The Effect of Waste Concrete Powder and Polypropylene Fiber on Geopolymer Foam Concrete with Base Ash Aggregate

This study investigates the mechanical, thermal, and durability performance of sustainable one-part alkali-activated foam concrete (AAFC) mixtures incorporating waste concrete powder (WCP) as a partial replacement for ground granulated blast furnace slag (GBFS), polypropylene fibers (PPF) as reinforcement, and bottom ash (BA) as the sole fine aggregate. Ten AAFC mixtures were designed by substituting GBFS with WCP at 10%, 20%, and 50% by binder weight and incorporating PPF at 0%, 0.5%, and 1% by weight. Sodium metasilicate powder served as the solid activator to ensure one-part activation, while a constant water-to-binder (W/B) ratio of 0.4 was maintained. The mixtures were subjected to both ambient and water curing regimes for up to 91 days. Experimental evaluations included flowability, compressive and flexural strength, oven-dry density, thermal conductivity, water absorption, sorptivity, drying shrinkage, high-temperature resistance (up to 900°C), and freeze–thaw durability. Results revealed that increasing WCP content reduced workability and mechanical strength due to its lower reactivity and higher porosity. However, the inclusion of 1% PPF significantly compensated for these drawbacks, enhancing strength, dimensional stability, and thermal resistance. The optimum performance was achieved in mixtures containing 10–20% WCP with 1% PPF, showing up to 217% improvement in compressive strength compared to WCP-only systems. Additionally, bottom ash contributed to reduced density and improved thermal insulation but slightly compromised strength due to its porous and angular nature. Microstructural analysis (SEM) confirmed the densification of the matrix in fiber-reinforced systems, especially under water curing, which facilitated gel formation and improved the interfacial transition zone. Strong correlations were observed between compressive strength and key durability indicators such as porosity, water absorption, and sorptivity (R² > 0.90). The findings suggest that incorporating WCP and PPF in AAFC with bottom ash aggregate offers a viable pathway for developing lightweight, low-carbon, and durable construction materials, promoting circular economy principles and reducing environmental impact.