# Structural Concrete from 100% Recycled Aggregates
## 1 Introduction  
The construction industry is one of the primary contributors to Greenhouse Gasses (GHGs), as it is responsible for 40% of the world's energy consumption, 30% of raw material use, 25% of solid waste, 12% of land use, and 33% of related GHG emissions [1, 2]. Concrete, which is the most common man-made material used worldwide due to the availability of its constituents, its structural performance, and its versatility in use, plays a large role in this field, with an average consumption of 1 m\({}^{3}\)/person/year in Western countries [3] and an annual average production of 4.1 billion metric tons in the last 10 years [4]. Concrete consists of cement (\(\approx\)15% by mass), fine and coarse aggregates (\(\approx\)35-40% each), and water (\(\approx\)8%), from which the largest contribution to its carbon footprint is due to cement. To reduce its GHG impact, a large and still very active body of research has been carried out in recent decades on substituting part of its natural aggregates (NAs) with recycled ones (RAs) of various origins, mainly from demolition debris [5, 6, 7, 8, 9, 10, 11, 12, 13]. The large variety of \"recycled aggregate\" (from concrete, mortar, tiles, clay bricks, marble, and stone, with these mixed in a variety of proportions) with different origins (and therefore with different pollutants), each with different mechanical and chemical properties, is an additional difficulty for research that still limits the spread of recycled aggregate concrete--RAC. Nevertheless, RAC is currently one of the most promising materials for improving the environmental impact of the construction industry: it could lead to less debris taken to dumps, less material collected from quarries and rivers, and less transport. RAC is known to exhibit mechanical and chemical properties that are poorer than those of Natural Aggregate Concrete (NAC). If we consider 100% RAC and compare it to an equivalent NAC (i.e., with same mix design, but all aggregates of that are of natural origin and brand new), we would find that its compressive strength is up to 45% and itstensile strength is up to 11% lower than those of NAC, while its density is not significantly modified (up to \(-6\%\) for 100% RAC). Their long-term behavior is also different: due to an increase in porosity, 100% RAC exhibits a carbonation depth that is increased up to 2.5 times, a shrinkage of up to 35%, and a creep that may be 50% higher than what is exhibited by NAC. The higher porosity of RAC is also responsible for its weaker chemical response: its permeability to chloride ions is almost doubled, its alkali-silica reaction is increased, and its freeze-thaw performance is 10 to 20 times poorer than that of NAC [14]. In addition, concrete produced with recycled aggregates may exhibit a strong alkali-silica reaction [14] due to the pollutants included in the debris, which would be a crucial issue for large structures exposed to environmental humidity. The improvement in RAC properties is under ongoing research; in spite of several procedures having already been studied and optimized (biodeposition, i.e., microbial carbonate precipitation, the addition of pozzolana and nanoparticles, polymer emulsion treatment, acid and thermal treatment, etc. [14]), RAC's application on an industrial scale is not yet economically feasible, due in large part to the low cost of NAC. If we consider just these data, it seems that 100% RAC could create more problems than it solves, which explains why RAC still has a limited spread and why codes accept only reduced fractions of RAs in concrete production [15; 16; 17]. In addition, the mechanical and physical drawbacks of RAC turn out to be not so severe if we consider the specific classes of structures where concrete is not exposed to the external environment, such as residential buildings, where it is protected by plaster and paint and also potentially by impregnating protective agents such as passivating grouts.