<p>Energy infrastructures typically generate significant amounts of heat, which jeopardizes the stability and long-term performance of structures. Current buffering materials, primarily sand-bentonite or bentonite mixtures, aim to mitigate these effects, but have limitations, particularly in terms of sustainability, cost, and long-term thermal stability. This study addresses the urgent need for advanced, thermally resistant, environmentally friendly, and cost-effective alternative buffer materials. The thermal efficiency, fracture mechanics, and toughness of sand (S)-bentonite (B) mixtures modified with calcium carbonate CaCO₃ additives – a sustainable and abundant material – have been investigated for the first time through this research. Thermal efficiency and fracture toughness values were determined at room and various elevated temperatures. Additionally, high-resolution visual examinations were conducted to observe material integrity and structural changes under thermal stresses. The test results showed that the S20 and S30 blends exhibit more stable and predictable thermal performance at 55 °C than pure sand at high bentonite content. Results showed that the S20 and S30 blends exhibited more stable and predictable thermal performance at 55 °C than pure sand at high bentonite content with 10 and 15% CaCO₃. Fracture test results suggest that thermal energy may slightly enhance the ductility or bonding efficiency of the CaCO₃ cementation between sand grains. Mode I fracture toughness (K_Ic) and fracture energy (G_f) increase with CaCO₃ content in sand-rich (90/10) composites. Conversely, for the bentonite-rich (70/30) mixture, increasing the temperature to 55 °C tends to decrease or maintain the toughness values, particularly at higher CaCO₃ concentrations.</p>