Alkali-activated slag substituted by metakaolin and dolomite at 20 and 50°C

Abstract

This work focuses on the reaction of dolomite fines (DM) in combination with metakaolin (MK) in alkali-activated slag (AAS), towards a better understanding of the hydrotalcite-type phase formation in this system. The strength, phase assemblage, and microstructure of AAS pastes with 40% slag substituted by a combination of MK and DM and cured at two temperature conditions (i.e., 20 °C and 50 °C) are studied. The results show that the DM at a less than 20% substitution ratio in AAS poses a filler effect that advances slag reaction; however, its potential chemical roles on supplementing magnesium and thus enhancing the hydrotalcite formation are highly limited. The incorporation of MK in AAS supplements aluminum but does not promote the hydrotalcite precipitation. The results suggest that the absence of portlandite in AAS restricts the dissolution and reaction of DM despite high alkalinity. The continued elevated temperature curing has a detrimental effect on the strength development of AAS with greater than 20% MK, likely due to the promoted transformation of strength-giving amorphous gel products into zeolitic crystals. The optimum percentage ratio between MK and DM lies around 1: 1 at a 40% replacement level in AAS, in term of strength achievement and stability. This study contributes to the materials selection and mixture design of slag-based alkali-activated composites with potentially enhanced chemical durability.