Setting Behavior and Phase Evolution on Heat Treatment of Metakaolin-Based Geopolymers Containing Calcium Hydroxide

Setting behavior in geopolymers remains poorly understood despite its importance for industrial applications. Prior research has shown that calcium hydroxide can influence setting times, but the mechanisms are not fully resolved. No prior work had resolved how calcium hydroxide affects phase evolution during heat treatment. This gap motivated the current investigation into the interplay between calcium hydroxide and geopolymer setting. The role of calcium hydroxide in modifying early-stage reactions is still debated. Some studies suggest it accelerates setting, but others report inconsistent effects. The transformation of geopolymer phases under heat remains an open question. This uncertainty drives the need for detailed structural analysis.

This study aimed to clarify the role of calcium hydroxide in geopolymer setting and phase evolution. The researchers focused on how calcium hydroxide dosage affects setting time and compressive strength. They sought to determine if phase changes depend on calcium hydroxide content. The motivation stemmed from inconsistent prior findings on calcium hydroxide effects. The team used XRD to track phase development during geopolymerization. They also examined how heat treatment influences the resulting phases. The goal was to link calcium hydroxide content with structural outcomes. The study aimed to provide a clearer picture of the setting mechanism.

The researchers prepared geopolymers with varying calcium hydroxide content. They measured setting times using standard methods for each mix. XRD analysis was used to identify phase changes during geopolymerization. Rietveld refinement helped quantify the amount of each phase present. High-temperature XRD tracked phase evolution during heat treatment. The team compared low and high calcium hydroxide doses in their samples. They analyzed compressive strength to assess mechanical performance. The methods combined structural and mechanical evaluations to address the research goals.

Setting times varied with calcium hydroxide content, showing no consistent trend. Low doses up to 2% improved compressive strength and accelerated setting. High calcium hydroxide doses led to rapid C-S-H gel formation in early stages. Some calcium hydroxide remained unreacted in high-dose samples. XRD revealed that geopolymer transformed into Si-rich nepheline during heating. C-S-H gel converted to wollastonite under high-temperature conditions. Wollastonite was also observed in low-dose samples after heat treatment. The results suggest that C-S-H gel forms alongside geopolymer regardless of calcium hydroxide content.

The study suggests that calcium hydroxide influences both setting and phase evolution in geopolymers. Low calcium hydroxide doses may enhance mechanical properties and setting speed. High doses lead to C-S-H gel formation but leave some calcium hydroxide unreacted. The transformation of geopolymer into nepheline and C-S-H into wollastonite was observed. These findings may help optimize geopolymer formulations for specific applications. The results suggest that phase evolution is not strictly dependent on calcium hydroxide content. The presence of C-S-H gel appears to be a consistent feature regardless of dosage. The study provides insights into the complex interactions between calcium hydroxide and geopolymer phases.