Abstract
In this study, waste fire clay (WFC) was used to replace up to 50% of the fine aggregate (FA) and cement in the production of concrete. Mechanical tests were carried out to determine the compressive strength (CS), splitting tensile strength (STS), and flexural strength (FS) of the concrete. The results showed that the use of WFC as a replacement of FA significantly improved the mechanical properties, with the highest performance achieved at a replacement level of 20%. At this level, FS increased by 25.6%, while improvements of 11.5, 18, 12.8, and 7.7% were recorded at replacement levels of 10, 30, 40, and 50%, respectively. At replacement levels above 30%, the CS values started to decrease, with a loss in strength of 4.2–10% compared to the reference sample. Replacement of cement with WFC had a negative effect on the mechanical strength of the concrete, resulting in significant decreases in CS values ranging from 12.2 to 68%. Similar decreases of up to 63% in STS values were also observed when cement was replaced. In the study, concrete properties were estimated using empirical formulae based on ACI 318 and Eurocode 2 (EC2) standards, together with autoregressive (ARX) models with external inputs; these models showed high agreement with experimental data and demonstrated strong applicability in the field. Scanning electron microscopy (SEM) analyses showed that matrix formation was improved at the optimum WFC ratio; however, hydration was negatively affected by void formation at high replacement ratios. These results demonstrate that WFC, when used at appropriate ratios, can serve as a valuable additive to improve concrete performance and contribute to sustainable construction applications. All results indicate that WFC, when used at appropriate replacement ratios, can improve the mechanical properties of concrete and contribute to the development of sustainable construction materials.