Biomass ash as supplementary cementitious material: microstructural mechanism underlying cement paste strength evolution

In order to achieve the effective utilization of biomass ash (BA) as a potential cementitious component, the present work explores the influence of BA, employed as a supplementary cementitious material, on the properties of cement paste. Meanwhile, the mechanism by which various BA replacement levels affect the mechanical strength of cement paste is analyzed via a series of microscopic characterization techniques. In this experimental program, BA was applied as a silica-rich admixture, and cement paste specimens were manufactured by substituting Portland cement with BA at mass ratios ranging from 0% to 50%According to the macroscopic test results, the fluidity of the fresh paste exhibits a slight decreasing trend with the elevation of BA substitution content. In terms of strength development, an initial increasing trend is observed, followed by a gradual reduction, and the optimal overall performance is achieved at a 20% replacement level; under the water-to-binder ratio of 0.6, the 28-day compressive strength reaches 36.11 MPa, showing a 29.6% increase compared with the control group (pure cement paste, 27.86 MPa), and the strength remains above 95% of the control group even at a 30% replacement level. Microscopic analytical results verify that BA possesses noticeable pozzolanic reactivity and is capable of participating in partial hydration reactions. During the hydration procedure, the silica-bearing mineral phases in BA are incorporated into the silicate chains and interlayer structures of C-S-H gels. Furthermore, the relative content of silicate phases within C-S-H gels and their polymerization degree are both remarkably enhanced with increasing BA replacement ratios. The impact of BA content on the pore structure of cement paste differs under varying water-to-binder (w/b) ratios, among which gel pores and mesopores are identified as the dominant factors responsible for the evolution of the overall pore system.