Early age hydration of model slag cement: Interaction among C₃S, gypsum and slag with different Al₂O₃ contents

A deeper insight into SO₃/Al₂O₃ ratio including the contribution of alumina in slag at early age is required to ensure a properly sulfated slag cement. In this paper, to investigate the effect of gypsum and alumina of slag, emphasis was laid on the hydration characteristics of C₃S-gypsum-slag system during the early age, of which slag was synthesized in the laboratory with varying Al₂O₃ contents from 3.69 to 18.19 wt%. The duration of dormant period during the hydration of C₃S depended on Al₂O₃ content of slag significantly; however, the amount of silicate reaction before the onset of aluminate reaction was independent of slag chemistry and gypsum content added. The rate of aluminate reaction was controlled by the availability of reactants, SO₄²⁻ and Al³⁺ ions in particular, which were sourced from gypsum and slag, respectively. Calcium monosulfoaluminate only occurred in mixture when slag contained a high amount of Al₂O₃ (18.19 wt% in this study) at early age, and its formation proceeded continuously at the expense of ettringite. Sulfur rich species incorporated in slag started to participate into aluminate reaction after the main hydration peak of C₃S, and it played a similar role to gypsum.