Supplemental Cementitious Materials (SCMs) are essential to decarbonizing the cement sector, as they significantly reduce the carbon footprint associated with conventional Portland cement production. However, the industrial by-products such as fly ash and blast-furnace slag from the thermal power plants and the steel industry, which are widely used for SCMs now, are forecasted to become less available due to the decarbonization of their sectors. Therefore, it would be necessary to investigate natural rock powder (NRP), such as natural pozzolan, abundant in reserves, for use as SCMs. This paper aims to clarify various properties of mortar when 15 different NRPs replace 25 wt.% of ordinary Portland cement (OPC), comparing mortars with OPC and fly ash. Fresh properties, compressive strength, chemical reactivity measured by ASTM C 1897 (R3 test), and expansion due to Alkali-Silica Reaction (ASR) measured by ASTM C 1567 in these mortars were experimentally evaluated. The result indicated that the setting time of all mortars with NRP satisfied the limit value defined for fly ash in the Japanese Industrial Standards (JIS) R 5213. Nine mortars with NRP of zeolite, andesite, porphyrite, tuff, granite, sandstone, shale, limestone, and crystalline limestone met the criteria for fluidity and activity index of fly ash for concrete as defined in JIS R6201. Furthermore, almost all mortars with NRP had less expansion due to ASR than OPC. Observations revealed that mortar using NRP with a lower specific surface area tends to have a lower water demand. Additionally, a higher reactivity of NRP was linked to a higher activity index and a lower expansion due to ASR.