Effect of the Sodium Silicate Modulus and Slag Content on Fresh and Hardened Properties of Alkali-Activated Fly Ash/Slag

Abstract

This paper presents the results of an experimental study performed to investigate the effect of activator modulus (SiO2/Na2O) and slag addition on the fresh and hardened properties of alkali-activated fly ash/slag (AAFS) pastes. Four activator moduli (SiO2/Na2O), i.e., 0.0, 1.0, 1.5, and 2.0, and five slag-to-binder ratios, i.e., 0, 0.3, 0.5, 0.7, 1.0, were used to prepare AAFS mixtures. The setting time, flowability, heat evolution, compressive strength, microstructure, and reaction products of AAFS pastes were studied. The results showed that the activator modulus and slag content had a combined effect on the setting behavior and workability of AAFS mixtures. Both the activator modulus and slag content affected the types of reaction products formed in AAFS. The coexistence of N–A–S–H gel and C–A–S–H gel was identified in AAFS activated with high pH but low SiO2 content (low modulus). C–A–S–H gel had a higher space-filling ability than N–A–S–H gel. Thus, AAFS with higher slag content had a finer pore structure and higher heat release (degree of reaction), corresponding to a higher compressive strength. The dissolution of slag was more pronounced when NaOH (modulus of 0.0) was applied as the activator. The use of Na2SiO3 as activator significantly refined the pores in AAFS by incorporating soluble Si in the activator, while further increasing the modulus from 1.5 to 2.0 prohibited the reaction process of AAFS, resulting in a lower heat release, coarser pore structure, and reduced compressive strength. Therefore, in view of the strength and microstructure, the optimum modulus is 1.5.

Preliminary Interpretation of the Induction Period in Hydration of Sodium Hydroxide/Silicate Activated Slag

“…The retarding effect of soluble Si on dissolution of slag then slowed down the reaction kinetics of sodium silicate activated slag, resulting in an induction period during the reaction process as shown in Figure 3b. The induction period has been also widely detected through isothermal calorimetry measurements by many studies in the literature [20][21][22][25][26][27][28]. Besides the reaction kinetics, the effect of soluble Si by slowing down the dissolution of slag also affects the microstructure formation.…”

“…According to the aforementioned calorimetric studies, among others in the literature like [25][26][27][28], an obvious different calorimetric characteristic can be found between the NaOH activated slag paste and the sodium silicate activated slag paste. As shown in Figure 1, the heat evolution rate curve of NaOH activated slag paste demonstrates no induction period, while that of sodium silicate activated slag paste show an obvious induction period.…”

Development of early age autogenous and thermal strains of alkali-activated slag-fly ash pastes

“…To meet the objectives, a modified AutoShrink device (Germann Instruments, 2015) is used for the quantification of both autogenous and thermal strains, which allows for applying temperature variations during the hardening. The deformation monitoring is accompanied by isothermal calorimetry conducted at three different temperatures, to monitor the reaction process (Ouyang et al, 2020;Joseph and Cizer, 2022) and to quantify the temperature activation behavior. Moreover, internal relative humidity (IRH) evolutions were measured to potentially correlate them with autogenous shrinkage results.…”

Compressive Strength of Sustainable Geopolymer Concrete Composites: A State-of-the-Art Review

“…Regarding the reviewed papers, the range of SiO2 was between 28 and 37 percent, Na2O was between 8 and 18 percent, and the proportion of H2O was between 45 and 64 percent, as shown in Table 2. These solutions generated a geopolymeric binder by activating and extracting Si and Al elements from the different source binder materials, as stated by Ouyang et al [56]. Several research studies have been published in the literature to highlight the effect of various parameters on the fresh, physical, mechanical, durability, and microstructural properties of FA-BGPC.…”