Effect of Waste Calcareous Opoka Rock on the Rheological, Physical, and Durability Properties of Thin-Layer Renders for External Thermal Insulation Composite Systems

Waste calcareous opoka rock (WCOR) was investigated as a mass-based replacement for 0.5–1.5 mm granulated dolomite aggregate in thin-layer silicate and mineral renders intended for external thermal insulation composite systems (ETICS). Eight formulations with replacement levels ranging from 0% to 100% were evaluated in terms of fresh-state properties, moisture transport, adhesion, freeze–thaw and sodium sulphate crystallisation resistance, and microstructure using SEM/EDS. Increasing WCOR content reduced fresh-mixture density and flow diameter while increasing resistance to manual surface texturing. In silicate renders, capillary water absorption decreased by 84%, from 1.55 kg/(m2·24 h) for the WCOR-free reference to 0.25 kg/(m2·24 h) at 75% replacement. Conversely, in mineral renders, it increased by 52%, from 17.2 to 26.2 kg/(m2·24 h) at complete replacement. At 100% replacement, freeze–thaw resistance increased from 150 to 308 cycles in silicate renders and from 105 to 251 cycles in mineral renders, while salt-crystallisation resistance increased from 25 to 52 and from 16 to 52 cycles, respectively. Adhesion increased from 0.27 to 0.30 N/mm2 in silicate renders but decreased from 0.31 to 0.19 N/mm2 in mineral renders. SEM/EDS showed matrix-dependent differences in local morphology and aggregate–binder interfaces. Multi-criteria assessment identified 75% replacement as the most balanced option for silicate renders, whereas 25–50% provided the preferred compromise for mineral renders. Overall, the influence and preferred content of WCOR were strongly matrix-dependent.