Thermal insulation and refractory performance of mortars containing lightweight expanded clay and quarry sand

The aim of this research was to enhance the high-temperature resistance of lightweight mortar by partially replacing 0–30% quarry sand (QS) with expanded clay (EC). Mechanical strength, physical properties and thermal insulation characteristics were investigated after 2 h of exposure to temperatures of 200–800°C. The results showed that an increase in EC content caused a reduction in thermal conductivity up to 30%, thus improving the mortar’s thermal insulation. Larger EC/QS ratios led to higher total porosity, with strong correlations among density, porosity and thermal properties. After heating to 800°C, the mass losses of the cement and EC were 1.87% and 1.08%, respectively, whereas the mass loss of the QS was 8.95%. Although compressive and flexural strength decreased with a higher EC content, the lightweight EC mortar still had better strength than ordinary mortar at high temperatures. Scanning electron microscopy analysis confirmed that EC limited high-temperature damage up to 800°C by reducing visible cracking, cement paste dehydration and portlandite decomposition. An increase in EC reduced violent spalling, enabling the production of lightweight refractory mortar. However, higher porosity increased mass loss and partial carbonation during curing accelerated thermal degradation. Moderate substitution (20% EC) provided the best compromise between lightweight performance and durability, whereas too much EC reduced the resistance to cyclic exposure.