Sustainable Self-Compacting Concrete: Investigating the Size Effects of Coal Bottom Ash as a Lightweight Aggregate

Abstract

The mining and processing of coal generate significant waste, often disposed of in landfills, with coal slag heaps in southwestern Algeria forming artificial mountainous structures. This study investigates the potential of reusing coal bottom ash (CBA) as a lightweight aggregate in self-compacting concrete (SCC), fully replacing natural fine and coarse aggregates. The research focuses on the effect of CBA aggregate size on the fresh and hardened properties of SCC. Experimental results show that replacing natural coarse aggregate (NCA) with coarse CBA (CCBA) enhanced self-compactability, while fine CBA (FCBA) reduced workability. The use of CBA resulted in lower dry densities (1920.6–2134 kg/m³), qualifying the mixtures as lightweight SCC according to EN 206-1 standards. Although compressive and flexural strength decreased and porosity increased due to the lightweight and porous nature of CBA, strength values met ACI standards for structural applications. Microstructural examination indicated a rise in porosity within the concrete matrix; however, a limited formation of supplementary calcium-silicate-hydrate (C-S-H) gel was observed, which contributed to partially offsetting the reduction in mechanical strength. This study demonstrates the viability of CBA, particularly as a coarse aggregate, for producing lightweight SCC, offering a sustainable solution for recycling industrial waste and developing eco-friendly construction materials with acceptable structural properties.