Recent interest in environment-friendly materials has led to the use of agricultural by-products as raw material for the production of many construction materials. Because of this the material that used in this study was coffee husk which are residues of coffee processing. Today; these materials are burnt on the field or dumped on land or in rivers. This research was therefore, amid to examine the potential of using coffee husk ash as partial replacement for ordinary Portland cement (OPC) in C-25 concrete production with investigation of optimum ratio of replacement and engineering properties of C-25 concrete. Coffee husk samples were collected from different coffee treatment centers in Zegi and it was carbonized at 550oC temperature for 3 hours in an oxygen deficient environment and its chemical properties were investigated. After that, the pastes containing ordinary Portland cement and coffee husk ash at 5, 10, 15 and 20 % of replacement were investigated with zero percent of coffee husk ash as a control mix. There are 5 different sets of mixes including the control mix used for investigations with a constant water-cement ratio of 0.5 and slump range between 25-50 mm for concrete having a compressive strength of 25 MPa with 5%, 10%, 15%, and 20% cement replacement using coffee husk ash. The test results revealed that the fresh concrete mixtures were decreasing workability with measured slump flow values of 15-35 mm and the initial and final setting times increases in the range of 67-118 minutes and 310-455 minutes respectively. Moreover, the hardened concrete samples obtained decreasing compressive strength results measured in the range of 35.1-22.7 Mpa for the 28th day sample but increases as the curing days increases whiles the water absorption of the concrete increases as the coffee husk ash increases but decreases as the curing days increases due to the porous nature of the coffee husk ash. It was observed that from the compressive strength the concrete would attain its design strength at 10%replacement level. From the microstructure of the concrete studied using Fourier Transform Infrared Spectroscopy and thermogravimetric analysis it was observed that the C-S-H gel occurred between 900-1100 cm-1 and at 500 oC respectively.