Cement production needs a significant energy content, needs several natural reserves including limestone, clay, and water, and is a significant contributor to greenhouse gas emissions. After replacing cement as a binder with by-products of industry like grass granulated blast furnace slag (GGBS) and fly ash (FA), they show good mechanical and durability properties. Total cement replacement with these by-products needs alkali-activated solutions (combination of sodium hydroxide (NH) and sodium silicate (NS)) to activate the reaction process and produce the final product called geopolymer concrete (GC). This study uses sintered fly ash aggregate (SFA) as coarse aggregate and sand as fine aggregate, with a mixture of FA and GGBS, to produce lightweight geopolymer concrete (LWGPC). An investigation has been carried out to determine the adequate solution absorption condition and molarity of NH solution so that LWGPC can achieve optimal compressive strength. Samples were prepared with different solution absorption conditions and, after that, with different molar ratios of NH. Samples were cured in ambient conditions. Compressive strengths (CS) of 3, 7, 28, and 56 days have been determined for the alkali-activated solution to the binder of 0.5, NS/NH 1.5, and a solution content of 207.5 kg/m3. Fresh density was obtained below 2000 kg/m3 for all the samples. A maximum CS of 42 MPa was obtained for 30 min solution absorption and a 12 molar solution of NH. All the LWGPCs satisfy criteria for structural lightweight concrete and can be acceptable for building construction purposes (CS > 17 MPa). Therefore, this proposed mixing using the given proportions can be used to design LWGPC.