Temperature and accelerator effects on microstructure and strength development of sprayed concrete containing high volume ground granulated blast furnace slag

The performance of sprayed concrete (SC) is significantly influenced by tunnel temperatures, particularly when incorporating Ground Granulated Blast Furnace Slag (GGBS), which despite its accepted sustainability benefits is often avoided due to its lower early-age strength. The combined effect of temperature and accelerators on GGBS activation also remains underexplored. This study therefore evaluates the effectiveness of a conventional SC liquid accelerator (LA) based on aluminium sulphate and a novel powder alkali-free accelerator (PA) based on calcium aluminate and calcium sulphate in activating (at an early age) SC containing high volumes of GGBS. Cementitious pastes and mortars with low (35 %) and high (70 %) GGBS replacement levels were tested under standard (20°C), and elevated curing temperatures (35°C, and 60°C) to assess their effect on microstructure and strength development. Full-scale spraying trials compared high-volume GGBS SC using PA to conventional SC with 100 % CEM I and LA. Tests included X-ray diffraction (XRD), scanning electron microscopy (SEM), ultrasonic pulse velocity (UPV), water accessible porosity (WAP), and compressive and flexural strength. Results showed that the LA enhanced early age strength in 35 % GGBS mixes, while the PA was more effective in 70 % GGBS mixes. Spraying trials confirmed that 70 % GGBS SC achieved 23 % and 8.5 % higher compressive strengths at 1 and 28 days, respectively, compared to conventional SC under standard curing (20°C). These findings demonstrate that, high volume GGBS SC, when combined with a suitable accelerator, is well suited for demanding tunnel environments with elevated temperatures, offering early strength gain, improved later-age performance, and a more sustainable approach to SC construction. • Powder accelerator enhances early-age activation of high-volume slag mixes, outperforming conventional liquid accelerator. • Elevated temperatures affect strength development through phase transformations; curing at 60°C reduces later-age strength. • 70% slag sprayed concrete demonstrates superior later-age strength in high-temperature tunnels, supporting sustainable tunnelling.