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The design of alkali-activated soil reinforcement grouts faces several challenges regarding the required rheological, mechanical and microstructural properties. In this type of applications, the wi...

At very low pH values materials based on hydraulic binders cannot be directly exposed to acid solutions due to leaching, hence the need for additional coatings. As these are generally characterized by relatively high cost and environmental impact, new solutions are being developed. In this study, grouts based on alkali-activated materials (activated slag and activated metakaolin with and without fly ash) have been considered as an alternative to be directly exposed to acid solutions at pH = 3.5, and compared with slag cement grout. The direct monitoring of mass and volumes was combined with the chemical analysis of the leachate and the observations of cross sections through SEM/EDS and X-ray tomography in order to understand the mechanisms involved in the leaching of studied materials. The cumulated amount of leached ions was converted into an equivalent mortar volume loss and compared with the direct measurement of the volume loss. A good correlation was found for slag cement and activated metakaolin, as leaching mainly affected the binding phase. For activated slag the volume loss was lower than expected due to the formation of new phases such as silica-rich gel. The higher volume loss of metakaolin-fly ash grout was attributed to the influence of partially reacted fly ash particles.

The evolution of technical and environmental requirements fosters the development of geopolymer based grouts for soil reinforcement. Geopolymer based materials actually have several advantages but the improvement of their rheological performances remains a challenging task, as the raw materials, especially sodium silicate and metakaolin, do not have favorable properties and the main chemical admixtures used to optimize cement-based materials have not been found compatible. As mix-design takes into account the performances in both fresh and hardened states, a comprehensive study is necessary to develop geopolymer grouts based on metakaolin-fly ash-stabilizer mixtures. Fly ash allowed reducing the viscosity and increasing the setting time and the stabilizer provided the fresh mixes with better homogeneity and stability. At constant liquid to solid ratio, the packing density of the powders actually increases with fly ash proportion. Fly ash affected the mechanical properties, especially at replacement rates of 40% and higher, due to combined effects of reduced reactivity and higher packing density of the powders. This could be shown by a new methodology combining the monitoring the elastic properties with isothermal calorimetry for understanding the early-age behavior and the distinction between the geopolymerization stages. The addition of xanthan gum had a beneficial effect on the stability of grouts, by acting on the activation solution without having any significant effect on the geopolymerization reaction.

The nanostructure of alkali-activated materials is now better understood. However, very few studies have been carried on grouts. These highly diluted systems are used to improved soil properties for instance. In this study, the evolution of their local structure was monitored via 29Si and 27Al NMR to understand the formation of the binding phase and its evolution at long-term. The results of this analysis were used to question the relationship between the evolution of the chemistry of binding phases and the observed changes in the mechanical properties at material scale. Metakaolin and slag were used, with the partial incorporation of fly ash to study its effects and the potential benefits of its use. The results show that while metakaolin-based systems are very stable at both scales (local and macroscopic), the structure of slag-based grouts tends to evolve, with a negative impact on their mechanical performance.