The urge to find alternative material to existing conventional ones and the need to bring down the cost of production have compelled researchers in Nigeria to intensify work on alternative local materials to cement in concrete and sandcrete production. The aims are to investigate their usefulness wholly as construction materials or partially as substitutes for cement. Sawdust Ash (SDA) is a construction-waste material found in abundance in Nigeria. The effective utilization of this material as a component in sandcrete block depends on the mix proportioning of the various component materials. A mathematical model was developed in this study and used to optimize the mix proportion that will produce maximum compressive strength of cement/sawdust ash sandcrete blocks using Scheffe’s four component second degree simplex lattice approach. The mathematical model compared favourably with the experimental data and the predictions from the model were tested with the student t-test and found to be adequate at 95% confidence level. The optimum value of compressive strength predicted by the model is 2.556 N/mm2. This strength corresponds to a mix ratio of 0.601: 0.94: 0.06: 5.97 (water: cement: sawdust ash: sand). With the model developed in this study, any desired strength of cement/ sawdust ash sandcrete block produced from ordinary Portland cement, SDA, water and sand mix proportions within the factor space of the simplex used herein can be evaluated and vise versa.