Effect of NBO/T of Biomass Ash on Compressive Strength of Hardened Cement: New Insights from Evaluation of Pore Structure and Hydration Products

Abstract

This study investigated the physical properties of hardened cement incorporating biomass ash (BA), considering the effect of the ratio of nonbridging oxygen atoms to tetrahedral-forming ions (NBO/T), a chemical indicator of ash. The reactivity of fly ash (FA) exhibited strong positive correlation with NBO/T, which can also serve as a reactivity indicator for other supplementary cementitious materials like slag. Chemical analysis revealed that BA possesses properties of both FA and blast furnace slag. During curing, the number of voids corresponding to small and large gel pores, formed by coagulated C–S–H, increased significantly, with sizes reaching tens of nanometers. Fourier transform infrared and aluminum nuclear magnetic resonance analyses showed that cement incorporating BA had higher compressive strength than coal ash-based cement, with higher contents of ettringite, C–A–S–H, and monosulfate after 28 days. This was attributed to the distinct compositions of coal and BA, alongside the usage of seawater and gypsum. Moreover, BA further enhanced the compressive strength owing to the influence of hydration products, including C–S–H. This positive correlation between NBO/T of BA and the compressive strength of the cured material suggests that NBO/T can serve as an indicator for estimating cement blending strength.

This paper is the English translation from the authors’ previous work [Takagi, R., Hayashi, T., Kamimura, K. and Saito, T., (2023). “Effect of NBO/T of biomass ash on compressive strength of hardened cement.” Cement Science and Concrete Technology, 77, 153-162. (in Japanese)].