materials Article The Design and Development of Recycled Concretes in a Circular Economy Using Mixed Construction and Demolition Waste Marcos Díaz González 1, *, Pablo Plaza Caballero 2 , David Blanco Fernández 1 , Manuel Miguel Jordán Vidal 3 , Isabel Fuencisla Sáez del Bosque 2 and César Medina Martínez 2   Citation: González, M.D.; Plaza Caballero, P.; Fernández, D.B.; Jordán Vidal, M.M.; del Bosque, I.F.S.; Medina Martínez, C. The Design and Development of Recycled Concretes in a Circular Economy Using Mixed Construction and Demolition Waste. Materials 2021, 14, 4762. https:// doi.org/10.3390/ma14164762 Academic Editor: Jean-Marc Tulliani Received: 5 July 2021 Accepted: 4 August 2021 Published: 23 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Construction Sciences, Metropolitan Technological University, Dieciocho, Santiago de Chile 161, Chile; dblanco@utem.cl 2 Department of Construction, Research Institute for Sustainable Territorial Development (INTERRA), University of Extremadura, 10003 Cáceres, Spain; pcaballerop@unex.es (P.P.C.); isaezdelu@unex.es (I.F.S.d.B.); cmedinam@unex.es (C.M.M.) 3 Department of Agrochemistry and Environment, Miguel Hernández University of Elche, 03202 Elche, Spain; manuel.jordan@umh.es * Correspondence: mdiaz@utem.cl Abstract: This research study analysed the effect of adding fine—fMRA (0.25% and 50%)—and coarse—cMRA (0%, 25% and 50%)—mixed recycled aggregate both individually and simultaneously in the development of sustainable recycled concretes that require a lower consumption of natural resources. For this purpose, we first conducted a physical and mechanical characterisation of the new recycled raw materials and then analysed the effect of its addition on fresh and hardened new concretes. The results highlight that the addition of fMRA and/or cMRA does not cause a loss of workability in the new concrete but does increase the amount of entrained air. Regarding compressive strength, we observed that fMRA and/or cMRA cause a maximum increase of +12.4% compared with conventional concrete. Tensile strength increases with the addition of fMRA (between 8.7% and 5.5%) and decreases with the use of either cMRA or fMRA + cMRA (between 4.6% and 7%). The addition of fMRA mitigates the adverse effect that using cMRA has on tensile strength. Regarding watertightness, all designed concretes have a structure that is impermeable to water. Lastly, the results show the feasibility of using these concretes to design elements with a characteristic strength of 25 MPa and that the optimal percentage of fMRA replacement is 25%. Keywords: recycled concrete; recycled mixed sand; recycled mixed gravel; mechanical properties; strength; watertightness 1. Introduction Climate change and global warming have become important issues that directly impact the world economy. This has led to countries developing laws and regulations that control the emission of CO 2 and the appropriate management of waste. In this context, the construction industry is responsible for 12% of all greenhouse gas emissions in the European Union (EU) and for generating ~25–30% of the solid waste produced every year in the EU [1], which equates to an average annual production of 800 million tonnes. In addition, as many as 534 and 200 million tonnes of construction and demolition waste (C&DW) are generated every year in the United States and China, respectively. Concrete is the most used material in the construction sector worldwide, with the EU and USA producing 165 and 150 million cubic metres, respectively, in 2018 [2]. This industry is characterised by requiring large amounts of natural resources, as this material is mainly composed of aggregates—60–75% of the volume of concrete—and, to a lesser extent, cement—10–15%. Its manufacturing process is responsible for ~8% of the total Materials 2021, 14, 4762. https://doi.org/10.3390/ma14164762 https://www.mdpi.com/journal/materials