Production, Characterization and Durability Assessment of Sintered Fly Ash Aggregate from Kyzylorda By-Product Hydraulic Ash for LightweightCementitious Composite

The growing accumulation of coal combustion by-products necessitates the development of sustainable approaches for their utilization in construction materials. This study investigates the production of sintered fly ash aggregate (SFAA) using hydraulic ash waste from the Kyzylorda Combined Heat and Power plant and evaluates its suitability as a coarse aggregate for lightweight cementitious composite. Hydraulic fly ash and clay from the Talsuat deposit were pelletized and sintered at 1100 °C. The physicochemical, mineralogical, and microstructural characteristics of the raw materials and produced aggregate were examined using X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The developed aggregate exhibited a bulk density of 1118 kg m−3, water absorption of 5.4%, crushing strength corresponding to grade M200, and frost resistance of at least F35. Mineralogical analysis revealed quartz and mullite as the predominant crystalline phases, while microstructural observations confirmed the formation of a stable porous aluminosilicate matrix. Chemical durability tests in alkaline, chloride, and sulfate media demonstrated high resistance to aggressive environments. Lightweight cementitious composite produced with the aggregate achieved an average density of 1657 kg m−3, compressive strength of 3.87 MPa, and water absorption of 16.0%, corresponding to density grade D1600 and strength class B3.5. The results confirm the feasibility of converting hydraulic ash waste into a durable lightweight aggregate suitable for structural-insulating lightweight cementitious composite, contributing to waste valorization, conservation of natural resources, and sustainable construction practices.