Lightweight Mortars Incorporating LECA, Perlite, and Crumb Rubber: Influence of SBR–PPF Modification on Material Performance

Developing lightweight cementitious materials incorporating recycled constituents while maintaining appropriate functional performance requires a balance between density, mechanical resistance, durability-related properties, and thermal efficiency. This study comparatively evaluates lightweight mortars incorporating expanded clay (LECA), expanded perlite, and recycled crumb rubber, used individually and in binary combinations, and examines the influence of combined styrene–butadiene rubber (SBR) latex and polypropylene fiber (PPF) modification. Ten mortar mixtures were produced, comprising five unmodified mixtures with a water-to-cement ratio of 0.40 and five corresponding mixtures containing 10% SBR and 1% PPF by cement weight, for which the added mixing water was reduced to account for the water supplied by the SBR latex. Flowability, compressive and flexural strength, hardened density, and water absorption were experimentally assessed, while thermal conductivity was estimated from oven-dry density correlations. The mortars exhibited densities between 760 and 1072 kg/m3, confirming their lightweight character. The SBR–PPF-modified LECA–perlite mixture achieved the highest 56-day compressive strength of 8.1 MPa, whereas rubber-containing mixtures generally exhibited the lowest strength levels, a trend consistent with mechanisms reported in the literature, including weak rubber–matrix interaction and increased internal defects or voids. Estimated thermal conductivity ranged from 0.19 to 0.28 W/m·K. The mixtures incorporating SBR and PPF together with reduced added mixing water exhibited improved flexural performance and later-age strength more clearly in LECA- and perlite-based systems than in rubber-dominated mixtures. The results demonstrate that aggregate selection governs the principal performance trade-offs in these lightweight mortars, while the combined modified mixture design can improve selected properties without fully overcoming the detrimental interfacial effects associated with high crumb rubber contents. The investigated mortars may therefore be considered candidates for further evaluation in non-load-bearing lightweight applications. The density-based thermal conductivity estimates provide only a comparative indication of potential thermal behavior; direct standardized measurements are required before thermal insulation performance or suitability for insulation applications can be established.