Degree of hydration in calcined alum sludge blends: Influence on hydration kinetics and hydrate evolution

This study investigated how calcined alum sludge (AS1600) influences hydration and hydrate evolution in cement pastes containing 5–20% cement replacement up to 90d. Degree of hydration (DoH) of cement was quantified using two thermogravimetric bound water approaches: the conventional Bhatty method and a Michaelis-Menten curve fitting method. Hydration mechanisms were further assessed using calcium hydroxide (CH) evolution, X-ray diffraction, early age ionic release by ICP-OES, and point EDS analysis. The results indicate the influence of alum sludge based on a replacement dependant manner. At 2d, AS5 showed higher DoH than the control (29.10% vs 20.45%), suggesting early acceleration associated with filler/nucleation effects. At higher replacement levels, CH accumulation decreased; by 90d AS20 produced only 9.25% CH compared with 21.32% for the control, while bound water remained comparable or higher. This behaviour suggests that part of the measured bound water originated from alumina rich hydrates rather than clinker hydration alone. Consequently, the Bhatty method overestimated DoH of cement in high AS1600 blends because of its fixed ultimate bound water assumption. Early-age ionic release showed faster sulphate depletion and sustained Al release at higher AS1600 contents, while XRD and EDS trends were consistent with enhanced AFm formation and a more Ca-depleted, Al-enriched reacted matrix. Overall, AS1600 should be evaluated as an alumina-rich SCM whose contribution to hydrate formation affects both hydration pathways and the interpretation of conventional TGA based DoH metrics.