Geopolymer concrete is reported as the best possible alternative to cement-based concrete. However, the limitation in the attainment of structural strength using ambient curing has restricted the use of geopolymer concrete for cast-in-situ construction. Therefore, the present study aims to develop ambiently cured geopolymer concrete using sillimanite sand as a replacement for river sand. The strength, durability, and microstructural properties of this concrete are evaluated. In the investigation, the fine aggregates of geopolymer concrete were replaced with 25%, 50%, 75%, and 100% sillimanite sand by weight. The test results revealed that the 8-M and 16-M molarity levels are feasible to attain the structural strength. Specifically, the 16-M molarity level with 50% replacement of fine aggregate with sillimanite sand exhibited higher geopolymerization. The suggested range of optimum replacement levels of fine aggregate with sillimanite sand is 25%–50%. An increase in percentage replacement beyond 50% indicates a reduction in compressive strength and durability properties. The microstructural analysis indicates dense interconnection between the gel networks due to the presence of sillimanite sand, uniform distribution of elements in the mix, and the presence of strong Na–Al–Si gel. The sillimanite-sand-containing geopolymer concrete developed using ambient curing resulted in a sustainable product along with higher mechanical and durability property.