This study was conducted to determine the effect of the concentration of basalt as a cement substitution material in mortar. Basalt was analysed by XRD and XRF. From the XRD results, the diffractogram shows the dominant phases, such as anorthite, augite, forsterite, and quartz or silicon oxide. SiO2, Al2O3, Fe2O3 are the main components in basalt, as can be seen from the XRF. With the concentration at 5% of the basalt before the calcination, the optimum compressive strength of the OPC and PCC

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... ment mortar was obtained. The highest compressive strength of the OPC cement mortar was 24.97MPa with a porosity of 2.4% and absorption of 1.29 %. Furthermore, the highest compressive strength of the PCC cement mortar was 22.55MPa with a porosity of 4.8% and absorption of 2.52 %. This result indicates that the substitution of composite cement with a basalt with a concentration of 5% can increase the compressive strength of the mortar.