Influence of alkaline and alkali-free accelerators on strength, hydration and microstructure characteristics of ultra-high performance concrete

Abstract

To broaden the application of ultra-high performance concrete (UHPC) in engineering fields with high-early-strength requirements, accelerators were used to accelerate the setting and hardening of UHPC. This study investigated how alkaline accelerator (NA) and alkali-free accelerator (AS) influence the strength, hydration and microstructure characteristics of UHPC. To achieve the proposed objective, the terms of setting time, strength, released ionic concentration, isothermal calorimetry, X-ray diffraction, scanning electron microscope, energy dispersive X-ray and mercury intrusion porosimetry were performed. The results show that NA and AS significantly increase the setting and hardening rate of UHPC, and their optimal dosages are 2% and 4%, respectively. 2% NA increases the 1 d compressive, splitting tensile and flexural strengths of UHPC by 55.8%, 62.2% and 56.5%, respectively, while 4% AS increases by 10.8%, 21.6% and 21.0%, respectively. However, their later strengths decrease slightly. The first exothermic peak values of NA-2% and AS-4% are about 3 times higher than the reference paste, and their main hydration peaks appear earlier by 38.2 and 23.3 h, respectively. NA and AS contribute to the rapid formation of hydration products and refine the pore structure of pastes.