Fluorgypsum binders with OPC and PFA additions, strength and reactivity as a function of component proportioning and temperature

Cementitious composites based on 50–80% fluorgypsum, 15–50% OPC and 0–15% PFA, were wet and dry cured for up to 180 days at 20 and 60 °C. All pastes developed and maintained strength up to 32 MPa, even under water. Characterization by thermal analysis and X-ray diffraction indicated that gypsum formed intensively in the first days, providing the initial strength jointly with a partial hydration of OPC, incomplete fluorgypsum reactions were observed. Wet curing at 20 °C gradually increased the strength and the nonevaporable water. At 60 °C, wet curing limited the gypsum formation and affected strength development, whereas dry curing resulted in the decomposition of the already formed gypsum and reduced the compressive strength. PFA additions were beneficial in some formulations in late periods, and its reactions were enhanced at 60 °C. The microstructure consisted of gypsum crystals surrounded by calcium silicates, and with localized areas of gypsum surrounding unreacted fluorgypsum.