**4. Chemical Properties**

The chemical characteristics of RCA affect the performance of the concrete. The durability of concrete should be affected by the presence of reactive chemicals such as chlorides, alkalis and sulphates in RCAs. The chemical properties of RCA are summarized below.

#### *4.1. Soundness*

Soundness is an indicator of the stability of aggregates to environmental influences, such as atmospheric influences [45,85–87]. The strength test for magnesium sulfates and sodium sulfates is mainly used to assess the integrity of the aggregate. RCAs usually pass magnesium sulfate strength tests, but do not pass sodium sulfate strength tests. For RCA and NCA, respectively, the strength loss of magnesium sulfate were 70% and 2.50% [88]. These figures are identical to the results obtained by several researchers worldwide [45,85–87]. It is also examined the reliability tests of sodium sulfates and found that the losses were 12.0% and 9.0%, respectively, for RCA and NCA [45,85–87]. Thus, the sodium sulfate leak test was in accordance with the standard. Moreover, the results are identical to those obtained by Lye et al. [87]. The failure of RCA is mainly associated with the distribution of pore sizes in aggregates.

#### *4.2. Reactivity of Alkali-Aggregates*

RCA concretes can undergo alkaline aggregate reactions (AAR) if the original concrete aggregate was prone to alkaline aggregate reactions. In addition, mortars adhered to RCA or the alkali content of the cement paste can have a significant impact on the AAR susceptibility of new concretes with RCA. The alkali content of RCA is related to the content of its solution. RCAs with more mortars lead to higher alkali content and are thus more susceptible to alkaline aggregate reactions.

## *4.3. Sulfate and Chloride Content*

RCA can have high sulphate content due to sulphate substances present in adhered cement slurries. It is investigated the content of water-soluble sulfates for RCA and NCA [87]. The researchers set the sulfate content of 0.032 g/L and 0.0250 g/L for RCA and NCA, respectively. The sulfate content of RCA depends on the amount of adhered mortar/cement paste. Higher sulfate content in RCA indicates that higher amounts of mortar will attach to RCA.

High levels of chlorides have been found in RCA, formed from bases with prolonged exposure to chloride-oriented antifreeze agents. RCA with higher chloride content can affect the durability of new concrete due to corrosion of the steel reinforcement [45,85–87]. Since increased corrosion of steel can lead to premature failure of reinforced concrete structures, RCA made from old concrete with chloride content higher than 0.04 kg/m<sup>3</sup> should not be used in new concrete. Furthermore, polymer materials, rubbers, plastics, joint seals, textiles, wood, and paper can be present in RCAs [45,85–87]. In concrete, these materials become unbalanced when thawed, dried.

#### **5. Physical Properties**

The concrete properties and mix proportions are influenced by the physical properties of the RCA. Basic properties such as absorption, pore volume, bulk density, specific gravity, texture and shape of RCA are generally inferior to those of natural aggregate due to impurities and the presence of residual slurry/paste [87]. The magnitude of the impact will vary depending on the amount and nature of the recycled slurries or pastes present in RCA. From the available literature [89–91], a summary of the main physical characteristics of RCA is shown in Table 2.


**Table 2.** Fundamental physical properties of NCA and RCA [67–69].
