Physicochemical, mechanical and optical characterisation of thermochromic gypsum composites

Thermochromic materials are emerging as a durable and effective alternative for adapting to environmental demands and improving interior comfort without raising the building's energy use. This study assessed the impact of organic thermochromic pigments on gypsum composites by examining the effect of pigment additions (1, 3, and 5 wt%) with different transition temperatures and colours. The suitability of the composites was evaluated through optical (reflectance spectra), morphological (XRD, SEM), and physico-mechanical characterisation (bulk density, surface hardness, flexural and compressive strength) to ensure that the pigments' properties remained stable after the gypsum set. The results demonstrate that incorporating thermochromic pigments causes a distinct colourimetric transition in gypsum with temperature fluctuations, producing a hue similar to that of the reference gypsum in its colourless form. Similarly, the pigments retain their integrity and shape when in contact with the gypsum matrix. Additionally, the dihydrate crystals can develop properly, although a slight increase in crystallite size is observed compared to the control sample. Each pigment type induces morphological and microstructural changes in the gypsum matrix, which influence both the physical and mechanical properties and the composites' rates and reflectance, resulting in different thermochromic behaviours. Specifically, the addition of both pigments slightly decreased the bulk density of the composites. Moreover, a 30% reduction in MOE US was observed with 5 wt% additions compared to the reference. This decrease, in turn, affected the mechanical strength, with maximum reductions of around 40% compared to the control sample. Adding the thermochromic pigments at 3% by weight seems to be the most practical solution for manufacturing gypsum-based prefabricated products because of the notable colour contrast, maintained mechanical properties, and the cost-efficiency of these raw materials.