Strength, carbonation, and microstructure of CO2-cured unfired building blocks: effect of curing conditions

Abstract

The reduction of CO₂ emissions in the construction industry has become an urgent requirement in pursuit of sustainable development goals. In this context, CO₂ curing of concrete emerges as a promising technological solution, contributing both to greenhouse gas sequestration and to product quality enhancement. This study focuses on evaluating the effectiveness of CO₂ curing on cement-based non-fired building blocks (NBB) grade M15 and manufactured by vibration-pressing technology. Experiments were conducted under varying curing conditions, including CO₂ pressure (2 bar) and exposure durations (6, 12, 18, and 24 hours) at ambient temperature, compared to traditional water-cured control samples. The evaluated parameters include compressive strength at 3, 7, and 28 days; flexural strength; water absorption; carbonation depth; and microstructural characteristics analyzed via SEM and XRD. The results show that CO₂-cured samples achieved 33–61% higher compressive strength at 3 days and 16–23% higher at 28 days compared to the control. Water absorption decreased from 11.3% to 8.5%, while carbonation depth reached up to 18 mm after 24 hours of curing. XRD analysis revealed strong formation of calcite and vaterite phases, while SEM images indicated a denser microstructure with pores filled by carbonation products. CO₂ curing is thus a feasible solution to improve the quality and durability of the grade M15 NBBs, while simultaneously contributing to greenhouse gas mitigation in building material production.