Long-Term Strength Development of Fly Ash-Based One-Part Alkali-Activated Binders

Current knowledge on alkali-activated materials focuses on short-term performance metrics. Few studies have examined long-term behavior of fly ash-based binders under ambient conditions. It was already known that activator dosage affects early-stage properties. However, the long-term implications of varying activator concentrations remained unclear. No prior work had resolved how water absorption and microstructure evolve over extended periods. This gap motivated the investigation into how activator levels influence durability. Researchers proposed that higher activator content could improve long-term strength. The need for sustainable construction materials drives interest in fly ash-based systems.

The study aimed to evaluate how anhydrous sodium metasilicate dosage affects long-term properties of fly ash-based OPAAB. The specific problem addressed was the lack of data on extended performance of these binders. The motivation was to identify optimal activator levels for durability. Researchers wanted to understand how activator concentration influences hardened density and strength. They also sought to clarify the relationship between water absorption and activator content. The goal was to assess soundness and microstructural evolution over time. By examining these factors, the team aimed to inform sustainable binder design. The findings could guide the use of fly ash in construction applications.

Researchers produced OPAAB samples using 8-16% sodium metasilicate activator by weight of fly ash. They tested hardened density, compressive strength, and flexural strength. Water absorption and efflorescence formation were also measured. Microstructural analysis was conducted using FESEM imaging. The study tracked property changes over extended curing periods. The activator was blended into powdered fly ash before hydration. Samples were cured under ambient conditions to simulate real-world use. The team analyzed how activator dosage affected long-term performance metrics.

The compressive strength of OPAAB increased with higher activator dosage up to 12%. Beyond 12%, compressive strength remained stable. Hardened density showed a strong correlation with compressive strength values. Water absorption decreased from 8% to 12% activator dosage. No further reduction occurred beyond 12% activator content. FESEM images revealed uniform solid matrices with micro-cracks present. Larger pores facilitated hydrate substance crystallization, such as (N,C)-A-S-H gel. Initial dissolution of the binder occurred within the first 30 minutes. At later stages, higher activator content reduced water absorption further.

The study suggests that activator dosage significantly influences long-term strength properties of OPAAB. Higher activator content up to 12% improves compressive strength and reduces water absorption. The correlation between hardened density and strength was confirmed. The microstructure evolved with larger pores promoting hydrate formation. The initial dissolution phase occurred rapidly within the first 30 minutes. Long-term soundness was maintained across all activator levels tested. The results propose that optimal activator content is around 12%. These findings may inform the design of durable alkali-activated materials.