However, the article focuses on improving the microstructure of the RCA without dividing improved regions. A summary of the physical properties of RCA and NA from various studies is presented in Table 1. This can be explained by the microstructure of the RCA, the presence of a weak ITZ, and adherent mortar.
6] analyzed the influence of carbonation treatment by comparing the properties of RCAs before and after treatment. In the same study, the SEM of the adhered mortar and ITZs in the RCA and CRCA were compared. Due to the lower WA, the microhardness of new ITZ in the carbonated NRCAs was higher than that of the non-carbonated sample.
Incorporating mineral mixtures is one of the alternative methods to improve the ITZ of RCA. One of the mineral mixtures that can be used to improve the performance of ITZ is silica fume. 42] investigated the effect of the two-step mixing approach on the microstructure of RCA.
The compressive strength of the samples increased due to the application of TM at various curing ages. Microhardness of fresh ITZ in the carbonated NRCAs was somewhat higher than that of the non-carbonated sample. Compressive strength of the specimens increased due to the application of TM at various curing ages.
Improvement of Adhered Mortar of RCA
45] evaluated the influence of carbonation treatment on RCA and mechanical properties of concrete. As shown in Figure 20, the microhardness of the cement paste increased in the aerated sample. As shown in Figure 20, the microhardness of the cement paste increased in the aerated sample.
This can be explained by the decrease in porosity, which led to an increase in the strength of the old ITZ and the old mortar. In addition, the obtained values indicate an increase in the microhardness of the old ITZ and the old mortar in the MRCA sample. However, the progress of the old ITZ compared to the old mortar was more significant.
Details of the mixing approach can be found in the previous section describing ITZ enhancement. During the experiment, the compressive strength of the sample improved compared to the sample without incorporated nanosilica. Finally, the authors pointed out that as the overall performance of the specimens improved, the adhesive properties of the mortar also improved.
However, most of the experiments, such as [2,46,47], compared the effect of processing on the overall performance of RCA without analyzing the effect on bonded mortar specifically. In addition, an improved bond on the surface of the aggregate was achieved due to the filling of voids in the adhesive mortar with the released CaCO3. 50] compared the performance of modified recycled fine aggregate (RFA) in mortar with unmodified RFA and natural fine aggregate (NFA) mortar samples.
50] compared the performance of modified recycled fine aggregate (RFA) in mortar with unmodified RFA and natural fine aggregate (NFA) mortar samples. According to Figure 26, the water absorption of M-mRFA decreased compared to RFA without modification. The results of the study show that with 2% and 3% sodium silicate, the compressive strength of the RAM sample increased by 25-28%.
At 3% sodium silicate, the highest value of compressive strength was found, which was about 14% higher compared to NAM. 65] investigated the use of mortar mixing approach (MMA) and sand envelope mixing approach (SEMA) as alternative ways to improve RCA properties.
Application of RCA in Concrete
MMA improved the compressive strength of RAC without any surface pretreatment, making the compressive strength equal to 31.7 MPa. The sand-enveloped-mixing approach (SEMA) is an alternative way to improve the compressive strength of RCA. Implementation of new mixing technique – coating using a pozzolanic powder such as FA, SF, blast furnace slag (BS) or their combination – improved the compressive and flexural strength of RCA.
The highest value of compressive strength was equal to 47.6 MPa after 28 days, while the flexural strength value was about 7.31 MPa after 28 days.
Different mixing methods were also evaluated to improve the properties of ITZ, although their improvement in properties is limited. It should be noted that the TM approach showed a significant increase in the compressive strength of RCA in comparison to DM. Based on previous studies on the improvement of adhesive mortar, the implementation of pozzolana addition such as FA, SF, and NSi can modify the surface of RCA, improve the properties of adhesive mortar by filling pores and voids, and produce C–S–H gel.
Compared with other pozzolanic materials, the utilization of nanosilica is the most effective way to improve the properties of adhesive mortar due to its high reaction rate and high specific area. However, although this method is effective in improving properties, the cost of using nanosilica is high, which makes it difficult to apply in practical applications. Polymer treatment is another method of treating the adhesive mortar that can improve workability and durability, serve as a filler and create a water-repellent layer.
However, most studies investigated the effect of polymer treatment on the overall improvement of RCA without detailed investigation of its effect on adhesive mortar, or only indirectly mentioning the possibility of improvement in the adhesive mortar area due to overall improvement of RCA properties. Using calcium carbonate biodeposition to treat RCA is also an effective method to improve the quality of adhesive mortar. This method is natural and has the lowest environmental impact thanks to the use of naturally occurring substrates for sowing.
The biodeposition method of calcium carbonate uses special bacteria which can accelerate the formation of calcium carbonate on the surface of RCA, thereby reducing the WA ability in the bonding mortar. However, the impact on compressive strength is not significant, the treatment is expensive and time-consuming, and its practical application requires more tests and researches before actual application. Application of sodium silicate solution is another treatment method to strengthen the bonding mortar because it reduces water absorption by filling pores and voids through C-S-H gel formation.
Compared to the above-mentioned methods, carbonation treatment is not only an effective method for increasing the properties of adhesive mortar, but it is also environmentally friendly. Therefore, this method is considered to be an effective and sustainable treatment for improving the properties and performance of adhesive mortar, but its drawback is that carbonization takes time. Table 8 summarizes the common advantages and disadvantages of each treatment method for ITZ and adhesive mortar that were mentioned by different researchers.
Conclusions and Recommendations
Studies by various researchers show that the main methods for improving the ITZ properties of RCA are carbonization, incorporation of mineral admixtures and different mixing approaches. CO2 treatment of recycled concrete aggregates to improve mechanical and environmental properties for unbound applications. Construction Reinforcement of recycled aggregate by microbially induced mineralization and calcium carbonate deposition - Influencing factors, mechanism and effect of reinforcement.
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