The role of bentonite in two-component grout: A comparative study

Two-component grout is the most used backfilling technology in shielded mechanised tunnelling. Bentonite is an important ingredient in the production of component A, since it plays a key role in the mortar stabilisation. Furthermore, recent studies have highlighted its involvement in the mechanical and elastic performance of the hardened grout after short curing times, while no information is available for long curing times. This study proposes an innovative test campaign, based on a comparison between a standard two-component grout mix design (reproduced with 5 different bentonites) and a special mix design, produced without using the bentonite. This interesting comparison highlights the important role of the bentonite and its activation time in properties related to fresh grout (component A) and hardened grout. Assessments of bleeding, viscosity, unit weight, gel time, surface compression strength (SCS), uniaxial compression strength (UCS) and tensile strength (T0) have permitted a deep quantitative analysis that, undoubtedly, highlights the importance of bentonite in this widespread backfilling grout technology.