Application of CO2-foamed lightweight grout with early strength and low density as a low-carbon material in shield synchronous grouting

Abstract

A popular topic in the field of underground engineering is to find synchronous grouting materials with excellent performance and environmental protection characteristics. In this study, CO2-foamed synchronous grouting (CSG) materials are prepared with cement, water, CO2 foams, fly ash, fine river sand and bentonite. The feasibility of the material is verified by a series of detailed tests and analyses. The properties and performance of the CSG materials are investigated. The effects of the ratios of water/binder (cement + fly ash), fly ash/cement, binder/fine river sand and bentonite/water on the response value of the synchronous grouting material performances are investigated with stepwise nonlinear regression analysis models and response plane diagrams. NSGA-II is used to solve the multiobjective problem to obtain an optimal mass ratio. The results indicate that the grout component proportion directly impacts the performance of CSG material. Under the optimal ratio conditions, the specific gravity of CSG is 19.42 % lower and the early strength is 52.46 % higher than that of traditional grouts, and other properties meet the basic performance requirements of grout. The microstructure and product analysis show that the presence of CO2 foams causes the carbonation reaction of the hydration products in the cement mortar to form calcium carbonate and silica gel. The carbonation reaction consumes free water, decreasing the setting time. Calcium carbonate and silica gel effectively increase the mechanical properties of the material and achieve the storage of CO2 in cement-based materials. The research strategy and optimal ratio proposed in this paper verify the effectiveness, practicality and environmental economy of CSG material as a synchronous grouting material, providing a new choice for tunnel excavation grouting.