# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 1 Introduction
The heating of the recycled concrete causes the dehydration of old cement stone, which leads to the separation of old mortar layer from the aggregate particles [31]. 3. Property improvement without recycled aggregate modification: * The mixing approach. This involves an atypical mixing of concrete components [32]. In the two-stage mixing approach (TSMA), the mixing water is divided into two portions. In the sand enveloped mixing approach (SEMA), sand and 75% of water are mixed at first, and then the cement is added. After further mixing, the remaining water and coarse aggregate are added. The aim of the approach is to generate a thin cement slurry on the surface of recycled concrete aggregate during the first water addition that covers pores and cracks of the recycled concrete aggregates. * The addition of fiber. The addition of short monofilament fibers in RAC is aimed to control the cracks opening and propagation by the \"fibre bridge effect\", thus overcoming the brittle failure mode of the RAC and increasing its tensile and flexural strength [33]  
The effect of the above-mentioned different approaches on the RAC properties has been investigated in terms of fresh properties (workability), physical and chemical properties (i.e., density, carbonation depth, and chloride ion penetration), mechanical properties (i.e., compressive, flexural, and splitting tensile strength as well as elastic modulus), and long-term performance (i.e., freezing-thawing resistance, alkali-silica reaction resistance, and creep and dry shrinkage). The bio-deposition significantly improves the elastic modulus and compressive strength of the final RAC. Specifically, the compressive strength increased by about 16% and 10% for RAC using recycled concrete aggregate and recycled mixed aggregate, respectively [34]. However, the bacterial culture medium may affect the concrete hydration process and long-term performance [21]. [PERSON] et al. [23] studied the improvement in compressive strength of recycled concrete aggregate constituent RAC through adding pozzolanic materials SF, FA, and NAS (pozzolana to cement ratio is 0.13). Increases in the compressive strength of RAC modified with SF, FA, and NAS were 55.2%, 39.4%, and 17.6%, respectively, over 28 days, i.e., higher compared with that of RAC without any modification. A small amount of nanoparticles can significantly improve RCA properties, but they can also increase the cost of concrete production. Furthermore, the dispersion of the nanoparticles in the composition is a critical issue and the proposed lab methods have limitations given the massive production of concrete [25]. The increases in compressive strength after 28 days for HCl, HNO\({}_{3}\), and H\({}_{2}\)SO\({}_{4}\) treatment were 34.6%, 25.3%, and 17.6% for a 100% RA replacement ratio, respectively. However, they are dangerous, expensive, and far from eco-friendly. Furthermore, they can deteriorate concrete and promote the corrosion of steel rebar. The mixing approach is considered a practical method [35] because it can be easily implemented, it has low costs, and it can lead to a significant strength enhancement (up to 20%) of the RAC compressive strength. [PERSON] et al. [36] applied chopped basalt fibers in RAC with a 20% replacement ratio with recycled concrete aggregate. Increasing the weight fibers content from 0% to 1.5% on cement increased the splitting tensile strength and flexural strength by 40% and 61%, respectively. However, the incorporation of fibers could reduce the workability of fresh RAC, especially when they have a high aspect ratio [36; 37]. [PERSON] et al. [38] reported that heating the recycled concrete aggregate to maximum 800 \({}^{\circ}\)C for 2 h can remove most of the attached old mortar; however, this treatment has a high-energy consumption. [PERSON] et al. [39], using recycled concrete aggregates soaked in 10% PVA solution for 24 h, reduced the water absorption of recycled aggregate concrete by approximately 70.0%.