*4.1. A General Overview*

Many studies have investigated the influence of the use of fly ash and CDW on the physical–mechanical properties of concretes, showing that the results improve the mechanical properties and durability in ages longer than 28 days, as presented in Table 5.


**Table 5.** Some recent studies on the mechanical and durability properties of concrete with fly ash.


<sup>1</sup> RCA = recycled concrete aggregate; <sup>2</sup> MRA = mixed recycled aggregate; <sup>3</sup> RMA = recycled masonry aggregate <sup>4</sup> RA = recycled aggregate; <sup>5</sup> FA = fly ash.

In general, the addition of fly ash in partial replacement of Portland cement presents a positive influence with respect to mechanical strength and durability in concretes with recycled aggregate from construction and demolition when compared to concretes with recycled aggregate and without fly ash at higher ages of healing. Kamal et al. [105] analyzed sample data where more than a thousand pieces of information was extracted from the literature, and through the nonlinear model, the effect of fly ash on the resistance properties of concrete was investigated. The study involved high cement replacement content by fly ash (up to 70%), different water/binder ratios, and a 90-day curing period. The authors observed, through mathematical models, that there is a good correlation between compressive strength and the water/cement ratio in cured concrete up to 90 days without fly ash, but no correlation was verified between compressive strength and water/binder ratio in concrete with cured fly ash up to 90 days. A good correlation was also verified between compressive strength and tensile and bending strength. According to the authors, compressive strength can be calculated through mathematical models constructed through the suggested methodology.

According to Limbachiya, Meddah, and Ouchgour [20], whenever recycled aggregate (RA) is added to concrete with 30% fly ash in partial replacement to natural aggregate, regardless of the content to be replaced, the tendency of durability and mechanical strength is to decrease, and shrink drying is increased.

According to Limbachiya, Meddah, and Ouchagour [20], as the content of substitution of natural coarse aggregate by recycled concrete aggregate increases (ARC), the strengths (compression and traction), and the modulus of elasticity decrease. On the other hand, shrinkage by drying increases. It is a consensus among all authors that the use of recycled aggregate in concrete decreases the mechanical properties and durability, but with replacement levels below 30% of natural aggregate by the recycled aggregate, the adverse effects are not so significant. The addition of fly ash in small proportions in concretes with recycled

aggregate tends to minimize the adverse effects, but the impact of fly ash is observed in concretes with higher ages, where, according Lorca et al. (2014), the fly ash reacts with calcium hydroxide released by the cement hydration product in older ages.

Some studies have developed mathematical models to estimate the mechanical strength of concrete with pozzolanic materials as well as with recycled aggregate [105–107]. Shakr Piro et al. [105] used five different models to estimate the compressive strength of concrete with carbon nanotubes. Therefore, the artificial neural network model, M5P tree model, nonlinear regression model, and multilinear model have been used. The variables used in the model were curing time in days, coarse aggregate content, water/binder ratio, cement intake in kg/m3, and carbon nanotube. As a methodology, the authors used information such as the reference concrete compressive strength and concrete with different contents of carbon nanotube. Based on sample data with 282 records analyzed statistically, the authors developed a multiscale model to estimate the compressive strength of the concretes. Another very relevant study developed by the authors [107] was the analysis of the correlation between compressive strength and electrical resistivity of concrete slag residue. The models used for this study were the multi logistic regression model, complete quadratic model, M5P tree model, and neural network. Barkhordari et al.'s [106] mathematical models have been used to estimate the compressive strength of concrete with fly ash. The models used were super apprentice algorithm, simple average, weighted average, and stacking employed. The database contained information from 270 samples that were collected and preprocessed. Next, some recent research on the use of fly ash in concretes with recycled aggregate will be presented.
