Chemical activation of kaolin-based clay bricks as a sustainable route to enhanced mechanical and thermophysical properties

This work examines the usage of various acid activators, including hydrochloric acid (HCl), sulphuric acid (H2SO4), phosphoric acid (H3PO4), and a combination of them, to improve the thermophysical properties of fired clay-based composite bricks modified-kaolin. To enhance these materials’ insulating capabilities, the main focus is on reducing their diffusivity, specific heat capacity, and thermal conductivity. Whereas, fibrous clays’ surface and catalytic properties were enhanced chemically via the addition of acid-activated kaolinite clay. The mechanical, thermophysical, morphological, shrinkage, density, porosity, microstructure, and shrinkage of each clay–kaolin(acid) composite were thoroughly examined, and the thermal conductivity performance was maximized. All of the peaks’ intensities in the XRD pattern increased in comparison to the untreated peak when varying acid types were added to the kaolin matrix. In the meantime, the addition of these activators caused the compositions’ apparent porosity (i.e., 29.15–29.47%) and compressive strength (i.e., 11.59–12.33 kg/cm2). The results demonstrate that treatment with these acids reduces thermal conductivity (i.e., 0.46–0.44 W/mk), and diffusivity, attributed to the increased porosity and altered microstructure of the bricks. Moreover, combining all three acids (H2SO4/HCl/H3PO4) resulted in the most significant improvements, yielding a composite with superior insulation capabilities. The aforementioned observations are attributed to the formation of two key mineral phases within the fired bricks: mullite and diopside. Mullite strengthens the bonding within the aluminosilicate framework, thereby enhancing the ceramic network and promoting a denser and more mechanically stable microstructure without causing a significant increase in porosity. Meanwhile, diopside also contributes to strength development and is widely recognized for its role in insulation ceramics due to its excellent thermal stability and chemical corrosion resistance.