Synergistic Effects and Mechanisms of Plant Ash and Activator on Geopolymer Gel Formation, Hydration Evolution and Mechanical Properties

Against the backdrop of promoting green buildings and a circular economy, the development of efficient, sustainable, and low-carbon cementitious materials is of great significance for reducing resource consumption and carbon emissions. In this study, plant ash (PA) was used as a partial cement replacement, and a series of alkali-activated composite cementitious materials (APAG) were prepared by regulating the dosages of PA and alkali activator (AA). The evolution of their workability, hydration behavior, and mechanical properties was systematically investigated. The results show that the incorporation of PA effectively delayed the setting process of the system; compared with P0, the initial and final setting times of P20 increased by approximately 302% and 100%, respectively, thereby mitigating the excessively rapid early-age reaction of the alkali-activated system while causing only a slight reduction in flowability. In contrast, the addition of AA shortened the setting time of APAG and led to a gradual decrease in fluidity. When the PA dosage was 20% and the AA dosage was 4%, APAG achieved a 28 d compressive strength of 57.8 MPa while maintaining good workability. Further analysis revealed a strong linear correlation between compressive strength and chemically bound water content under different PA and AA dosages, indicating that the reaction degree is a key factor governing macroscopic mechanical performance. Microstructural characterization confirmed that the incorporation of PA and AA significantly altered the reaction pathways and the morphology of hydration products, providing a reasonable microstructural explanation for the evolution of macroscopic properties. These findings provide valuable insights into the high-value utilization of biomass waste and the broader application of green cementitious materials.