*4.2. Mechanical Properties*

#### 4.2.1. Compressive Strength

Thus, concretes with the addition of fly ash in partial replacement to Portland cement may present lower compressive strength compared to conventional ones in the smallest ages. Below will be a few studies of the combined effect of different levels of fly ash and with 50% recycled aggregate in the compressive strength in concrete at 28 days of curing (Figure 3).

**Figure 3.** Compressive strength of concrete with: recycled aggregate of concrete (RCA), mixed recycled aggregate (RMA), and ceramic bricks (RA). Data from Zong et al. [7], da Silva and Andrade [17], Kou and Poon [32], Shaikh [33], Sunayana and Barai [34] and Kurad et al. [37].

The addition of fly ash in partial replacement to Portland cement tends to reduce the compressive strength of concrete in the smallest ages. According to Mehta [84], this is because the oxides when reacting with water and Ca(OH)2 form a layer of C-S-H around the particle making it difficult to access the oxides of the inner part. With this, the hydration heat of the pozzolanic reaction forms more slowly making the development of resistance slower.

According to Kurad et al. [37], the actual decrease in the combined fly ash effect and recycled aggregate concrete (RCA) on compressive strength is less than the sum of the individual fly ash effect and RCA, especially after 28 days of cure. According to the authors, one of the factors that led to this behavior is the pozzolanic reaction between silicon dioxide (SiO2) of fly ash and calcium hydroxide (Ca(OH)2) of RCA. With the increase of Ca(OH)2 due to the increasing reason for the incorporation of recycled aggregate, fly ash SiO2 will have more Calcium Oxide (CaO) in the not hydrated particles of the old cement to produce more C–S–H, which is the main contributor to the development of concrete resistance.

In 2017, Kurad et al. [37] verified the effect of incorporating high volumes of recycled concrete aggregates and fly ash on the mechanical strength of new concretes. The authors produced concrete with axial compressive strength of 20 MPa at 28 days. At the same age (28 days) the concretes were crushed, and only after 10 months, the recycled concrete aggregate (RCA) was used in the mixture as coarse and fine aggregates for the production of new concretes. The sum of the main oxides of the fly ash used in this study (SiO2 + Al2O3 + Fe2O3) was 86.1% of the total mass. The axial compressive strength designed for the original concrete was 37 MPa, and the slump of the cone was 80 ± 10 mm in all mixtures.

In this study, the authors replaced Portland cement with fly ash in the following proportions: 0%, 30%, and 50% of fly ash, and the binder consumption were 350, 235, and 140 kg/m3, respectively. The authors produced concrete with 0% and 100% replacement content of natural aggregate by recycled sand. Some of the concrete mixtures were repeated with the addition of a 1% superplasticizer.

The authors observed that, in general, the replacement of natural aggregate with recycled aggregate (RCA) is prejudicial to the compressive strength. The incorporation of RCA as fine aggregate is more detrimental than its incorporation as coarse aggregate. The authors also observed that when incorporating fly ash in mixtures with recycled aggregate (RCA), the compressive strength of concretes has the tendency to decrease at early ages. However, the rate of increase of concrete strength is directly proportional to the rise in incorporation levels of RCA and FA.

According to Kurad et al. [37], the decrease of the combined effect of the joint employment of fly ash and RCA is smaller than the individual effect of the components, especially after 28 days of curing. According to the authors' study, one of the factors that led to this behavior is the pozzolanic reaction between the Silicon Dioxide (SiO2) of the fly ash and Calcium Hydroxide (Ca(OH)2) present in the RCA. With the increase of Ca(OH)2 due to the increasing ratio of incorporation of recycled aggregate, the SiO2 of the fly ash will have more Calcium Oxide (CaO) from the extra particles of the old cement to produce more C-S-H, which is the main reason for the development of concrete strength [37].

According to Corinaldesi and Moriconi [108], concretes with a recycled concrete aggregate present an improvement in the interfacial transition zone (ITZ) as a result of the internal curing effect due to water being returned by the recycled aggregate particles, which have high porosity, and in the C-S-H particles, probably contained in recycled aggregates coming from the old mortar [108]. According to the authors, recycled aggregates also have Ca(OH)2 particles that should help improve the pozzolanic activity of fly ash. To analyze the TZs between natural aggregate and cement paste in conventional concrete and between recycled concrete aggregate and cement paste in RCA concrete and fly ash, the authors used the scanning electron microscopy (SEM) technique. The results showed that the ITZs of the concrete mixes made with fly ash and recycled concrete aggregates were better than those of the original concrete.

Corinaldesi and Moriconi [108] verified that the high amount of old cement particles increases the Ca(OH)2 content, and the fly ash also contains large amounts of SiO2. Soon the amount of CSH increments and fills the ITZ and works on the interfacial connection among aggregates and paste. This behavior was also observed by other authors [20,106,109].

In 2017, da Silva and Andrade [17] produced concretes with mixed recycled aggregate consisting of 8% ceramic, 13% natural aggregate, and 79% concrete. The sum of the main oxides of the fly ash used in this study (SiO2 + Al2O3 + Fe2O3) was 80.6%. The Portland cement used in this research was similar to ASTM C 150 III, whose levels of substitution of natural coarse aggregate by recycled aggregate proportions employed in the experiment were 25%, 50%, 75%, and 100%, with 15%, 20%, 25%, and 30% of cement substitution by fly ash. The axial compressive strength of the reference concrete at 28 days was 54.1 MPa, and the slump of the cone was approximately 80 ± 10 mm in all mixtures. The w/binder ratios employed were 0.40, 0.45, 0.50, 0.55, and 0.65. Based on a nonlinear regression model, the authors observed that by replacing natural coarse aggregate with mixed recycled coarse aggregate, the growth rate of axial compressive strength of the concretes without fly ash between the ages of 28 and 91 days was low for the w/b ratio of 0.4. However, as the replacement content of Portland cement by fly ash is increased, the growth rate of compressive strength increases significantly, and as the w/b ratio is increased, this growth rate is even higher (Table 6).

**Table 6.** Influence of the ratio w/b in the growth rate of the concrete compressive strength 25% RCA.


According to da Silva and Andrade [17], the pozzolanic reaction between fly ash and Ca(OH)2 in concretes with mixed recycled aggregate showed significant improvements in mechanical properties, tending to approach the reference concretes at older ages. The results verified in this study are in agreement with those observed by Kurad et al. [37].

The addition of fly ash in concretes with clay brick waste with a strength of approximately 10 MPa as coarse aggregate replacing natural coarse aggregate was the subject of a study by Zong, Fei, and Zhang [7]. The sum of the principal oxides in the fly ash (SiO2 + Al2O3 + Fe2O3) was 86.91% of the total mass. The cement employed in this study was ordinary Portland cement. The proportions used in this study were 30%, 40%, and 50% recycled aggregate, and 15% fly ash. A high-performance polycarboxylate admixture was used for water reduction. The water content in the concrete was the standard amount of water required for reference concrete plus additional water based on the increased water absorption of the recycled aggregate. The authors observed a significant reduction in the density of the concretes with recycled aggregate compared to the reference concrete.. According to the authors, this behavior is related to the low density of the recycled aggregate of ceramic material. According to the authors, the reference concrete had a density of 2476 kg/m3, while concretes with 30, 40, and 50% of recycled aggregate content presented 2352 kg/m3, 2316 kg/m3, and 2175 kg/m3, respectively. According to the authors, the reference concrete had a density of 2476 kg/m3, while concretes with 30, 40, and 50% of recycled aggregate content presented 2352 kg/m3, 2316 kg/m3, and 2175 kg/m3, respectively. According to the authors, the reduction in density of the concretes with recycled aggregate is related to the low density of the recycled aggregate of ceramic material.

Fei and Zhang [7] observed that the reduction in mechanical strength was more significant in concretes with 50% recycled aggregate and 15% fly ash compared to the reference concrete, which was 44% at 28 days of curing. Based on the authors' results, the concrete with only 15% fly ash presents compressive strength of 48 MPa and flexural

strength of 11 MPa. For concretes with 15% fly ash and 30, 40, and 50% recycled aggregate, the compressive strength was 42 MPa, 35 MPa, and 28 MPa, respectively, while the tensile strength was 4.8 MPa, 4.6 MPa, and 3.9 MPa, respectively. According to the authors, this behavior occurred because the recycled aggregate used in this study presents much lower strength than the natural aggregates.

In a general way, it can be noticed that the reduction in the mechanical strength of concrete when recycled aggregate is used is closely related to the type of waste. Recycled concrete aggregates tend to present higher mechanical resistance in comparison to recycled aggregates from clay bricks, and consequently present a higher strength decrease. The addition of fly ash in concretes with recycled aggregate has a very significant contribution because it produces an excellent pozzolanic reaction in the long term and has a pore filling effect due to the fine particles of the ash. The higher the content of adding fly ash in partial replacement to Portland cement, the lower the degree of reaction in the initial ages of cure [110].
