# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 1 Introduction
Different levers can be implemented to make the C&DW recycling an economically attractive option that includes levies on virgin materials, taxes on landfilling waste, and agreements and commitments between stakeholders in the value chain [6]. Specifically, to make the recycled aggregates competitive with virgin materials, it is crucial to increase their market value and manufacture products that perform as well as those made with virgin materials, or at least meet high-performance and durability criteria. The amount of recycled materials in cementitious composites and their applications is regulated by national standards. Furthermore, up to 20 percent substitution of virgin aggregates is allowed in new concrete because it does not damage the durability and the resistance of the cementitious materials. More than 50 percent substitution of virgin aggregates is only suitable for certain applications [12]. The possibility of using higher replacement rates of recycled aggregate from building demolition in concrete would represent a key step in increasing its high-quality recycling, due to the huge volumes of concrete yearly produced and being the fine and coarse aggregate accounting for 60-75% of concrete total volume [13; 14; 15]. Therefore, the scientific community has sought to close the gap in knowledge related to the topic. Attributing the loss of fresh and hardened state properties of RAC to the higher porosity of sorted RA has been met with unanimous consensus due to its inherent defects, such as the existing of additional interfacial transition zones (ITZ) [16; 17; 18]. These pores absorb water during mixing and have any load carrying capacity in the cured concrete. They also reduce the durability of RAC, because channels facilitate the penetration of detrimental substances, such as chloride ion and carbon dioxide, which both promote the corrosion of steel rebar and sulphate ion. The mechanical properties and long-term performance of RAC can be improved by enhancing the properties of the ITZ to facilitate the reuse of RCA in construction projects. In the most recent papers, three group of approaches have been proposed and investigated:  
1. Reduction in the recycled aggregate porosity: * Accelerated carbonation. The old mortar layer on the RAs contains a high amount of calcium hydroxide and calcium silicate hydrate. The method involves a carbonation reaction with CO\({}_{2}\) under the presence of water. The corresponding carbonated products show a volume expansion and partially occupy or even close the porosity of the RAs [19; 20]. * Biodeposition. The biodeposition is defined as microbial carbonate precipitation that can be achieved by ureolytic bacteria B. Megaterium [21] and B. Sphaericus [22]. The concrete pores can be filled with the insoluble calcium carbonate. * The addition of pozzolana. Silica fume (SF), fly ash (FA), and nano-SiO\({}_{2}\) (NAS) have been investigated for RA modification throughout the literature [23; 24]. In this case, the pores are filled by the calcium silicate hydrate (C-S-H) gel produced by the pozzolana reaction with Ca(OH)\({}_{2}\). * The addition of nanoparticles. Nanoparticles have much smaller size than cement grains. They not only act as filler in the ITZ, pores, or cracks, thus reducing the porosity, but also participate to the hydration process of the cement as nucleation centers for the crystallization [25; 26]. * Polymer emulsions. When polymer emulsion is applied to RA, the polymer molecules can fill up the pores and cracks in the RA, especially in the attached mortar and the ITZ and the aggregate surfaces can be sealed [27; 28] 2. Reduction in the old mortar layer on recycled aggregate surface: * Acid treatment. Pre-soaking of recycled concrete aggregate in HCl, H\({}_{2}\)SO\({}_{4}\), HNO\({}_{3}\), and (CH\({}_{3}\)COOH) solution at room temperature for programmed time reduces or removes the old mortar layer, as the alkaline mortar can be dissolved in acid solutions [29; 30]. * Thermal treatment.