Increasing infrastructure development has led to the massive use of commercial Portland cement and natural aggregates, which can threaten environmental sustainability. Commercial Portland cement production produces quite high CO2 emissions, which can exacerbate global warming. As a solution, fly ash waste from coal combustion in steam power plants containing aluminosilicate materials can be used as raw material for geopolymer mortar, which functions as an alternative to commercial Portland cement mortar. Fly ash-based geopolymers can be modified with rice husk ash, a waste product from rice husk combustion because it contains silica, and can also be modified with High-Density Poly Ethylene (HDPE) plastic waste to reduce the use of natural aggregates. Superplasticizers can be added to geopolymers to increase their mechanical strength. This study aims to examine the effect of variations in the addition of rice husk ash, the addition of superplasticizer, and the addition of HDPE plastic waste on the compressive strength and microstructure of the resulting geopolymer mortar. The results showed that the addition of rice husk ash as a substitute for fly ash reduced the compressive strength of geopolymer mortar, where geopolymer mortar with 100% fly ash content had the highest compressive strength at 28 days of 50 MPa. The addition of 1.5% superplasticizer did not relatively affect the compressive strength of geopolymer mortar. The addition of 3% HDPE as a substitute for natural aggregates increased the compressive strength of geopolymer mortar at 28 days by 54.01 MPa. The results of SEM tests on both geopolymer mortar with 100% fly ash content, geopolymer mortar with the addition of rice husk ash, and geopolymer mortar with the addition of HDPE plastic waste showed the formation of a continuous phase of aluminosilicate gel. Meanwhile, the FTIR test on geopolymer mortar with 100% fly ash content, geopolymer mortar with the addition of rice husk ash, and geopolymer mortar with the addition of HDPE showed the presence of dominant Si-O-Si and Si-O-Al functional groups.