Effect of Preformed Polymeric Microspheres on the Frost Resistance of Low-Clinker Cementitious Composites with Fine Recycled Aggregate

Achieving adequate frost resistance in cementitious composites made with low-clinker binders remains challenging, as conventional air-entraining admixtures often show limited effectiveness in such systems. This study examines an alternative approach that involves incorporating preformed polymeric microspheres to create a stable air-void system and enhance freeze-thaw durability. Cementitious composites were prepared using a low-clinker binder containing fly ash and ground granulated blast furnace slag (GGBFS) as supplementary cementitious materials, with natural sand partially replaced by fine recycled aggregate derived from concrete waste. The influence of polymeric microspheres on workability, compressive strength, pore structure, and frost resistance was evaluated. Compared to the reference mixture (32.8 MPa), the mortar modified with polymeric microspheres exhibited clearly higher compressive strength-about 25% greater after 28 days-while the AEA-modified mixture showed a slight reduction. Total porosity measured by MIP was 18% for REF, 19% for AEA, and 17% for PPMThe results showed that adding polymeric spheres initially introduced a network of discrete voids that improved the material's resistance to early freeze-thaw cycles. However, due to the prolonged hydration of the low-clinker system, hydration products progressively filled the initially created voids after the partial degradation of the polymeric spheres. Consequently, the air-void system gradually disappeared, leading to a loss of frost resistance at later ages. After 100 cycles, the PPM mixture exhibited a 75% loss in flexural strength and a 35% loss in compressive strength, whereas the AEA mixture retained its durability, with compressive strength loss limited to 6%. This finding suggests that, although early tests may indicate improved performance, the long-term durability of low-clinker cementitious composites incorporating fine recycled aggregate cannot be reliably enhanced by preformed polymeric spheres alone.