# The Properties and Durability of Self-Leveling and Thixotropic Mortars with Recycled Sand
## 1 Introduction  
The construction industry consumes around 40% of all extracted natural resources, produces large amount of waste from building demolition, and consumes a lot of energy [1]. Several approaches have been carried out to mitigate the environmental impact of construction practices, such as investigating on more sustainable binders [2, 3] and reusing demolition waste [4, 5]. The generation of construction and demolition waste (C&DW) comprises the largest waste stream in the European Union (EU). It totaled 374 milliontons in the EU in 2016, excluding excavated soil [6]. Their management is driven by the increasingly environmental concerns about the over-exploitation of aggregates from quarries or rivers, landfill shortages, and rising transportation and landfill costs. Construction and demolition represent a priority area in the EU according to the Circular Economy Action Plan (EC 2015) for closing the loop, and EU countries set a 70% recovery target of C&DW within 2020 [6]. While most EU countries are on track to meet or exceeded the recovery target of 2020, it should be emphasized that most of the recovery practices result in a down-cycling [6]. They are actually based on low-value backfilling operations, such as filling holes on construction sites and low-grade recovery (involving the use of recycled and crushed cement or stones (aggregates) in road construction). The revised Waste Framework Directive (WFD 2008/98/EC, amended 2018/851) encourages measures which promote and implement selective demolition and sorting systems for C&DW in order to enable the removal and safe handling of hazardous substances and to facilitate reuse and high-quality recycling. Analyses of the costs and benefits of reusing demolition waste as recycled aggregates have been published in recent years, not only involving economic considerations but also environmental benefits. [PERSON] [7] reported that the long-term net benefit of producing coarse recycled concrete aggregate (CRCA) from waste concrete was positive with a value of USD 22,334,116 per year, while that of crushed coarse natural aggregate (CNA) had a negative net value of USD \(-\)31,841,109 per year. [PERSON] [8] reported cost savings of 63.13% for recycled aggregate concrete (RAC) produced from the recycling of concrete waste to make fresh concrete at the same demolition site, rather than producing conventional concrete made with CNA. According to [PERSON] et al. [9], the long-term cost of producing one ton of coarse recycled concrete aggregate is about 40% less than that for coarse natural aggregate. Furthermore, they calculated that the environmental benefit of producing one ton of recycled concrete aggregate was approximately 97% higher than that for natural aggregate. [PERSON] [10] and [PERSON] [11] summarized the major environmental impacts and consequences of sand exploitation based on the analysis of existing literature associated with this issue. The very high volume of sand being currently extracted has a serious negative impact on rivers, deltas, and coastal and marine ecosystems, such as the loss of land through river or coastal erosion, the lowering of water levels, and the loss of sediment supply, thus damaging fauna and flora at significant levels. However, technical and commercial barriers for circular concrete recycling can be summarized as follows:  
* A lack of quality/purity in recycled materials;
* A lack of national instructions for recycling in new concrete in some countries;
* Low replacement rates in low-grade concrete allowed by Standards;
* A lack of documented information available regarding the origins of waste;
* Variable quality of feedstock;
* Contamination risks;
* A lack of trust of end-users in the quality/purity of recycled materials;
* The low price of virgin materials;
* The processing costs of demolition waste to secure high-quality material for recycling.