Influence of Fly Ash on the Fresh and Hardened Properties of a Cement-Based Grout for Mechanized Tunneling

This work is focused on studying the influence of fly ash on the fresh and hardened properties of a grout formed by Portland cement and sodium-activated bentonite. Fly ash partially replaced the cementitious material as 10, 30 and 50% of binder weight. This mixture constitutes a part of a two-component grout used for fulfilling the annular gap between the tunnel structure and the surrounding soil which occurs during tunnel excavation by using a tunnel boring machine. The remaining part is a chemical admixture for accelerating the grout hydration and is not the focus of this study. For the fresh state, viscometry tests were carried out imposing increasing and decreasing shear rates. Although fly ash improved the rheology by increasing the partial replacements, bentonite still highly influenced the thixotropy and shear thinning behavior was obtained in all grouts. For the hardened state, hydration and mechanical tests were used. Fly ash delayed the set time proportionally to its content and increased the mechanical compression response in the long term at 92-day age test.