*4.3. Durability Properties*

#### 4.3.1. Water Absorption

According to Zhang et al. [115], there are several aggressive agents present in nature that contribute to the deterioration of the concrete structure, reducing its service life, whose action is associated with climatic and environmental conditions, such as CO2, Cl−, O2, and H2O. In addition, Ca(OH)2 gives concrete high alkalinity, maintaining the pH of the mixture between 12 and 13, but it is a leachable product and in contact with CO2 present in the environment, and relative humidity of approximately 60–80% initiates the carbonation process, which reduces the pH to values close to 8–9, leaving the concrete exposed to chemical attacks. Limbachiya, Meddah, and Ouchagour [20] produced concretes with 0%, 30%, 50%, and 100% ratio of natural coarse aggregate to recycled concrete aggregate and with 30% fly ash in place of Portland cement CEM I 42.5 N. The mixtures were classified into three grades according to the 28-day design compressive strength (C20, C30, and C35 considering a 28-day design compressive strength of 20, 30, and 35 MPa, respectively).

The method employed for testing water absorption in concretes by the authors consisted of measuring the rate at which water, through a known surface area, flows into the capillary network of concrete pores with a fixed scale of 10 min. The estimation of volumetric flow is obtained by measuring the length of flow along a capillary of known size. The initial surface absorption (ISA) of the various mixtures was determined in 150 mm cubes.

The authors observed that the initial surface absorption (10 min) versus RCA content for all investigated mixtures showed an increase in the initial surface absorption (ISA) as the replacement content of the recycled concrete aggregate was increased. However, the authors observed that concretes with 30% fly ash in partial replacement of Portland cement showed a reduction in water absorption when compared to concretes without fly ash. According to Limbachiya, Meddah, and Ouchagour [20], this behavior is linked to the pozzolanic reaction and the pore structure refinement that reduces the water flux. The same behavior was verified by da Silva and Andrade [17].

To analyze the water absorption in concretes with recycled aggregate and 10% ultrafine fly ash (UFFA), Shaikh [48] used 10% ultrafine fly ash (UFFA) and two levels of substitution (25% and 50%) of coarse recycled aggregate from construction and demolition waste (CDW) in partial replacement of natural aggregate. The CDW used was constituted of approximately 78% concrete, 13% bricks, 2.3% asphalt, and 5.7% other materials. The water absorption of the recycled aggregate was 4.88%. The sum of the fly ash oxides (SiO2 + Al2O3 + Fe2O3) was 95.5%, with a surface area of 2.51 m2/g. Concrete water absorption over a period of 6 h was adjusted by linear regression, and, to describe the absorption, the slope of the equation was used. The absorption rate (mm) for the investigated concretes at 7, 28, and 90 days.

Based on the analysis through linear regression based on R2 values greater than 0.98 for all mixtures, the authors observed that the water absorbed in the concrete increased as recommended in Fick's first law equation [33]. The concretes with recycled aggregate presented higher water absorption than the reference concretes. According to Shaikh [33], the rate of water absorption through concrete is a function of the permeability of the pore structure where, due to capillary increase, the rate of water percolation is controlled mainly in unsaturated concretes. Based on the results obtained the authors noticed that as the age of cure increases, the rate of water absorption of recycled aggregates decreases. This behavior is related to the continuous hydration reaction and the formation of calcium silicate hydrate (C-S-H), which generally fills the micropores in the matrix [33]. It was also found that the addition of 10% UFFA significantly reduced the water absorption rate of concretes containing recycled coarse aggregate and natural coarse aggregate at all ages [33]. The high pozzolanic activity, the secondary C-S-H due to the pozzolanic reaction of ultrafine fly ash (UFFA) with calcium hydroxide (CH), as well as the fineness of fly ash, may have significantly contributed to the reduction of water absorption [17,116].

In general, studies have shown that recycled aggregates are, for the most part, more permeable than natural aggregates. The higher the replacement level of natural aggregate by the recycled aggregate, the greater the water absorption. The addition of fly ash in concretes with recycled aggregate reduces the negative effect that the recycled aggregate causes in the concretes due to the refinement of the porous capillary network that makes these concretes denser, improving their mechanical resistance and reducing the flow of water through the concretes.
