Phase evolution and performance of sodium sulfate-activated slag cement pastes

Abstract

This study evaluates the reaction kinetics, phase assemblage, and microstructure evolution of Na₂SO₄-activated slag cements produced with three commercial slags. The main reaction products identified are ettringite and calcium aluminosilicate hydrates, alongside a poorly crystalline SO₄^2- intercalated Mg-Al-layered double hydroxide (LDH) phase. Results revealed that the Al₂O₃ slag content alone does not correlate with the cement performance. While pastes made with a higher Al₂O₃ content slag exhibit faster reaction kinetics, those made with a slag with a higher Mg/Al ratio developed superior compressive strength and reduced porosity over extended curing periods. Thermodynamic modelling simulations indicate that sulfate consumption occurs via ettringite and LDH phase formation, influencing the slag reaction degree, pH value, and porosity reduction in these cements. This research highlights the critical role of slag composition in controlling microstructure and, consequently, performance of sodium sulfate activated slag cement pastes.