RSM-based Optimized Mix Design of Alkali-activated Slag Pastes Based on the Fresh and Hardened Properties and Unit Cost

Alkali-activated slag (AAS) is a relatively new class of materials with superior properties to cementitious materials. Numerous factors affect the properties of AAS mixtures, and knowing the optimal levels of each factor can help in se-lecting the mix design that meets specific requirements. Optimizing AAS pastes based on the fresh and hardened prop-erties and unit cost, then suggesting a model predict its properties has not been examined yet. In this study, the fresh and hardened properties of AAS paste were optimized using the full factorial design of experiments and response surface methodology (RSM). Effects of the main factors including the type and concentration of the alkaline solution (AS), modulus of sodium silicate (MSS), and sodium silicate (SS) to AS ratio on the properties of AAS pastes were studied experimentally. The results displayed that among the considered factors, the SS to AS ratio and type of AS have the most impact on the improvement of compressive strength of the mixtures. Finally, the optimized mixture was obtained based on maximum compressive strength and flowability while targeting the minimum cost. This mixture contained a NaOH alkaline activator with a concentration of 6 molars, MSS of 2.29, and SS to AS ratio of 0.4.