Influence of Binder Type and Expanded Perlite Replacement Level on the Residual Performance of Cement Mortars after High-Temperature Exposure

The residual performance of expanded perlite-modified mortars after high-temperature exposure remains insufficiently understood when binder type and replacement level are evaluated together under a water-management procedure suitable for porous lightweight aggregate. This study compares CEM I 42.5 N (R1) and CEM II/B-S 42.5 N (R2) mortars containing 0–40 vol.% expanded perlite under an ISO 834-based heating phase followed by 2 h isothermal holds at 100–1000 °C. Residual strength, mass loss, microstructural observations, and thermogravimetric analysis were used to assess degradation. In expanded perlite-modified mixtures, saturated surface-dry conditioned aggregate was included in the nominal total water budget; therefore, free mixing water decreased with replacement level, and comparisons with the 0% reference mortars include the combined effects of aggregate replacement, saturated surface-dry water management, and superplasticizer omission. Expanded perlite reduced oven-dry density and unheated 28-day strength. A marked degradation threshold occurred at approximately 400 °C, associated with hydrate dehydration and expanded perlite–paste interfacial damage. Above 800 °C, both series showed severe strength loss, with flexural strength more temperature-sensitive than compressive strength. Among modified mortars, the 30% expanded perlite mixture with CEM II/B-S 42.5 N gave the most favorable 400 °C residual performance, retaining 82.1% flexural strength and 79.9% compressive strength, with residual strengths of approximately 4.5 and 29.9 MPa. CEM II/B-S 42.5 N generally provided higher absolute compressive residual capacity, although its retention-based advantage was not universal.