Avaliação técnica e ambiental do concreto de cinza volante álcali-ativada com adição de cimento portland

Abstract

Alkali-activated concrete based on fly ash is a sustainable alternative to conventional cement; however, it has low reactivity, and the use of Portland cement as a supplementary material can promote curing at ambient temperature. In this context, the present study investigated the mechanical performance of alkali-activated concrete with partial replacement of fly ash by Portland cement. Initially, the characterization of fly ash, used as a precursor, and the aggregates was carried out. Then, two pastes were prepared using sodium hydroxide and sodium silicate as activators, varying the replacement of fly ash mass by Portland cement at 5% and 10%. The alkali-activated paste was characterized in the fresh state through the Kantro mini-slump test, the flow index in the Ford Cup, and unit mass. In the hardened state, compressive strength was determined, and the microstructure was analyzed using Scanning Electron Microscopy (SEM). The concrete was composed of 35% paste and 65% aggregates. The workability of the concrete was evaluated through the slump test. Then, cylindrical specimens were molded to determine compressive strength and analyze the microstructure. Additionally, the environmental impacts of the produced alkali-activated concretes were investigated. The pastes showed spread values ranging from 125.12 mm to 136.34 mm and flow time between 101 and 185 seconds. The yield stress values were 4.6 and 2.8 Pa. The compressive strength results indicated good mechanical performance, with values of 46.45 MPa and 55.40 MPa for 5% and 10% Portland cement, respectively. The microstructural analysis revealed the presence of unreacted fly ash particles and the formation of a gel network. The concretes exhibited high workability, with slump values of 25.80 cm and 24.50 cm. The average compressive strength of the concretes was 40.51 MPa and 45.35 MPa. The environmental impact analysis indicated that the produced concretes resulted in a reduction of 55% to 60% in both CO₂ emissions and embodied energy compared to conventional concrete. The microstructure showed the formation of a geopolymeric gel coexisting with C-S-H and C-A-S-H gels. Additionally, the concretes exhibited low water absorption, with a rate of 1.09 g/cm². The results confirm that the partial replacement of fly ash with Portland cement in alkali-activated concretes is satisfactory for producing cementitious materials with good strength and lower environmental impact.