Effect of hydroxyethyl cellulose on reaction kinetics, microstructure and mechanical performance of alkali-activated slag

To address the excessively rapid reaction and short workable time of alkali-activated slag (AAS) systems, this study investigated the effects of hydroxyethyl cellulose (HEC) on the workability, reaction kinetics, mechanical properties, and microstructure of sodium silicate/sodium hydroxide-activated slag paste. Flowability, consistency, setting time, and isothermal calorimetry were employed to evaluate the regulatory effects of HEC on fresh-state properties and early-age reaction behavior. XRD, FTIR, SEM-EDS, and MIP were further used to characterize the evolution of reaction products, microstructural morphology, and pore structure in the hardened matrix. The results showed that HEC significantly reduced paste flowability while prolonging the setting time. Meanwhile, the intensity of the main heat-release peak and the cumulative heat release at early ages decreased with increasing HEC dosage, indicating that HEC delayed the early reaction and structural build-up of the AAS system. The mechanical performance exhibited a non-monotonic trend with increasing HEC dosage, and the optimal overall performance was obtained at 0.3 wt.% HEC, where the 28 d compressive and flexural strengths reached 71.3 MPa and 8.7 MPa, respectively. Microstructural analyses showed that HEC did not change the main types of reaction products. However, an appropriate HEC dosage was associated with a denser and more homogeneous microstructural appearance, reduced porosity, and refined pore structure, whereas excessive HEC weakened strength development, probably due to excessive viscosity enhancement and delayed reaction-product formation. Overall, HEC exhibited a dosage-dependent regulatory effect on sodium silicate/sodium hydroxide-activated slag paste, characterized by early-stage reaction retardation and moderate later-age microstructural densification. Under the conditions investigated in this study, 0.3 wt.% HEC provided the best balance between reaction regulation and mechanical performance. This work provides useful guidance for the use of cellulose ether admixtures in alkali-activated cementitious materials.