Effect of Untreated Bottom Ash on Mechanical, Thermal, and Shrinkage Properties of Foam Concrete

Fly ash (FA) has been widely recognized as a cementitious material for bulk use in conventional concrete. However, the application of fly ash in foam concrete suffers from high drying shrinkage, poor thermal insulation, and susceptibility to cracking. In contrast, bottom ash (BA), which also is a solid waste, has inherent advantages: it is lightweight, porous, and highly absorbent, and can achieve internal curing effects. This study investigated the use of untreated bottom ash as a partial replacement for fly ash, along with the incorporation of polypropylene fibers (PPFs), while maintaining a constant paste volume. The impact of varying BA content (0%–90% replacement by volume) and PPF (0% and 0.5% by volume) on foam concrete’s workability, density, mechanical performance, water absorption, thermal conductivity, shrinkage, pore structure, and cracking resistance was systematically examined. Results indicate that moderate BA content (60% replacement) enhances workability, reduces shrinkage by as much as 40%, and improves cracking resistance due to its internal curing effect. However, excessive BA content decreases compressive and flexural strength due to increased porosity and reduced hydration reactivity. The inclusion of 0.5% PPF significantly improves cracking resistance, eliminating visible cracks, but slightly increases shrinkage and water absorption due to fiber-induced voids. Microstructural analysis confirms that the optimal BA–FA ratio promotes better encapsulation of BA particles, reducing defects and enhancing matrix integrity. This study demonstrates the synergistic potential of BA and PPF in producing reduce environmental impact, enhanced mechanical properties foam concrete with improved durability and cracking resistance.