Concreto permeável contendo resíduos de caulim e metacaulim: propriedades físicas, hidráulicas, mecânicas e adsortivas

Abstract

The intensification of urban soil impermeabilization has aggravated problems related to drainage and stormwater quality, requiring construction techniques capable of simultaneously performing hydraulic, structural, and environmental functions. In this context, the concept of multifunctional performance of pervious concrete stands out, being associated with the ability to combine hydraulic permeability, mechanical strength, and adsorptive capacity within a single material. From this perspective, kaolin residues, both in the fine fraction (FKR) and coarse fraction (CKR), present potential for application in pervious concrete mixtures, since kaolin-based materials are recognized for their adsorptive capacity. In addition, it is estimated that, for every ton of processed kaolin, approximately 80% becomes waste. This scenario represents a significant environmental liability due to the occupation of large disposal areas and the environmental degradation associated with mining activities, causing impacts on soils and adjacent water bodies. Given this context, this study evaluated the multifunctional performance of pervious concretes containing metakaolin, FKR, and CKR through an integrated experimental approach, including mechanical, hydraulic, and adsorption analyses of metallic ions (Al³⁺ and Fe²⁺) present in simulated contaminated stormwater. Initially, the chemical, mineralogical, physical, and morphological characterization of the materials was performed. Subsequently, the study was developed based on a preliminary analysis of cement pastes containing metakaolin replacements and additions (10% and 20%) relative to cement mass. The cement pastes with the best mechanical performance were later investigated in reference pervious concrete mixtures composed of natural aggregate and 10% and 20% metakaolin replacements relative to cement mass. Thereafter, concrete mixtures containing 10% metakaolin replacement relative to cement mass and additions of FKR and CKR at contents of 10%, 20%, 30%, and 40% relative to natural aggregate mass were analyzed. The pervious concretes were evaluated in both fresh and hardened states. Physical characterization included void ratio tests, two-dimensional image analysis obtained through Python programming, three-dimensional analysis using X-ray micro-computed tomography (micro-CT), texture analysis, and permeability tests. Mechanical properties were determined through compressive strength, flexural tensile strength, and abrasion mass loss tests. Additionally, durability against carbonation was evaluated. To integrate the different investigated properties, a multicriteria analysis based on dimensionless normalization was employed, allowing the identification of mixtures FKR10 and CKR10 as those presenting the best overall performance by balancing permeability, mechanical strength, and adsorptive capacity. The results demonstrated that additions of 10% FKR and CKR promoted moderate refinement of the paste–aggregate matrix, increased mechanical strength, and maintained permeability at adequate levels for pavements subjected to light traffic loads. In contrast, higher CKR contents (30% and 40%) resulted in mixtures with greater internal heterogeneity, less cohesive interfaces, and higher abrasion mass loss. The mixtures containing residues exhibited significant adsorptive performance during the initial hours of testing (First Flush), with Al³⁺ and Fe²⁺ removal efficiencies ranging from 25% to 48%. Therefore, based on the obtained results, it was possible to highlight that 10% contents of FKR and CKR may be considered the most technically viable for application in pervious concrete. Thus, the use of these residues may contribute to the circular economy, constituting a technically feasible and sustainable solution.