• A geopolymer coating was designed for 400 − 800 °C by varying FA, NaOH/KOH, and A/P ratios. • Optimizing the alkaline/precursor ratio improved compressive strength at high temperatures. • KOH and 0.55 A/P result in impressive compressive strength, resistance to cracking, and reduced mass loss. • The concrete-coating composite was evaluated for 10 mm and 25 mm overlays. • The 25 mm-thick coated specimen showed significant concrete protection at 400 °C. Concrete cover protects the reinforcement to optimize its performance, but it spalls when exposed to temperatures above 400 °C. Therefore, this work was designed to protect concrete with a geopolymer-based coating that withstands temperatures up to 800 °C. Initially, a fly ash (FA)-based geopolymer coating was developed by varying the Sodium (Na) and Potassium (K) content of the alkaline solution and the alkaline-to-precursor ratio (A/P). Their strength was evaluated at 400 and 800 °C, then the optimized proportion was selected for coating application. Afterward, the coating was applied to the substrate concrete with thicknesses of 10 and 25 mm. After heating the coated specimens to 400 °C, the estimated strength of the encased concrete was compared with the bare concrete strength and the coating strength. Moreover, the cracking and interfacial bonding were also assessed and correlated with the residual strength. The study revealed that K-based activators exhibit good resistance to elevated temperatures, with strength improving by up to 120% at 800 °C. As a coating, it provides a strong barrier against thermal shock to the concrete when the thickness is 25 mm. Even the 10 mm thickness provided protection; it failed, but the concrete’s residual strength didn’t drop significantly.
Daud et al. (2026) studied this question.