## 1 Introduction
Therefore, valorizing this waste in concrete represents a promising and sustainable alternative. While CS has been extensively investigated for its use as aggregates in concrete [31; 33; 34; 35; 36; 37; 38], its potential as a SCM has received comparatively less attention [28; 39; 40]. The 15% vol CS replacement level was selected based on prior experimental work [28; 41] that identified this dosage as optimal in balancing reactivity,mechanical performance, and sulfate resistance. Preliminary internal tests also showed that higher CS contents increased early strength loss and plasticity issues, particularly in the presence of RCA. On the other hand, in response to the challenges associated with CDW, its valorization has been explored as an environmentally driven strategy and, in some cases, an economically viable one. This approach involves the use of appropriate crushing and screening equipment to produce recycled aggregates (RA) for use in the production of mortar, concrete, and pavement base or sub-base layers [10; 12; 42]. RAs sourced from CDW generally exhibit lower mechanical strength, higher porosity, and greater water absorption compared to natural aggregates (NAs) These limitations are mainly attributed to a weaker interfacial transition zone (ITZ), the presence of microcracks generated during the crushing process, and the residual adhered mortar [43]. For this reason, various international standards have established specific criteria for RAs, considering factors such as their type (originating from concrete, mortar, fired clay bricks, etc. ), density, water absorption, and other properties [44; 45]. Currently, numerous studies have focused on addressing the limitations of RAs for use in concrete through the application of chemical, mechanical, and thermal treatments, or combinations thereof, and through optimized mixing protocols such as two-stage or pre-saturated mixing [46; 47]. Several studies have shown that the mechanical and durability shortcomings of RCA can be mitigated through surface treatments [48; 49], pozzolanic coatings [50; 51; 52], or pre-saturation technique [53], among others, all of which enhance the interfacial transition zone (ITZ). Concrete's workability and strength can also be improved by optimized mixing protocols that employ stabilizers, water-reducing admixtures, or graded RCA blending [54; 55]. These strategies, however, increase processing complexity and cost. Assessing the performance of untreated RCA therefore remains crucial--especially in contexts like Chile, where advanced treatment facilities are often unavailable or economically unfeasible. In this study, RCA is used in its untreated state and combined with CS to establish a practical baseline for their joint behavior in sulfate-rich environments, understanding that this baseline can be further improved with the use of enhanced RCA. In addition to the responsible use of by-products and waste-derived materials in concrete design, ensuring the long-term durability of this composite material is a key factor for its sustainability. Among the most aggressive environments that can compromise its long-term performance is sulfate attack [56; 57]. The external sulfate attack (ESA) results from chemical and physical processes that cause the degradation of concrete's performance [57; 58]. When sulfates penetrate the concrete, they react with certain phases formed from the hydration of cement and form ettringite and/or gypsum [59; 60], Expansive compounds that cause strains in the concrete, leading to cracking and spalling, which ultimately results in its degradation [61]. One of the most detrimental forms of ESA is caused by magnesium sulfate (MgSO\({}_{4}\)). In addition to forming gypsum through its reaction with portlandite (Ca(OH)\({}_{2}\)), it can also produce brucite (Mg(OH)\({}_{2}\)). Furthermore, MgSO\({}_{4}\) attacks calcium silicate hydrates (C-S-H), leading to the formation of magnesium silicate hydrates (M-S-H) that lead to severe cement matrix deterioration, cracking, and weakened ITZs [60; 62].