Cement production harms the environment in many ways. Various industrial by-products like grass-granulated blast furnace slag (GGBS), silica fume (SF), and fly ash (FA) utilisation are also necessary to reduce the effect on the environment. Using industrial by-products in making cement-free mortar significantly decreases the harmful effect. These are highly rich aluminosilicate materials. They need an alkali solution to activate the binding process, mainly a combination of NaOH and Na2SiO3, and start the polymerisation process. This study uses various combinations of these materials and the property enhancer material metakaolin (MK). An investigation has been conducted to determine the adequate proportion of FA, GGBS, MK, and SF to determine the maximum compressive strength of geopolymer mortar. Samples were prepared with different proportions of materials and cured in ambient conditions. FA, GGBS, MK and SF proportions are 50–60%, 20%, 10–15%, and 10–15%, respectively. Compressive strengths of 3, 7 and 28 have been determined for the alkali-activated solution to the binder of 0.5, and the ratio of Na2SiO3/NaOH has been kept at 1.5 and 16 molar NaOH solution used in making mortar. A maximum compressive strength of 43 MPa was obtained for a combination of 20% GGBS, 15% MK, 10% SF, and 55% FA. This study effectively describes the compressive strength variation of different combinations of by-product materials.