Recycling of Chemical Industry Waste in Cementitious Mortar: Mechanical Performance and Durability Evaluation of Anhydrous Calcium Sulphate-Based Mixes

The chemical industry’s by-products offer a viable alternative to natural raw materials, presenting an opportunity to advance circular economy practices that are beneficial for both industrial processes and environmental sustainability. This study explores the innovative application of anhydrous calcium sulphate (ACS), a by-product from the hydrofluoric acid production process, as a substitute for fine aggregate in cementitious mortars, diverging from its traditional use as a cement replacement. Integrating ACS into mortar mixes reduces the extraction of non-renewable natural sand, thereby mitigating environmental impact. Four different mortar mixes with varying proportions of ACS substitution for cement and fine aggregates were prepared, and their properties were compared with those of a standard control mix devoid of ACS. The findings reveal that while all modified mixes maintained adequate consistency and mechanical strength suit- able for structural applications, the freeze–thaw durability was markedly reduced, correlating with higher ACS content. This investigation underlines the potential of ACS-enhanced mortars to contribute to sustainable construction methods, highlighting the need for further research into optimising mix design to balance performance with environmental gains.