Activation of fluorogypsum with calcium hydroxide and aluminium sulfate

A composite activator, comprising calcium hydroxide and aluminium sulfate, is formulated to activate the hydration reactivity of fluorogypsum. The effects of the composite activator on the setting time, strength, hydration rate, and microstructure of the fluorogypsum paste are investigated with pure fluorogypsum paste as a control. With the increase of the content of aluminium sulfate, the setting time and compressive and flexural strengths of fluorogypsum are shortened significantly at first and then gradually prolonged, and the compressive and flexural strengths present the trends of first increasing and then decreasing. The fluorogypsum paste with the composite activator of 1.3% slaked lime and 2% aluminium sulfate octadecahydrate has the shortest setting time and highest strengths, when 28-day compressive and flexural strengths are improved by 112% and 174.5%, respectively. The composite activator greatly accelerates the transformation of anhydrite into dihydrate gypsum by forming ettringite, which promotes the formation of prismatic dihydrate gypsum crystals at early curing ages and layered dihydrate gypsum crystals at long-term curing ages. Moreover, the incorporation of the composite activator reduces the porosity of the hardened fluorogypsum paste while concomitantly increasing the average pore size.