This study investigates the compressive strength of ordinary Portland cement mortar prisms with rice husk ash (RHA). Pit sand–cement, and sea sand-cement mortar prisms were separately prepared using cement-sand mix ratio 1:3, and RHA composition of 0–44.44%. The optimum strength of the pit sand-cement mortar prisms after 1, 2, 7, and 28days was recorded at 11.11% RHA by the mass of the cement. However, the compressive strength of the sea sand-cement mortar prisms with 11.11% RHA decreased on the 7th and 28th day. Thus the inconsistencies in the compressive strength indicate that, sea sand is not a good candidate for constructions.