Abstract The waste foundry sand was alkaline activated into geopolymers to reduce their environmental pollution. The waste foundry sand was blended with aluminium slag and alkali activated to produce geopolymers. Aluminium slag was used to supplement WFS with alumina because as it has low content of alumina. The main parameters tested were the effect alumina content, concentration of alkali, liquid to solid ratio, and temperature. The optimum synthesis parameters were a geopolymer prepared with 2% aluminium, 3 M NaOH solution, liquid solid ratio of 0.15 and curing temperature and time of 80°C and 5 days respectively. The optimum parameters yielded the highest UCS of 4.7 MPa. The developed aluminium slag modified waste foundry sand geopolymers met the minimum requirements for ASTM C126-99 and ASTM C216-07a.