Dielectric Properties and Numerical Modelling of Microwave Heating of Portland Cement/Fly Ash Blends

Due to its volumetric heating mechanism, microwave is increasingly being recognized as a potential alternative low-energy heating technique for curing cementitious products. However, its heating capacity, among other factors, heavily relies on the dielectric properties of the raw materials, which, in turn, can be affected by moisture content, temperature and the degree of hydration. Unfortunately, current understanding on these issues is still very limited. Consequently, it is difficult to predict the heating profile of the cementitious products heated in microwave, even though a good knowledge of this is essential to optimize the microwave operation condition in order to ensure the quality of cementitious products. In this paper, the dielectric properties of Portland cement/fly ash blends were investigated during a 24-hour hydration period under 25oC, 40oC and 60oC at a frequency of 2.45GHz. The Portland cement was replaced by fly ash at the levels of 0% and 55% by weight at water-to-binder ratios of 0.25 and 0.35, respectively. Setting times were also measured in order to interpret the changes of the dielectric properties at different stages of hydration. Based on the obtained dielectric properties data, heating models were developed using two different software packages, namely, ANSYS and COMSOL, which were then validated by the heating profiles obtained in a tailored microwave system using FBG sensor and thermal imaging techniques. It showed that both simulated results overestimated the real temperature within the sample. However, COMSOL can give a better prediction of the overall temperature profile, whilst ANSYS is more reliable in predicting single temperature. Further investigation is still needed in order to identify which modelling technique is the best option for predicting both the temperature profile and the temperature within cementitious materials cured under microwave