# Novel Processing Methods of Low-Clinker Multi-Component Cementitious Materials--A Review
## 4 Microwave Treatment (MW-T) Methodology
### MW-T for Recycled Aggregates  
The MW-T of concrete has the potential to provide a brand-new, extremely effective, and energy-efficient method of recycling aggregates. Microwave-assisted aggregate recovery from saturated concrete can be more effective and efficient, with significantly less performance loss [152]. Increasing the water content of crushed concrete could produce better aggregate recycling outcomes for industrial applications. Better recycling results can be obtained by watering or purling on crushed concrete first, then moving it to microwave heating equipment to begin the separating process (Figure 14). In comparison to the traditional mechanical separation procedure or the microwave heating method, the approach was more practical and convenient for the environment.  
For concrete, temperature-gradient-induced cracks began in the specimen's middle region and spread to the specimen surface, and the beginning and growth of cracks induced by temperature gradients were dependent on the temperature difference [103]. Due to the differences in properties between aggregate and mortar, cracks were formed along the interface between the mortar matrix and the aggregates. Based on the outcomes of the experiment, higher microwave input does not result in a larger temperature difference between mortar and aggregates; however, it results in more severe interface damage (Figure 14). The size and strength of the concrete will also impact the outcome of the aggregate recovery. The microwave heating cavity's feature can be more suitable for concrete with particular proportions. It still needs to be clarified how to choose the most efficient heating parameters when other concrete and aggregate qualities are considered. The proper heating conditions are required to achieve a better separation effect while minimizing aggregate property damage.  
The effectiveness of MW-T for the surface modification of recycled aggregates was studied by [PERSON] et al. [153]. The surface-modified coarse aggregate (SMCA) and the aggregate interface were heated differently to over 100 \({}^{\circ}\)C thanks to MW-T (Figure 15),  
Figure 14: Diagram of concrete’s microwave heating system (with permission from [103]).  
which enables selective heating. Based on this finding, it was revealed that microwave heating successfully heated the iron oxide blended with the SMCA. The temperature of SMCA concrete containing iron oxide, a dielectric substance, increased higher than it did for original coarse aggregate (OCA) concrete when heated with microwaves. The concrete started to microcrack, particularly when it was heated for longer than 180 s, and the highest temperature was over 400 \({}^{\circ}\)C.  
This was accompanied by an increase in void size distribution brought on by a dehydration reaction of the hydrates. This, in turn, led to the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) decomposing, weakening and decreasing the strength of the cement paste. The recycled coarse aggregate (RCA) that was recovered with microwave heating for 180 s had around 2-3% paste and fine aggregate. The recovered RCA was extremely comparable to the recovered RCA from OCA concrete, suggesting the potential for recovering high-quality RCA. It has been discovered that microwave heating weakens the binder that includes a dielectric substance, enabling the effective recovery of RCA.