Mechanical and Microstructural Performance of Different Plaster Mortars Used to Protect Interior Concrete Exposed to...

The physical, mechanical, and microstructural degradation that occurs in concrete elements exposed to high temperatures constitutes an important research topic in terms of the fire safety and service performance of structures. In this study, the behavior of plaster mortars produced with cement-based and ground granulated blast furnace slag (GGBFS)-based binders, as well as normal concrete (interior concrete) specimens coated with these mortars, under high-temperature exposure was experimentally investigated. After the prepared mortars and plastered concrete specimens were exposed to temperatures of 100, 300, 500, and 700 °C, changes in compressive strength, ultrasonic pulse velocity (UPV), water absorption, mass loss, and bulk density were evaluated. Analysis of the experimental results showed that high temperatures, particularly 300 °C and above, caused significant performance losses in all binder systems. Although calcium aluminate cement-based mortars developed high early-age strength, they exhibited more pronounced strength losses under elevated temperatures, whereas geopolymer-based binders demonstrated more stable performance at low and medium temperatures. Additionally, a strong correlation (R² ≈ 0.89) was observed between UPV and compressive strength in the plastered interior concrete specimens. At 500 and 700 °C, the plastered concrete specimens exhibited 9.8-13.3% and 10.4-18.1% higher residual compressive strength, respectively, compared with the unplastered control specimens. At 700 °C, the PC-based plaster provided the highest improvement in residual compressive strength (18.1%), while the G-Na system exhibited the lowest water absorption, which was 19.5% lower than that of the control concrete. Plastered concrete specimens retained their mechanical and physical properties better than the control (unplastered) concrete specimens at all temperature levels, and this finding was further supported by microstructural analyses. The results indicate that plaster systems produced with different binders are effective in limiting thermal damage to the interior concrete.