Refractory castables cured at low temperatures—Spalling risks and testing

Refractory concrete typically protects industrial furnaces from high temperatures. However, explosive spalling can cause severe problems in CAC (calcium aluminate cement) based castables during the furnace’s first drying and heating cycle. This study is motivated by a real-world case of the spalling of biomass boiler cured long before the first heating. It develops a “harsh” laboratory testing protocol to simulate the curing conditions that may emerge in real-world practice and cause unexpected spalling failure. This manuscript presents the results of conventional and low-cement refractory castables hardened for an extended period (120 h) in a moist environment at different curing temperatures: 10 °C, 20 °C, and 35 °C, when different CAC hydrates can form. The prolonged curing period offered causes the densification of the concrete structure due to an increase in the degree of hydration. The results of explosive spalling tests, mass loss rate, and porosity measurements indicate a significant risk of spalling failure in low-cement castable samples cured at 10 °C in such conditions. The study also involves low-cement castables with different contents of polypropylene fibers (ranging from 0 to 0.10 wt%) to demonstrate the sensitivity of the proposed testing protocol, verifying the castable spalling resistance. The practical implications of this research in refractory engineering and construction are significant, as it provides a new approach to testing and improving the performance of refractory castables if a prolonged period is expected before the first heating is realized. • This study investigates calcium aluminate cement-based refractory castables. • These refractories are prone to explosive spalling during the first heating cycle. • A real-case spalling of the biomass boiler cured at 10 °C motivated this study. • A harsh testing protocol is proposed for stimulating these adverse conditions. • It can verify the castable spalling resistance if prolonged curing is anticipated.