Rapid Immobilisation of Chemical Reactions in Alkali-Activated Materials Using Solely Microwave Irradiation

Efflorescence is a major concern in alkali-activated materials (AAMs), affecting their potential practical applications and challenging the preservation of the material in a time-frozen state for post-characterisation purposes. Efflorescence is a time-dependent phenomenon that may stop with complete water removal. Therefore, this study mixed chemically and physically different secondary raw (slag, fly ash, glass wool, and rock wool) and nonwaste (metakaolin) materials with Na-silicate solution in ratios that facilitated efflorescence and mixtures that could prevent it. The material was cured at 40 °C for 6 days. On the 7th day, half of the intact samples were additionally treated with low-power microwaves (2.45 GHz at 100 W) until complete dehydration to rapidly and gently influence the chemical reaction. The impact of microwave-irradiation-induced dehydration on the specimens was thoroughly examined on day 7, both chemically and physically, and compared with that of a set of samples that were not subjected to dehydration. Chemical alterations were examined using Fourier transform infrared spectroscopy, which revealed the removal of water. Further, X-ray diffraction revealed no changes in crystallinity upon irradiation. The irradiation-increased porosity was evaluated through Mercury Intrusion Porosimetry, whereas the mechanical resilience of the materials was evaluated through compressive strength tests and synchronised with the porosity development. Pores in the irradiated samples that required higher temperatures or longer curing times (glass wool, fly ash, and metakaolin) were spherical; otherwise, dehydration resulted in elongated cracks. This result provided side knowledge on the determination of finalisation of the curing of AAM, which cannot be determined by any surface-penetrating test. Efflorescence formation was evaluated on 1-year-old dehydrated samples, maintained at room temperature, before and after exposure to a water droplet using a low-vacuum scanning electron microscope. This explained the reason why the surface of AAMs exposed to water became slippery and the effect of the water droplet on the surface. The microwave treatment method successfully accomplished the complete dehydration of AAMs within a few minutes, effectively inhibiting reactions in the AAMs, terminating further development of the aluminosilicate network, and partially removing unbonded alkali elements from the chemically unfinished AAM system. Therefore, microwave irradiation can be used to evaluate the chemical compositions of time-evolving materials during dehydration. If the dehydrated samples are stored in a waterless environment, post-characterisation of the AAMs is possible at any later time. The procedure of stopping the reactions offers significant advantages over conventional techniques which require physical processing, such as grinding or milling, the use of solvents, and more time.