•Investigating the effect of high-volume WCP on the properties of sustainable recycled mortar.•Utilizing thermal modification to improve the performance of mortar with high-volume WCP.•Suggesting the optimal thermal modification temperature for WCP and its mortar. Utilizing waste concrete powder (WCP) to prepare eco-friendly cementitious materials provides an effective approach to recycling concrete waste, and thus, this work focuses on the properties and modification of sustainable recycled mortar including high-volume WCP. The WCP with an irregular microstructure consists of abundant inert components, while the thermal modification treatment on WCP promotes the formation of new active components. Incorporating untreated WCP decreases the hydration products and increases the pore size, but the thermal modification on WCP benefits the micro-properties of newly-prepared paste. The addition of WCP prolongs the setting time, while the thermal modification on WCP up to 900 ℃ shortens the setting time; besides, the mortar with WCP after 1200 ℃ activation has excellent fluidity and shrinkage resistance. The mechanical strength decreases and water transport increases following the growing substitution rate of WCP, and an obvious degeneration appears as high-volume WCP incorporates. At the identical replacement ratio of WCP, the properties of mortar with 300–900 ℃ thermally-modified WCP are superior to those of mortar including untreated WCP. For instance, the compression strength and capillary absorption coefficient of mortar including 50% WCP after 900 ℃ activation are 26.4% higher and 46.1% lower relative to mortar including untreated WCP. However, there is hardly apparent improvement on the performance of newly-prepared mortar including WCP modified by 1200 ℃.