Hydration, microstructural development and dimensional stability of cementitious materials made of portland cement clinker and fluoroanhydrite

The production of Portland cement accounts for 5 to 8% of the global anthropogenic CO2 emissions. In the past years, the cement industry has attempted to reduce this contribution by the use of mineral additions on the cement or in the concrete mix. The typical additions in cement are materials with pozzolanic properties such as fly ash, silica fume, etc. Other attempts have been done using materials obtained as byproducts from other industrial processes. This is the case of the chemical gypsums i.e. calcium sulfates. Calcium sulfate can be obtained in the production of several acids such as boric (borogypsum), hydrofluoric (fluorgypsum or fluoroanhydrite), phosphoric (phosphogypsum), etc. or in desulfurization process in coal burning power plants. In this work, the reaction mechanisms of a binder made of 60% fluoroanhydrite and 40% Portland cement clinker have been studied. This binder is currently being produced in Mexico in the preparation of aerated concrete mixes intended to be used in low-cost housing. This work aims to gain insight on the mechanisms of reaction of the binder made of fluoroanhydrite- Portland cement clinker on the one hand and the dimensional stability aspects related with this binder and its application in the construction industry on the other. Model mixes prepared with different proportions of materials were also studied to understand the effect of the fluoroanhydrite on the properties of the binder. The hydration kinetics of the binder was studied by means of isothermal calorimetry and chemical shrinkage. The microstructural development was studied by a combination of techniques such as SEM, XRD, mercury intrusion and others. The dimensional stability of the binder was approached mainly by length change tests carried under different conditions. As this product is a calcium- sulfate based binder the results under high relative humidity conditions were highlighted. The reaction kinetics of the binder were found to be affected by the impurities present in the fluoroanhydrite; hydrofluoric acid traces particularly retard the hydration of both fluoroanhydrite and the clinker. Other aspects such as aging and the ethanolamine-based grinding agent used on the production of the binder were found responsible of the delayed reaction of the binder compared to Portland cement clinker. The hydration of the binder in water immersion is not very different from that observed under medium relative humidity; however, the dimensional stability of the binder is importantly affected by this parameter. The composition of the binder was found to play an important role.