


## 2.3.2. Fluidity Test

Based on the GB/T 8077-2012 standard [39], the fluidity of designed mixtures was measured using a truncated cone (h = 60 mm, dtop = 36 mm, dbottom = 60 mm) at room temperature. Firstly, the fresh pastes were filled into a mold. After about 15 s, the mold was lifted vertically. After another 30 s, the diameters of the paste spread were measured along two perpendicular directions. The average diameter was interpreted as the fluidity of the pastes.


## 2.3.3. Compressive Strength Test

The paste samples were poured into prismatic molds of 40 mm × 40 mm × 160 mm dimensions, followed by shaking them manually. Afterwards, plastic foils were used to seal the surfaces of specimens to avoid any moisture losses. Then, the specimens were put in a standard room (20 ± 2°C and RH > 95%) for 1 day. After demolding them, the specimens were kept in the standard room until tests for the age of strength were conducted. Compressive strength values were tested at 1, 3, 7, 14, and 28 days by means of a YAW-300 compressive strength analysis apparatus at a loading rate of 1.0 kN/s. The mean values of six samples were interpreted as the compressive strength of AAS pastes for each age.


## 2.3.4. Drying Shrinkage and Mass Loss Tests

According to the Chinese standard JCT 603-2004 [40], the fresh pastes were poured into prismatic molds of 25 mm × 25 mm × 280 mm dimensions. The further procedure was the same as described in Section 2.3.3 until the demolding of specimens. The lengths of specimens were first determined, and then they were put in isothermal chambers (20 ± 2°C and RH = 50 ± 5%). The drying shrinkage and mass of specimens were tested every day until 28 days. The drying shrinkages were monitored by using a comparator with a measuring accuracy of 0.001 mm. Meanwhile, the mass values were measured by means of an electronic balance having an accuracy of 0.1 g.


## 2.3.5. Microstructure Analytical Methods

After compressive strength tests at the corresponding curing time, the samples from the crushed prisms were kept in a mixture of alcohol and acetone (1 : 1 v/v, respectively) for one week to stop the hydration process. Afterwards, the samples were taken out of the solution and dried in an oven at 60°C under vacuum. Some of the samples grounded into powder were passed through an 80µm sieve and, then, were used for XRD and FTIR spectroscopy analyses. Rest of the samples were used for SEM/EDS and MIP tests. In order to determine the hydration products and reaction mechanism of AAS pastes, the phase and characteristics of the bonds of raw material and products were determined using XRD (D8 ADVANCE manufactured by AXS, Germany) and FTIR spectroscopy (FTIR-650 spectrometer from China) analyses, respectively. The morphology and elemental analysis of the hydration products were determined using the SEM/EDS (ZEISS MERLIN Compact FE-SEM, Germany) technique. The pore structure of AAS was analyzed using MIP (AutoPore IV 9500, Micromeritics Instrument Ltd., USA).


## 3. Results and Discussion

### 3.1. Setting Times

Figure1 shows the effects of silicate moduli and Na2O contents on the initial and final setting times of AAS pastes at room temperature. When the Na2O contents increased from 6% to 10%, the initial and final setting times increased at the silicate modulus from 1.2 to 1.8, respectively. However, the setting times decreased for the silicate modulus of 1. Moreover, the optimum silicate modulus was 1.4 for longer setting times. For a higher Na2O content and silicate moduli in the alkaline activator, Ca from the decomposition of slag tends to form Ca(OH)2 rather than C-S-H due to the excess of OH− concentration in the earlier stage, resulting in the prolongation of time of Ca2+ precipitation [6, 41, 42]. The setting time of AAS pastes changed slightly, showing that all of the pastes can be used as the building material of urgent construction or repair due to their fast setting property. These faster setting properties of AAS were ascribed to the rapid formation of initial C-S-H in the sodium silicate activator [10]. Another study [43] reported that the initial and final setting times of AAS were 76 and 106 min, respectively, when the activator of water glass was used. In short, silicate moduli and Na2O contents of the alkaline activator have a combined effect on the setting properties of AAS pastes.Figure 1
The setting times of AAS pastes: (a) initial setting time; (b) final setting time.
(a)
(b)


### 3.2. Fluidity

Based upon the silicate moduli and Na2O contents, the fluidity values of fresh pastes are shown in Figure 2. The fluidity of AAS pastes increased with the increase in silicate moduli, suggesting that increasing the silicate modulus can effectively improve the functioning of AAS pastes, which increased slightly once the silicate modulus went beyond the value of 1.4. The slag activated by 8% Na2O presented higher fluidity than those activated by 6% and 10%. The M1N10 specimen has no fluidity as a lot of reaction products are produced in a very short time, which is related to the shortest setting time of M1N10 (Figure 1). A previous research [44] has reported that the nature and concentration of the activator have vital effects on the rheological performance of AAS pastes. They proved that their rheology depends on both the silicate modulus and Na2O concentration for AAS pastes activated with water glass. Wang et al. [1] reported that the slump flow increased as the alkali agent increased from 4% to 8% in liquid-to-solid ratios of 0.35, 0.40, and 0.45 in the alkali-activated ladle furnace slag geopolymer. These results were mainly attributed to the stickiness of alkali metal silicate solution.Figure 2
The effects of the silicate moduli and Na2O contents on the fluidity of AAS pastes.


### 3.3. Compressive Strength

The effects of silicate moduli and Na2O contents on the compressive strength of AAS pastes within the studied periods are presented in Figure 3. For the Na2O content of 8%, highest compressive strength was observed, whereas beyond this content of Na2O (e.g., 10%), the strength decreased. Some previous studies [45, 46] have shown that the excess content of Na2O in the alkali activator is used for charge balancing the Al3+ in the process of polymerization reaction. On the contrary, when the Na2O contents were 6% and 8%, the specimens with the silicate moduli of 1.4 and 1.6 exhibited relatively good compressive strengths at the early and later stages of the process. Moreover, all the specimens showed higher early strengths (more than 55 MPa after 1 d), whereas the highest 1-day compressive strength obtained was 102 MPa at room temperature. The compressive strength values after 1 day lied within the range of 55%–91%, which were close to those obtained after 28 days. This indicated that the sodium silicate solution-activated slag pastes can enhance the early compressive strength development. The high early compressive strength can be suitable for emergency construction and repair works. Aydin and Baradan [10] showed that the C-S-H gel was rapidly formed for improving the early strength. Puertas and Torres-Carrasco [47] concluded that the gel formed by the activation of sodium silicate had a more condensed structure during the early stages.Figure 3
The effects of silicate moduli and Na2O contents on compressive strength of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


### 3.4. Drying Shrinkage

The effects of silicate moduli and Na2O contents on the drying shrinkage before 28 days are shown in Figure 4. As can be observed from Figure 4, the drying shrinkage of AAS pastes increased as the silicate modulus increased. The drying shrinkage values of specimens with Ms = 1.2, 1.4, 1.6, and 1.8 were 1.12, 1.43, 1.59, and 2.01 times that of specimens with Ms = 1 with 8% Na2O content after 28 days, respectively. In addition, when the silicate modulus was 1 and 1.2, the drying shrinkage increased as the Na2O contents increased from 6% to 8%, after which the value decreased for 10% Na2O content. However, the drying shrinkage increased with the increase in Na2O content for the silicate moduli of 1.4 and 1.8. A previous study [1] has shown that the drying shrinkage increased with the increase in alkali agent from 4% to 8% at the liquid-to-slag ratios of 0.35, 0.40, and 0.45. In addition, Komnitsas et al. [45] reported that the excess KOH did not react with the raw material. Neto et al. [48] concluded that the higher content of sodium silicate resulted in a higher drying shrinkage of AAS pastes. Based upon these results, it can be concluded that the silicate moduli played a more important role in the drying shrinkage of AAS pastes than the Na2O contents.Figure 4
The effects of the silicate moduli and Na2O contents on drying shrinkage of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


### 3.5. Mass Loss

Figure5 shows the mass loss (expressed as the mass percent of the total specimen) of AAS pastes for different Na2O contents (6%, 8%, and 10%) and silicate moduli (Ms = 1, 1.2, 1.4, 1.6, and 1.8) during the drying shrinkage testing within 28 days. Regardless of the Na2O content of the specimens, the mass loss increased as the silicate modulus increased. In addition, the mass loss decreased with the increase in the content of Na2O. A previous study [13] has shown that the water loss decreased as the content of Na2O increased from 4% to 6% in AAS cement. This may indicate that more free water existed in specimens having a lower content of Na2O and higher silicate modulus. However, Lee et al. [49] have reported contrary results and concluded that the higher dosage of sodium silicate resulted in the loss of less evaporable water, as more water took part in the polymerization reaction. According to a previous study [50], there was a strong relationship between the drying shrinkage and mass loss for Portland cement. However, for AAS pastes, the relationship was not clear, although the drying shrinkage and mass loss of AAS pastes followed the same trend due to the effect of silicate moduli, whereas the mass loss was found not to be directly related to the drying shrinkage due to the effect of Na2O content.Figure 5
The effects of the silicate moduli and Na2O contents on the mass loss of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


### 3.6. XRD Analysis

Figure6 shows the XRD diffractograms of AAS pastes for various Na2O contents and silicate moduli at an early age. All the hydration products of the specimens showed some weak peaks, which can be identified as omphacite ((NaCa) (AlMg)SiO6), larnite (Ca2SiO4), poorly crystalline C-S-H, and calcite (CaCO3). In addition, calcium aluminum silicate (Ca2Al2SiO7) was only found in the M1N10 specimen, whereas the peaks of C-S-H and calcite overlapped with each other. There was almost no significant difference between the crystal phases of the hydration products in the specimens for different contents of Na2O or silicate moduli. These results were in accordance with those obtained in a previous study [51], in which the researchers also concluded that there were no obvious peaks, indicating that significant quantities of crystalline hydration products were not formed at room temperature. However, Rashad et al. [52] concluded that the intensity of amorphous hump increased as the Na2O content increased from 3.5% to 6.5%, which resulted in a higher compressive strength. Additionally, there was no indication to prove the existence of Ca(OH)2 compared to the ordinary Portland cement paste [33].Figure 6
The XRD diffractograms of AAS pastes at different Na2O contents (a) and silicate moduli (b).
(a)
(b)


### 3.7. FTIR Spectroscopy Analysis

The FTIR spectra of AAS pastes are shown in Figure7. In pastes, the reaction products had the stretching and bending vibrations of H-O-H groups at 3458 cm−1 and 1651 cm−1, respectively. Both the wave numbers shifted towards the larger wavelength as compared to the raw slag (at 3435 cm−1), indicating that more hydration products were formed with the progress of the polymerization process after one day. The band located at 1454 cm−1 was related to the existence of CO32− and corresponded to the production of calcite [53], which was in agreement with the XRD results discussed in Section 3.6. The asymmetric stretching vibration of Si-O bonds was located at 964 cm−1, which represented C-S-H in the hydration product. The bands at 675 cm−1 were assigned to Al-O bonds, which implied that Al replaced Si in the silicon-oxygen tetrahedron [54]. The wavenumber of the Al-O bond was lower after the activation than that of the unreacted slag (at 694 cm−1), which indicated that the reaction of AAS included the dissolution of Al3+ from the slag and the recombination of Al3+ with silicate anions [33]. The bands at 461 cm−1 were attributed to Si-O bending. The results were in accordance with the information of C-(A)-S-H, which was identified using the XRD analysis. It can be concluded that the sodium silicate activator had a significant effect on the degree of polymerization of AAS pastes at an early age.Figure 7
FTIR spectra of AAS pastes: (a) Na2O contents; (b) silicate moduli.
(a)
(b)


### 3.8. SEM/EDS Analysis

The microstructures of AAS pastes synthesized using different contents of Na2O and silicate moduli are shown in Figures 8 and 9. As can be observed from Figures 8(a) and 8(c), the hardened AAS pastes with 6% and 8% Na2O consisted of rod-like particles on the surface of the gel, which were the main reaction products (such as C-(A)-S-H and N-A-S-H) [28, 55]. Moreover, the AAS paste of M1.2N8 was denser than that of M1.2N6, which was in accordance with the result obtained regarding the development of compressive strength and discussed in Section 3.3. This was due to that the strength of M1.2N8 was higher than that of M1.2N6 after 28 days. The products of AAS pastes mainly consisted of Si, Ca, Na, and Al, while the amount of Mg produced was relatively low. The chemical elements of hydration products were tested using energy dispersive X-ray spectroscopy (EDS), and the results of Ca/Si were found to be 0.961 and 1.171 for M1.2N6 and M1.2N8 specimens, respectively. It may be concluded that the activator has higher pH for larger Na2O contents, and vice versa. The increase in pH can accelerate the solubility of Ca in the dissolved slag. The silicate modulus has a more pronounced effect on the degree of polymerization of AAS. In addition, silicate modulus also determined the structure and morphology of reaction products [56]. SEM images of AAS pastes with different silicate moduli (Ms = 1, 1.2, 1.4, 1.6, and 1.8) and Na2O content of 8% are shown in Figures 8(b)–8(f), respectively. The network of fine particles and layered silicate minerals were noticeably observed for the M1N8 specimen. The M1.2N8 sample showed rod-like morphology of the reaction products, and the matrices looked rather dense. The alkali-activated products presented a homogeneous and close-grained structure (calcium silicate hydrate gel), whereas the Ca/Si ratio of the gel was found to be 1.4 for the M1.4N8 specimen. In addition, a network of fine particles was also observed in the M1.6N8 specimen. Meanwhile, the paste was close-grained hydration products and made the structure more compact compared with the M1N8 specimen. The surface of the M1.8N8 specimen was covered with a significant number of rod-like particles and porous structure, resulting in weak bonding strength. These results were in agreement with the development of mechanical properties, as the compressive strength of the M1.8N8 specimen was lower than those of M1.4N8 and M1.6N8 specimens. Rattanasak et al. [57] reported that the difference in gel formation affected the binding capacity and strength of geopolymer binder. Moreover, there were little crystalline products in all the samples.Figure 8
SEM images of AAS pastes for 28 days: (a) M1.2N6. (b) M1N8. (c) M1.2N8. (d) M1.4N8. (e) M1.6N8. (f) M1.8N8.Figure 9
EDS images of AAS pastes corresponding to Figure8: (a, b) M1.2N6. (c) M1N8. (d) M1.2N8. (e) M1.4N8. (f) M1.6N8. (g) M1.8N8.The EDS analysis revealed that the hydration products have lots of O, Na, Al, Si, and Ca elements, whereas the Ca/Si ratio was in the range of 0.8–1.4. The main product of AAS pastes was C-S-H (or N-A-S-H) gel, which was amorphous at room temperature. In addition, Na and Al elements were found to be present in the structure of calcium silicate hydrate gel, as observed by the EDS. Meanwhile, the atomic percentages of Na and Al were found to be 1.45–27.77% and 2.99–11.06%, respectively. A previous study [58] concluded that a higher silicate modulus of the activator can accelerate the rate of polymerization and form lower crystalline products.


### 3.9. MIP Analysis

Table3 presents the pore sizes of AAS pastes. The pore size was tested between 5 and 360000 nm using AutoPore IV 9500. Therefore, there were three types of pore sizes in the AAS pastes, namely, the mesopores, macropores, and voids and microcracks. The effect of silicate modulus on the pore size distribution of AAS pastes is shown in Figure 10. Figure 10(a) shows that the pore volume decreased with the increase in silicate modulus, except for the M1.8N8 specimen, indicating that a lower silicate modulus resulted in a larger pore volume. Figure 10(b) indicated that the volume of mesopores increased, whereas the volume of macropores decreased with the increase in silicate modulus. The larger volume of mesopores resulted in higher drying shrinkage values [33], which was consistent with the development of drying shrinkage, as discussed in Section 3.4. It can also be seen that the increase in silicate modulus led to a lower volume of macropores along with smaller voids and microcracks (M1.8N8 specimen had the largest voids and microcracks), which revealed that more hydration products had filled the larger pores, thus making it denser with the increase in silicate modulus at the same curing ages [59]. The porosities and pore size distribution of M1.6N8 and M1.4N8 were almost the same, which could explain the fact that both of them had relatively higher compressive strengths as mentioned above in Section 3.3.Table 3
The classifications of pores in the AAS pastes (according to the International Union of Pure and Applied Chemistry system).
Micropores
Mesopores
Macropores
Voids and microcracks
Pore size (mm)
<2.5
2.5–50
50–10000
>10000Figure 10
Pore size distribution of AAS pastes with different silicate moduli: (a) cumulative intrusion; (b) porosity.
(a)
(b)


## 3.1. Setting Times

Figure1 shows the effects of silicate moduli and Na2O contents on the initial and final setting times of AAS pastes at room temperature. When the Na2O contents increased from 6% to 10%, the initial and final setting times increased at the silicate modulus from 1.2 to 1.8, respectively. However, the setting times decreased for the silicate modulus of 1. Moreover, the optimum silicate modulus was 1.4 for longer setting times. For a higher Na2O content and silicate moduli in the alkaline activator, Ca from the decomposition of slag tends to form Ca(OH)2 rather than C-S-H due to the excess of OH− concentration in the earlier stage, resulting in the prolongation of time of Ca2+ precipitation [6, 41, 42]. The setting time of AAS pastes changed slightly, showing that all of the pastes can be used as the building material of urgent construction or repair due to their fast setting property. These faster setting properties of AAS were ascribed to the rapid formation of initial C-S-H in the sodium silicate activator [10]. Another study [43] reported that the initial and final setting times of AAS were 76 and 106 min, respectively, when the activator of water glass was used. In short, silicate moduli and Na2O contents of the alkaline activator have a combined effect on the setting properties of AAS pastes.Figure 1
The setting times of AAS pastes: (a) initial setting time; (b) final setting time.
(a)
(b)


## 3.2. Fluidity

Based upon the silicate moduli and Na2O contents, the fluidity values of fresh pastes are shown in Figure 2. The fluidity of AAS pastes increased with the increase in silicate moduli, suggesting that increasing the silicate modulus can effectively improve the functioning of AAS pastes, which increased slightly once the silicate modulus went beyond the value of 1.4. The slag activated by 8% Na2O presented higher fluidity than those activated by 6% and 10%. The M1N10 specimen has no fluidity as a lot of reaction products are produced in a very short time, which is related to the shortest setting time of M1N10 (Figure 1). A previous research [44] has reported that the nature and concentration of the activator have vital effects on the rheological performance of AAS pastes. They proved that their rheology depends on both the silicate modulus and Na2O concentration for AAS pastes activated with water glass. Wang et al. [1] reported that the slump flow increased as the alkali agent increased from 4% to 8% in liquid-to-solid ratios of 0.35, 0.40, and 0.45 in the alkali-activated ladle furnace slag geopolymer. These results were mainly attributed to the stickiness of alkali metal silicate solution.Figure 2
The effects of the silicate moduli and Na2O contents on the fluidity of AAS pastes.


## 3.3. Compressive Strength

The effects of silicate moduli and Na2O contents on the compressive strength of AAS pastes within the studied periods are presented in Figure 3. For the Na2O content of 8%, highest compressive strength was observed, whereas beyond this content of Na2O (e.g., 10%), the strength decreased. Some previous studies [45, 46] have shown that the excess content of Na2O in the alkali activator is used for charge balancing the Al3+ in the process of polymerization reaction. On the contrary, when the Na2O contents were 6% and 8%, the specimens with the silicate moduli of 1.4 and 1.6 exhibited relatively good compressive strengths at the early and later stages of the process. Moreover, all the specimens showed higher early strengths (more than 55 MPa after 1 d), whereas the highest 1-day compressive strength obtained was 102 MPa at room temperature. The compressive strength values after 1 day lied within the range of 55%–91%, which were close to those obtained after 28 days. This indicated that the sodium silicate solution-activated slag pastes can enhance the early compressive strength development. The high early compressive strength can be suitable for emergency construction and repair works. Aydin and Baradan [10] showed that the C-S-H gel was rapidly formed for improving the early strength. Puertas and Torres-Carrasco [47] concluded that the gel formed by the activation of sodium silicate had a more condensed structure during the early stages.Figure 3
The effects of silicate moduli and Na2O contents on compressive strength of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


## 3.4. Drying Shrinkage

The effects of silicate moduli and Na2O contents on the drying shrinkage before 28 days are shown in Figure 4. As can be observed from Figure 4, the drying shrinkage of AAS pastes increased as the silicate modulus increased. The drying shrinkage values of specimens with Ms = 1.2, 1.4, 1.6, and 1.8 were 1.12, 1.43, 1.59, and 2.01 times that of specimens with Ms = 1 with 8% Na2O content after 28 days, respectively. In addition, when the silicate modulus was 1 and 1.2, the drying shrinkage increased as the Na2O contents increased from 6% to 8%, after which the value decreased for 10% Na2O content. However, the drying shrinkage increased with the increase in Na2O content for the silicate moduli of 1.4 and 1.8. A previous study [1] has shown that the drying shrinkage increased with the increase in alkali agent from 4% to 8% at the liquid-to-slag ratios of 0.35, 0.40, and 0.45. In addition, Komnitsas et al. [45] reported that the excess KOH did not react with the raw material. Neto et al. [48] concluded that the higher content of sodium silicate resulted in a higher drying shrinkage of AAS pastes. Based upon these results, it can be concluded that the silicate moduli played a more important role in the drying shrinkage of AAS pastes than the Na2O contents.Figure 4
The effects of the silicate moduli and Na2O contents on drying shrinkage of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


## 3.5. Mass Loss

Figure5 shows the mass loss (expressed as the mass percent of the total specimen) of AAS pastes for different Na2O contents (6%, 8%, and 10%) and silicate moduli (Ms = 1, 1.2, 1.4, 1.6, and 1.8) during the drying shrinkage testing within 28 days. Regardless of the Na2O content of the specimens, the mass loss increased as the silicate modulus increased. In addition, the mass loss decreased with the increase in the content of Na2O. A previous study [13] has shown that the water loss decreased as the content of Na2O increased from 4% to 6% in AAS cement. This may indicate that more free water existed in specimens having a lower content of Na2O and higher silicate modulus. However, Lee et al. [49] have reported contrary results and concluded that the higher dosage of sodium silicate resulted in the loss of less evaporable water, as more water took part in the polymerization reaction. According to a previous study [50], there was a strong relationship between the drying shrinkage and mass loss for Portland cement. However, for AAS pastes, the relationship was not clear, although the drying shrinkage and mass loss of AAS pastes followed the same trend due to the effect of silicate moduli, whereas the mass loss was found not to be directly related to the drying shrinkage due to the effect of Na2O content.Figure 5
The effects of the silicate moduli and Na2O contents on the mass loss of AAS pastes. (a) 6% Na2O. (b) 8% Na2O. (c) 10% Na2O.
(a)
(b)
(c)


## 3.6. XRD Analysis

Figure6 shows the XRD diffractograms of AAS pastes for various Na2O contents and silicate moduli at an early age. All the hydration products of the specimens showed some weak peaks, which can be identified as omphacite ((NaCa) (AlMg)SiO6), larnite (Ca2SiO4), poorly crystalline C-S-H, and calcite (CaCO3). In addition, calcium aluminum silicate (Ca2Al2SiO7) was only found in the M1N10 specimen, whereas the peaks of C-S-H and calcite overlapped with each other. There was almost no significant difference between the crystal phases of the hydration products in the specimens for different contents of Na2O or silicate moduli. These results were in accordance with those obtained in a previous study [51], in which the researchers also concluded that there were no obvious peaks, indicating that significant quantities of crystalline hydration products were not formed at room temperature. However, Rashad et al. [52] concluded that the intensity of amorphous hump increased as the Na2O content increased from 3.5% to 6.5%, which resulted in a higher compressive strength. Additionally, there was no indication to prove the existence of Ca(OH)2 compared to the ordinary Portland cement paste [33].Figure 6
The XRD diffractograms of AAS pastes at different Na2O contents (a) and silicate moduli (b).
(a)
(b)


## 3.7. FTIR Spectroscopy Analysis

The FTIR spectra of AAS pastes are shown in Figure7. In pastes, the reaction products had the stretching and bending vibrations of H-O-H groups at 3458 cm−1 and 1651 cm−1, respectively. Both the wave numbers shifted towards the larger wavelength as compared to the raw slag (at 3435 cm−1), indicating that more hydration products were formed with the progress of the polymerization process after one day. The band located at 1454 cm−1 was related to the existence of CO32− and corresponded to the production of calcite [53], which was in agreement with the XRD results discussed in Section 3.6. The asymmetric stretching vibration of Si-O bonds was located at 964 cm−1, which represented C-S-H in the hydration product. The bands at 675 cm−1 were assigned to Al-O bonds, which implied that Al replaced Si in the silicon-oxygen tetrahedron [54]. The wavenumber of the Al-O bond was lower after the activation than that of the unreacted slag (at 694 cm−1), which indicated that the reaction of AAS included the dissolution of Al3+ from the slag and the recombination of Al3+ with silicate anions [33]. The bands at 461 cm−1 were attributed to Si-O bending. The results were in accordance with the information of C-(A)-S-H, which was identified using the XRD analysis. It can be concluded that the sodium silicate activator had a significant effect on the degree of polymerization of AAS pastes at an early age.Figure 7
FTIR spectra of AAS pastes: (a) Na2O contents; (b) silicate moduli.
(a)
(b)


## 3.8. SEM/EDS Analysis

The microstructures of AAS pastes synthesized using different contents of Na2O and silicate moduli are shown in Figures 8 and 9. As can be observed from Figures 8(a) and 8(c), the hardened AAS pastes with 6% and 8% Na2O consisted of rod-like particles on the surface of the gel, which were the main reaction products (such as C-(A)-S-H and N-A-S-H) [28, 55]. Moreover, the AAS paste of M1.2N8 was denser than that of M1.2N6, which was in accordance with the result obtained regarding the development of compressive strength and discussed in Section 3.3. This was due to that the strength of M1.2N8 was higher than that of M1.2N6 after 28 days. The products of AAS pastes mainly consisted of Si, Ca, Na, and Al, while the amount of Mg produced was relatively low. The chemical elements of hydration products were tested using energy dispersive X-ray spectroscopy (EDS), and the results of Ca/Si were found to be 0.961 and 1.171 for M1.2N6 and M1.2N8 specimens, respectively. It may be concluded that the activator has higher pH for larger Na2O contents, and vice versa. The increase in pH can accelerate the solubility of Ca in the dissolved slag. The silicate modulus has a more pronounced effect on the degree of polymerization of AAS. In addition, silicate modulus also determined the structure and morphology of reaction products [56]. SEM images of AAS pastes with different silicate moduli (Ms = 1, 1.2, 1.4, 1.6, and 1.8) and Na2O content of 8% are shown in Figures 8(b)–8(f), respectively. The network of fine particles and layered silicate minerals were noticeably observed for the M1N8 specimen. The M1.2N8 sample showed rod-like morphology of the reaction products, and the matrices looked rather dense. The alkali-activated products presented a homogeneous and close-grained structure (calcium silicate hydrate gel), whereas the Ca/Si ratio of the gel was found to be 1.4 for the M1.4N8 specimen. In addition, a network of fine particles was also observed in the M1.6N8 specimen. Meanwhile, the paste was close-grained hydration products and made the structure more compact compared with the M1N8 specimen. The surface of the M1.8N8 specimen was covered with a significant number of rod-like particles and porous structure, resulting in weak bonding strength. These results were in agreement with the development of mechanical properties, as the compressive strength of the M1.8N8 specimen was lower than those of M1.4N8 and M1.6N8 specimens. Rattanasak et al. [57] reported that the difference in gel formation affected the binding capacity and strength of geopolymer binder. Moreover, there were little crystalline products in all the samples.Figure 8
SEM images of AAS pastes for 28 days: (a) M1.2N6. (b) M1N8. (c) M1.2N8. (d) M1.4N8. (e) M1.6N8. (f) M1.8N8.Figure 9
EDS images of AAS pastes corresponding to Figure8: (a, b) M1.2N6. (c) M1N8. (d) M1.2N8. (e) M1.4N8. (f) M1.6N8. (g) M1.8N8.The EDS analysis revealed that the hydration products have lots of O, Na, Al, Si, and Ca elements, whereas the Ca/Si ratio was in the range of 0.8–1.4. The main product of AAS pastes was C-S-H (or N-A-S-H) gel, which was amorphous at room temperature. In addition, Na and Al elements were found to be present in the structure of calcium silicate hydrate gel, as observed by the EDS. Meanwhile, the atomic percentages of Na and Al were found to be 1.45–27.77% and 2.99–11.06%, respectively. A previous study [58] concluded that a higher silicate modulus of the activator can accelerate the rate of polymerization and form lower crystalline products.


## 3.9. MIP Analysis

Table3 presents the pore sizes of AAS pastes. The pore size was tested between 5 and 360000 nm using AutoPore IV 9500. Therefore, there were three types of pore sizes in the AAS pastes, namely, the mesopores, macropores, and voids and microcracks. The effect of silicate modulus on the pore size distribution of AAS pastes is shown in Figure 10. Figure 10(a) shows that the pore volume decreased with the increase in silicate modulus, except for the M1.8N8 specimen, indicating that a lower silicate modulus resulted in a larger pore volume. Figure 10(b) indicated that the volume of mesopores increased, whereas the volume of macropores decreased with the increase in silicate modulus. The larger volume of mesopores resulted in higher drying shrinkage values [33], which was consistent with the development of drying shrinkage, as discussed in Section 3.4. It can also be seen that the increase in silicate modulus led to a lower volume of macropores along with smaller voids and microcracks (M1.8N8 specimen had the largest voids and microcracks), which revealed that more hydration products had filled the larger pores, thus making it denser with the increase in silicate modulus at the same curing ages [59]. The porosities and pore size distribution of M1.6N8 and M1.4N8 were almost the same, which could explain the fact that both of them had relatively higher compressive strengths as mentioned above in Section 3.3.Table 3
The classifications of pores in the AAS pastes (according to the International Union of Pure and Applied Chemistry system).
Micropores
Mesopores
Macropores
Voids and microcracks
Pore size (mm)
<2.5
2.5–50
50–10000
>10000Figure 10
Pore size distribution of AAS pastes with different silicate moduli: (a) cumulative intrusion; (b) porosity.
(a)
(b)


## 4. Conclusions

Based on the experimental results, the following main conclusions can be drawn from the study:(1)
The initial and final setting times were in the range of 6.1–28.2 min and 9–36 min, respectively. The fluidity of AAS pastes was between 147 and 226 mm.(2)
AAS pastes also exhibited higher early compressive strengths, whereas the highest 1-day compressive strength was found to be 102 MPa at room temperature.(3)
Both the drying shrinkage and mass loss increased as the silicate modulus increased during the curing process. However, with the increase of Na2O content, the drying shrinkage increased, while the mass loss decreased when the silicate modulus went beyond the value of 1.2. The mechanism of drying shrinkage was not entirely due to the mass loss and had a strong relationship between the volume of micropores and hydration products.(4)
The dominant reaction products of AAS pastes were amorphous C-S-H and omphacite. The hydration products have high amounts of O, Na, Al, Si, and Ca elements, whereas the products of the M1.4N8 specimen exhibited a homogeneous and close-grained microstructure. Furthermore, the Ca/Si ratio was found to be ca. 1.4.(5)
Lower values of porosity and volume of voids and microcracks were observed for the specimen with silicate moduli of 1.4 and 1.6 having 8% Na2O, which explained the excellent mechanical performance of the specimen.



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*Source: 8407380-2018-12-16.xml*