Using industrial waste as a construction material reduces the consumption of natural materials and saves the environment. Copper slag (CS) and rice husk ash (RHA) treated with geopolymer have been developed as composite materials. Geopolymer is a good alternative binding material compared to traditional binders (lime and cement). It may be cost-effective and have low carbon emissions. It may contribute to environmental conservation. In this paper, triaxial (unconsolidated undrained) tests have been performed on a developed composite material. Deviator stress at failure (kPa), elastic modulus (MPa), and modified failure envelopes have been examined. Triaxial tests were performed at three different confining pressures, i.e., 50 kPa, 100 kPa, and 150 kPa. Specimens have been tested at different periods of curing, i.e., 3, 7, 14, and 28 days. Results indicate that deviator stress at failure (kPa), elastic modulus (MPa), and shear strength parameters (c and ϕ) increased with increasing the curing period and confining pressure. It is concluded that geopolymer is a good binder for CS and RHA. Developed composite materials may be used as construction materials for flexible pavement (sub-base) layers.