Engineering Properties of Alkali-Activated Concrete Using Natural Pozzolans and Coal Combustion Byproducts as Prospective Precursors

The use of nontraditional aluminosilicate binders as substitutes for traditional precursors in producing alkali-activated concretes (AACs) has become essential due to the steady decline in the availability of fly ash and ground granulated blast furnace slag. This study examines the engineering performance of AACs utilizing 11 nontraditional aluminosilicate materials, including low-purity calcined clays, ground bottom ashes, volcanic ashes, and fluidized bed combustion ashes. The investigation covers various engineering properties, such as compressive strength, splitting tensile strength, flexural strength, elastic modulus, and drying shrinkage. The findings indicate that these binders show promising potential as precursors for AAC production, with compressive strengths ranging from 30 to 49 MPa at 28 days, comparable to ordinary portland cement (OPC) concrete. However, the splitting tensile strength (10–19% of compressive strength), flexural strength (3.3 to 4.3 MPa), and modulus of elasticity (5–24 GPa) were lower than those of traditional concrete. These properties were compared with prediction models from existing design codes and literature, emphasizing the need for further research to develop accurate models for nontraditional AACs. The significant drying shrinkage, reaching values up to 3,000 με in some of these AACs, could be reduced by 37–43% using existing shrinkage mitigation methods. Nonetheless, as mercury intrusion porosimetry (MIP) revealed, the finer pore size distribution accounts for the higher compressive strength and greater drying shrinkage observed compared to conventional concrete.