Effect of the Retarder on Initial Hydration and Mechanical Properties of the “one-step” Alkali-activated Composite Cementitious Materials

This paper studied the effects of different retarders on the performance of the “one-step” alkali-activated composite cementitious material (ACCM) which is composed of ground granulated blast slag(GGBS) and fly ash(FA), and analyzed its mechanical properties, hydration mechanism, and retardation mechanism. The effects of retarders on the hydration products, mechanical properties, and hydration kinetics of ACCM were investigated using XRD, SEM, FTIR, EDS, and thermoactive microcalorimetry. The results showed that Na₂B₄O₇·10H₂O (B) delayed the exotherm during the alkali activation process and could effectively delay the setting time of ACCM, but the mechanical properties were slightly decreased. The setting time of ACCM increased with the increase in SG content, but the mechanical properties of ACCM decreased with the increase in SG content. C1₂H₂₂O₁₁ (CHO) could effectively delay the hydration reaction of ACCM and weakly enhanced the compressive strength. H₃PO₄ (HP) at a concentration of 0.05 mol/L had a certain effect on ACCM retardation, but HP at a concentration of 0.07 and 0.09 mol/L had an effect of promoting the setting and hardening time of ACCM.