Impact of curing conditions on the mechanical properties and micro characteristics of alkali-activated material synthesized from industrial waste soda residue

Alkali-activated materials (AAM) are considered a type of green cementitious material because they are produced without the grinding and calcining required for cement production, significantly reducing carbon emissions. However, their production typically involves the use of commercial alkalis, which are often corrosive, irritating, and expensive. As an industrial waste from sodium carbonate production, soda residue (SR) has a large reserve and is challenging to dewater and utilize, making its disposal a widespread technical challenge. In this paper, considering the alkaline nature and high salt content of SR, a soda residue activated granulated blast-furnace slag cementitious material (SAG) was synthesized directly using undried and unground SR slurry as the alkali activator, with a mass ratio of SR to granulated blast-furnace slag (GBFS) of 30:70. The effects of ambient temperature (AT) curing, water curing, various heat curing temperatures (45°C, 60°C, 75°C, and 90°C), and different heat curing durations (6 h, 12 h, 18 h, and 24 h) on the properties of SAG were investigated. Specifically, the effect of curing conditions on the mechanical properties of SAG was analyzed through unconfined compressive strength (UCS) testing of mortar specimens. The impact of curing conditions on the phase composition and changes in SAG was characterized using XRD, TG, and FTIR, while the effect on the microstructure of SAG was characterized using SEM. The results showed that SAG achieved the highest UCS (3 d/56 d: 28.1 MPa/36.7 MPa) when subjected to heat curing at 75°C for 12 h. The UCS increased significantly by 3022 % and 321 %, respectively, compared to those under AT sealed curing. This was attributed to the fact that SAG under heat curing produced more C-(A)-S-H gels and hydrocalumite crystals, optimizing pore structure and resulting in a denser microstructure. However, exceeding the threshold temperature of 75°C led to an excessively rapid hydration reaction that adversely affected the further dissolution of GBFS and the uniform distribution of hydration products, thereby diminishing the positive effects of heat curing on UCS. In addition, excessively prolonging the heat curing time exacerbated the development of microcracks within the SAG, leading to a deterioration of UCS over age. In this paper, a novel AAM was synthesized using undried and unground industrial waste without the addition of commercial alkali, and the impact of curing conditions on their properties was studied. This broadens the range of applications for SAG, fills a gap in research on the impact of curing conditions on AAM without commercial alkali, and provides a theoretical basis for the low-carbon utilization of industrial waste. • Expanding the application of alkaline industrial waste for the preparation of AAM. • A substantial improvement in the mechanical properties of SAG has been achieved. • The mechanism of the impact of curing conditions on SAG was revealed. • Strength deterioration due to over-extension of holding time was discussed. • The raw materials did not need to be dried and milled.