# Novel Processing Methods of Low-Clinker Multi-Component Cementitious Materials--A Review
## 4 Microwave Treatment (MW-T) Methodology
### MW-T of Cementitious Materials/Concrete and SCMs
The electric field concentration in the center and surface heat loss from convective heat transfer between the air and the surface are responsible for the observed trends of the lowest temperature at the edge and the highest temperature near the center. A small variation might be ascribed to the unequal composition of the concrete in the cross-section for various specimens. Furthermore, as opposed to rapid MW-T, the delay in heat conduction also increases the temperature differential between the center and the surface. The overall temperature was higher in the heating scheme with the higher maximum temperature. In light of this, a higher temperature of the concrete structure on average will arise from a higher maximum temperature. Because of the wide variances in their dielectric constants, which cause differences in their thermal responses, the microwave heating sensitivity of aggregate and cement paste are likewise highly different from one another. Separating the aggregates and mortar in concrete happens instantly once the microwave power reaches a particular level. This feature can be very useful for the beneficiation of the recycled concrete aggregate because its adverse effects on the ST of concrete can be attributed to the excessive content of the attached mortar [133]. Concrete mortar-aggregate separation using MW-T outperforms conventional heat treatment techniques [119; 128; 129; 131]. Since it can increase the amount of electromagnetic energy converted to thermal energy, the temperature difference at the mortar-aggregate interfaces can increase the microwave input power [104; 105; 106; 107; 108; 128; 129; 130; 131]. When MW-T is applied to freshly laid concrete, water is removed, capillary pores close, and the material becomes denser. Although most microwave radiation is used to cure concrete, the radiation period is typically lengthy (more than 1 h with high power) [134]. In terms of effective temperature control, the occurrence of cracks owing to shrinkage, the surface  
Figure 11: The porosity of mortar under different curing regimes (with permission from [129]). condition of concrete after curing, economic efficiency, and CO\({}_{2}\) emissions, microwave heat curing performed superiorly to traditional steam curing [135]. However, verification and augmentation based on actual data are required to produce environments suitable for the varied sizes and shapes of forms. In their investigation of the impact of MW-T as a substitute for steam curing on the preparation of concrete, [PERSON] et al. [135] found that using microwaves can result in excellent performance in terms of preventing concrete bleeding, preventing surface separation after MW-T, reducing CO\({}_{2}\) emissions, and saving energy costs when compared with conventional preparation systems (Figure 13). Figure 12: Cross-sectional temperature profile measured at various MW-T in the diagonal direction (with permission from [132]). Figure 13: Comparison of traditional steam curing with MW-T (open access status) [135]. The effect of MW-T on the production of concrete and mortar was studied by [PERSON] and [PERSON] [136]. They developed an effective process for concrete (w/c = 0.33) that produced unexpected results in terms of 4.5 h fc reaching 35.4 MPa and the 7-day strength being 57.0 MPa (in the case of w/c = 0.40, the strength reached 29.5 MPa at 4.5 h and 48.0 MPa at 7 days). Applying a microwave could remove free water while the slurry is still in its plastic state, reducing the water-cement ratio of the mixture, which can make up for the strength loss during the heating process [136; 137; 138]. Microwave pre-curing may also reduce the porosity of mortar and increase its fc. MW-T can potentially be used in various building processes, such as concrete pre-casting and repair, to obtain extraordinarily high early strengths while conserving much energy. The number of CH crystals, the size of the CH face (001), and the preferred orientation of CH all decreased as a consequence of microwave curing, as found by [PERSON] et al. [139].