Effect of a Synthetic Nano-CaO-Al2O3-SiO2-H2O Gel on the Early-Stage Shrinkage Performance of Alkali-Activated Slag Mortars

Abstract

The relatively high shrinkage of the alkali-activated slag (AAS) has restricted its application as a widely-used building material. This research attempts to study the effect of a hydrothermally synthesized C-A-S-H gel, which has a similar composition to that of the main AAS product, on the shrinkage performance of the AAS. The C-A-S-H nano-particles were mixed into AAS mortars in a proportion ranging from 1 wt % to 5 wt % using two different methods, and the drying and autogenous shrinkage ratio of modified AAS mortars were measured at various ages. The effect of C-A-S-H on their microstructures was also characterized. Results obtained indicated that the addition of C-A-S-H gel to AAS mortars had reduced their drying and autogenous shrinkage, with the addition of 3 wt % reaching the maximum reduction. However, the added amount was not directly proportional to the decrease of shrinkage; the proportion of early-stage drying shrinkage of AAS mortars was greater than that of autogenous shrinkage; the dispersion method was slightly better than the dry mixing method in both shrinkage reduction. MIP results suggested that the addition of C-A-S-H gel had reduced the total porosity and the average pore size of AAS mortars, optimized their pore structure distribution, and significantly reduced the volume of mesopores (<0.05 µm) which resulted in high shrinkage, while the adding method had no significant effect on the pore size distribution of AAS mortars. SEM results showed that the addition of C-A-S-H gel can reduce the crack width of mortars, obtain a dense and uniform matrix structure, increase the density, and effectively suppress both shrinkage deformation of the system, whereas the adding method has no obvious effect on the crack width of the mortar. This research provides a novel approach of the AAS shrinkage reduction and structure refinement, shedding lights on nano-material modification of the AAS.

Insights into the Microstructure of Hydrothermal Synthesized Nanoscale K2O-Al2O3-SiO2-H2O Particles

“…Low crystallinity K-A-S-H gels and zeolite crystalline powders were synthesized and the differences and similarities of their (micro) structures were analyzed. Our previous studies [3,7] have demonstrated the synthetic C-A-S-H gel can be regarded as a chemical intensification of the hardening effect in alkali-activated binders, which also refine the pore structures of the alkali-activated binders and provide a novel approach of alkali-activated binders shrinkage reduction at an early stage. Meanwhile, these gels and the hydration products of alkali-activated materials have similar composition and morphology.…”

“…The proportions of nano-SiO 2 , nano-Al 2 O 3 , KOH, and deionized water are shown in Table 1. Nanoscale K-A-S-H gel was synthesized according to our previous hydrothermal method [3,4]. The samples are named as follows: using K-A-S-H1.0 1 -1d-95 as an example, the K-A-S-H1.0 indicates a K 2 O/SiO 2 ratio in the K-A-S-H system of 1.0, the subscript indicates a SiO 2 /Al 2 O 3 ratio of 1, the 1d indicates a curing time of 1 day, and the 95 indicates that the material was cured at a temperature of 95 • C.…”

Investigation of the Effect of Blended Aggregate on the Strength and Drying Shrinkage Characteristics of Alkali-Activated Slag Mortar

“…Various methods have been attempted to reduce the drying shrinkage of AAS. For example, the addition of fiber [60][61][62][63], pozzolanic material [57][58][59][60][61][62][63][64][65][66][67], an expansion agent [68][69][70], nano-materials [57,[71][72][73], a shrinkage-reducing agent (SRA) [32,34,[74][75][76][77], superabsorbent polymer [78,79], and methods for increasing the curing temperature [23,42,58,80,81] have been attempted. Previous studies on drying shrinkage reduction experiments and studies have shown a certain level of shrinkage reduction effect.…”