Abstract: The study investigated the effects of construction waste powders exposed to elevated temperatures on the properties of cement mortar. The waste powders were obtained from demolished granite and clay blocks after more than 15 years of their service life. The exposure temperatures were 200 °C, 400 °C, 600 °C, and 800 °C. The heat-treated and untreated waste powders replaced cement in mortars at 10% and 20% by weight. The use of untreated recycled granite and clay powders adversely affected the mechanical strength and transport properties of the cementitious mixtures. On the other hand, the effects of thermal exposure varied for the two powder materials. As the exposure temperature increased, the performance of mixtures containing granite powder gradually deteriorated, while mixtures containing clay powder improved. The efficiency was both worst and best at 800 °C. For instance, a mixture containing 10% clay powder treated at 800 °C exhibited compressive strength equivalent to the reference mortar with no waste material. In contrast, under the same conditions, the compressive strength of the mixture containing granite powder was 33% lower. These results indicate that identifying the type and characteristics of recycled materials is essential for their utilization and application of enhancement methods