Fast setting and high early strength alkali-activated fly ash synthetized with pre-polymerized suspension combined with ultrafine fly ash at ambient temperature

Alkali-activated fly ash (AAFA) can offer both significant performance benefits and environmental advantages in corrosive and high-temperature environments. However, the slow setting and limited early strength development of AAFA at ambient temperature strict its use to pre-cast elements that must be cured using elevated temperatures. In order to address this limitation, this experiment pursued a distinctive approach to produce a pre-polymerized suspension (PPS) using ultrafine fly ash (UFA) that was activated with NaOH, thereby generating numerous nano-scale zeolite crystals and amorphous N-A-S-H gels. PPS was used alongside UFA in conjunction to create AAFA. Combining PPS in the AAFA mixture, while cutting UFA usage by 50%, led to a 30-minute initial setting time and a 15 MPa compressive strength after 3 days, which was significantly improved compared to the nearly 3 days required by the control sample to initially set. This remarkable result was attributed to the accelerated dissolution of active Si/Al ions from UFA, as well as the nucleation template effect of nano-scale zeolite and N-A-S-H gels within the PPS that speeded up the product's polyreaction, and resulted in a dense and quickly developed hardened structure. By using PPS, the dependency on UFA and its accompanying high energy consumption was dramatically reduced. Additionally, the fast-setting and high early strength properties of the optimized AAFA paste make it ideal for in-situ construction, thereby extending the usage of fly ash resources.