Macroscopic and microscopic properties of alkali-activated ultra-high performance concrete at ambient and elevated temperatures: Effects of single and combined curing regimes

This study systematically investigates the influence of various curing regimes (including standard, single and combined methods) on the macroscopic and microscopic performance of alkali-activated ultra-high performance concrete (A-UHPC) under both ambient and elevated temperatures. The objective is to address the challenge of optimizing curing conditions for A-UHPC, particularly in fire-exposed environments where thermal stability and durability are critical yet insufficiently studied in previous works. In this work, standard, single (dry heat, steam, hot water), and combined (dual-stage) curing methods were investigated. Mechanical behavior was evaluated through compressive and flexural strength tests, ultrasonic pulse velocity, and water absorption, while microstructural features were examined using SEM, XRD, FTIR, TGA and MIP analyses. The results suggest that combined curing methods, especially the sequence of hot water followed by steam curing (CC3), tend to enhance early alkali activation, improve matrix compactness, and support better mechanical performance at room temperature. Under high-temperature exposure (up to 800 ℃), CC3-cured samples generally showed more favorable results in terms of structural stability, mass loss, and residual density, although performance varied depending on specific conditions. In contrast, specimens cured by dry heat alone displayed relatively high porosity and lower strength, likely due to insufficient early alkali activation. Overall, the findings highlight that the choice and sequencing of curing regimes may play an important role in improving both early-age and high-temperature behavior of A-UHPC. By demonstrating how single and combined curing enhances both mechanical performance and microstructural stability, this work provides insights for developing durable, fire-resistant alkali-activated materials suited for infrastructure operating under elevated temperature conditions.