Effect of Na versus Ca Sulfate Salts on the Hydration of Calcium Sulfoaluminate Clinker

This paper examined how the amount (5% or 20%) and type (CaSO₄ or Na₂SO₄) of sulphate salt affect the hydration of calcium sulphoaluminate clinker (KCSA). The mechanical behavior of the pastes was determined at 1, 3, 28, and 90 days, the heat flow and total heat were measured with isothermal conduction calorimetry, and the reaction products were characterized using X-ray diffraction (XRD), differential thermal analysis/thermogravimetry (DTA/TG) and scanning electron microscopy (SEM). The results obtained indicated that both the amount of sulphate salt (5% or 20%) and its type (CaSO₄ or Na₂SO₄) affect the hydration kinetics, type of reaction products formed, and development of mechanical strength. The incorporation of CaSO₄ has a positive effect on the development of the mechanical strength of KCSA. However, Na₂SO₄ can also produce adverse side effects at older ages. The presence of Na₂SO₄ increases pH values, which partly destabilizes the ettringite formed, thereby favoring carbonation and thenardite precipitation, which can cause the specimens to crack and break.