Evaluation of the effect of curing conditions on compressive strength and microstructure of alkali-activated binders

Alkali-activated binders (AABs) are increasingly researched as sustainable alternatives to ordinary Portland cement (OPC), providing comparable strength. In recent years there has been a growing interest in studies focusing on the development of suitable composites and their fabrication methods. Composites' properties are significantly affected by temperature, humidity, formulations, and constituent types. Therefore, it is essential to understand these factors comprehensively to obtain the desired adequacy to use. This article aims to assess the impact of (i) temperature through ambient curing (25°C) and thermal curing (50°C for 24 hours), (ii) exposure to air (isolated or exposed to the air atmosphere), and (iii) susceptible to external humidity (air contacted or submerged) as curing conditions. The evaluation encompassed compressive strength tests at 1, 28, 63, and 91 days, along with microstructural evaluation through morphology analysis and incidence of voids. Digital image processing (DIP) through ImageJ software was employed. The AAB proposed consisted of fly ash (FA) and steel slag from the Basic Oxygen Furnace (BOF) process as precursors, with sodium hydroxide (NaOH) and sodium silicate (NaSi2O3) as alkali activators, applied in mortar production. Compressive strength results revealed that mortar subjected to thermal curing, isolated from air contact and external humidity, exhibited the highest strength outcome, with 46.05 MPa. Microstructural analysis indicated the formation of hydrated aluminosilicate gels and the presence of voids, including partially reacted particles and microcracks. The DIP analysis of the sectional area demonstrated that void incidence under 50 µm2 predominantly did not affect compressive strength.