Alkali-activated Fly Ash Manufactured with Multi-stage Microwave Curing

Alkali-activated fly ash (AAFA) manufactured with conventional thermal curing (85oC) shows comparative mechanical properties to Portland cement (PC) based system, demonstrating its potential as a future low-carbon and environmentally friendly alternative cementitious material. However, the environmental benefit which could have been achieved from the low carbon nature of AAFA is often offset by the higher energy consumption from thermal curing. Since microwave heating is usually considered as a low-energy and low-carbon heating method, some pilot attempts have been made in the literature to cure AAFA with domestic microwave ovens. However, due to the lack of control on the temperature and humidity in domestic microwave ovens, splashing of fresh AAFA pastes and microcracks of hardened AAFA are two main reasons causing the unsatisfactory performance of microwave cured AAFA. In this study, a custom-made microwave oven equipped with proportional-integral-derivative (PID) control was employed to cure AAFA samples. The microwave power was automatically regulated through the real-time temperature feedback from the fibre Bragg grating (FBG) sensor embedded in the AAFA sample. The microwave curing regime consisted of four curing stages at four different temperatures, namely 65oC, 85oC, 105oC and 125oC. The similar temperature profile was applied on the AAFA cured in a conventional thermal oven. Compressive strength, reaction products and microstructure of AAFA samples with two curing methods were examined by the analytical techniques of XRD, FTIR, NMR, MIP and SEM respectively. The results showed that AAFA under microwave curing exhibited higher compressive strength with less energy consumption than that under the conventional thermal curing. The microwave curing favoured the formation of Al-rich N-A-S-H gel in AAFA and a more compacted microstructur