Effect of synthetic CaO-Al2O3-SiO2-H2O on the early-stage performance of alkali-activated slag

C(-A)-S-H (CaO(-Al2O3)-SiO2-H2O) gel is the main product and source of strength in alkali-activated slag systems. Previous studies on C-A-S-H have focused mostly on the characterization of the structure and composition of the products. In this study, synthetic C-A-S-H particles with a similar composition as gels produced by alkali-activated slag were prepared by a hydrothermal method and were added to the alkali-activated slag systems, and the synthetic can be regarded as a chemical intensifiers of hardening (CIH). The effects of the synthetic C-A-S-H on the hydration process, the early-stage mechanical properties, and the microstructure of the alkali-activated slag were investigated by hydration heat tests, scanning electron microscopy (SEM), mercury intrusion porosimetry tests (MIP), and other methods. The results showed that the synthetic C-A-S-H shortened the setting time, promoted the formation of an acceleration period at an early stage, and increased the hydration heat released during the acceleration period. The synthetic C-A-S-H improved the flexural and compressive strength of the alkali-activated slag to a certain extent and the improvements in the flexural strength were more significant than the improvements in the compressive strength; the flexural strength of the sample with the added 3% synthetic C-A-S-H with 3-d curing increased by 16.25%. Microstructure analysis indicated the synthetic C-A-S-H also reduced the porosity of the alkali-activated slag-hardened paste and optimized the pore size distribution. This study of the early-stage performance provides a new approach and predictions for the late-stage performance and in-depth modification mechanisms of alkali-activated slag systems.