# Photocatalytic Recycled Mortars: Circular Economy as a Solution for Decontamination
## 1 Introduction
Another widely studied application is in the manufacture of concrete. There are studies that support its use, even for structural concrete [26; 27]. The use of recycled aggregates in the production of concretes and mortars has the following competitive advantages: (i) decreased extraction of aggregates from rivers, coasts, and quarries; (ii) exclusion of aggregates from landfills, reducing the volume of waste to be treated; and (iii) implementation of the circular economy model and approach to set recycling targets [14]. In most of these studies, only the coarse fraction was used [7; 11; 12; 28]. It was concluded that concrete strength decreased when recycled concrete was used and the reduction could be as low as 40% [29]. No decrease in strength was reported for concrete containing up to 20% fine or 30% coarse recycled aggregates, but beyond these levels, there was a systematic decrease in strength as the content of recycled aggregates increased [29]. Of the different types of RAs, recycled crushed concrete (RCA) is the most widely used in the manufacture of new concrete [9; 26; 27; 30; 31; 32]. Even the Spanish concrete standard (EHE-08) [16] allows the use of up to 20% RCA, but only referring to fractions greater than 4 mm. Because of the lower density of RMA, concrete made with it has lower density and higher water absorption than reference concrete [19]. Regarding the influence of recycled coarse and fine fractions, studies on recycled concrete incorporating fine recycled aggregate (FRA) from CDW did not obtain satisfactory results [19; 29; 30; 32; 33]. [PERSON] [29] studied the incorporation of FRA, and showed that properties such as density, dynamic modulus of elasticity, and compressive strength were reduced, the latter resulting in a decrease of 10% and more than 15% with a 50% and 100% incorporation ratio, respectively, at 90 days, in agreement with the results by [PERSON] et al. [33]. Because of these differences, the use of fine fractions in concrete should not be dismissed, but more research on it is needed. For this reason, the main application of FRA is in recycled mortar, for which the requirements are more tolerant. Within this group, there are different uses depending on where it will be placed: masonry mortar, interior or exterior rendering mortar, or mortar for the manufacture of paving blocks. Most of the investigations carried out to date were aimed at incorporating FRA in mortar for masonry [34; 35; 36; 31]. However, this research in particular is intended for the use of recycled mortar for any application in contact with the atmosphere, such as wall cladding or paving blocks. With the results obtained, it is hoped that its use can be further expanded. As in the case of recycled concrete, the use of mixed FRA (MFRA) in mortar has been found to be harmful to the properties of mortar [14; 36]. However, this research aims to prove its use as suitable if it meets a series of quality requirements, normally linked to correct CDW treatment. In addition to the circular economy, the mortar manufactured in this research is intended to provide a second benefit for environmental sustainability: atmospheric decontamination and reduced carbon footprint. The atmospheric pollution produced by accelerated population growth and industrialization can cause serious damage to the health of both people and ecosystems, and even to infrastructures and historical heritage. Among the main harmful emissions are nitrogen oxide gases (NO and NO\({}_{2}\), commonly known as NO\({}_{\text{x}}\)) generated by transportation and various industries. These gases have high toxicity and can cause serious problems in human health, as well as environmental problems (acid rain, photochemical smog, destruction of the ozone layer, etc.) [37]. It is essential to reach the highest air quality that does not create a risk to people's health and does not cause deterioration or permanent damage to ecosystems.