# Environmentally Friendly Smart Construction--Review of Recent Developments and Opportunities
### Use of Recycling Materials in Construction
#### 4.4.3 Recycled Aggregate Concretes  
Another way to achieve circular waste management and conservation of natural resources is to use construction and demolition waste (CDW) for new concrete structures by reprocessing the construction debris into recycled concrete aggregates [270]. In general, the environmental advantages of recycling CDW are not evident as the environmental impact depends on several factors, such as transportation to and from recycling plants, the type of processing technology and demolition methods. For the best environmental results, the selective demolition (deconstruction) should be used and CDW content should be assessed and classified [271]. There are two types of recycled aggregates used to produce the recycled aggregate concretes (RACs): a recycled coarse aggregate (RCA) and recycled fine aggregate (RFA). The main source of RCA is bricks, concrete and ceramic waste, while RFAs are normally obtained from glass. The microstructure and mechanical behaviour of RAC are controlled by the presence of two Interfacial Transition Zones (ITZs): the old ITZ between the conventional coarse aggregate and the adhered (old) mortar, and the new ITZ between the adhered (old) mortar and the new cement matrix. The old ITZ, being highly porous and having pre-existing cracks, makes the microstructure more fragile. Mechanical tests have shown that failure in RAC concrete generally occurs through the old ITZ, indicating that the old mortar has a higher tendency to crack compared to the new mortar. However,the presence of the old mortar does not significantly affect the bond strength between the recycled aggregate and the new cement mortar [272]. The strength of RAC is directly proportional to the strength of the recycled material. UCS in RAC is about 20-25% lower than in OPCC, while the tensile splitting strength in RAC is only reduced by about 10%. Shrinkage and creep deformations are increased in RAC by 25% and 30%, respectively [273]. The RAC modulus of elasticity decreases as the content of the recycled aggregate increases. This effect is even more pronounced when the recycled aggregate has a higher amount of adhered old mortar [272]. The thermal expansion and bond strength of RAC differ minorly from the ordinary Portland cement concrete (OPCC). The major difference between RAC and OPCC is in their absorption capacity, which is 2.3-4.6 times higher in RAC. The high porosity of recycled aggregates increases water absorption, which in turn reduces durability, mechanical strength and workability of RAC [273, 274]. Several methods to improve RAC characteristics have been proposed. The majority of methods are devised to remove the old mortar attached to the surface of the aggregate, which can be carried out by ultrasonic cleaning, microwave heating or pre-soaking in acid solutions. Other methods include coating of old mortar with pozzolans or polymer emulsions, direct carbonation and biodeposition of calcium carbonate. It was also found that by changing mixing methods, the water absorption can be reduced, and the RAC properties can be adjusted. The available improvement methods are summarised in Table 2 following review papers [272, 273, 274]. The geopolymers discussed in the previous section can also be used to eliminate some of the negative effects by improving the bond strength [275, 276, 277]. The lack of RAC performance can also be compensated for by using fibres obtained from agricultural or municipal wastes [277, 278]. To date, the RFA aggregates attract less attention as their recycling and pre-treatment are more complex. The microstructural performance of RFAs is essentially similar to RCAs featured by high water absorption and a presence of old mortar [274, 279]. The RFA from fine ground glass though can improve durability and make RAC properties more uniform; however, extra care should be taken of the curing process to control the alkali-aggregate