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RESEARCH PRODUCT

Interactions ciment-organofunctional silanes : implications on hydration and on mechanical properties of the modified systems

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Nowadays concrete is the most attractive option for the construction sector. This is because concrete itself is a low cost, low energy and low environmental impact material. Moreover, concrete structures are very durable and high load bearing. This is achieved by incorporating steel, because concrete itself is a very low tensile strength material. Chemically, the weakness originates in the cohesive nature of cement used for concrete making. Nanoscale experimental investigations and numerical simulations showed that cohesion of cement paste is caused by short range surface forces acting between calcium silicate hydrates (C-S-H) in the interstitial solution. This thesis addresses the possibility of engineering the bonding between hydrates in order to tune the mechanical properties of cementitious materials. We aim at introducing long range cohesion forces between hydrates in addition to the existing ones. This should potentially lead to an increase in strength and toughness. The strategy chosen was to hybridize the cement prior to hydration with organofunctional silanes. Two possible methods of silanization were investigated and the modified products have been characterized. The first method consisted in dry blending cement powder to silanes. It is shown that by doing so, cement pastes and mortars exhibit improved workability. In addition, we have observed that silane agents strongly affect the hydration kinetics, mainly by retarding the hydration of silicates and reducing their degree of hydration. As a consequence, severe strength loss was evidenced in all standard mechanical tests. This was related to excessive dosage of silane to cement imposed to reach good mix homogeneity during hybridization.A second silanization methodology was developed in order to allow diminishing the dosage of silane without facing inhomogeneity mix issues. It is shown that by adsorbing silane from organic solvents we gain a better understanding of silane-cement interactions. In addition, the adsorption data provide indirect means to help characterize the modified substrates. It was found that silane-cement interactions strongly depend on the type of the solvent used as vehicle media. The surface coverage has also been calculated and is far from being monolayer because both chemically bonded and physically adsorbed species are assumed to be present. This further influences the properties of the modified cements. In terms of hydration kinetics, stronger retarding effects of silicates hydration are always associated to silanes displaying lower surface affinity, but stronger surface bonding. In terms of rheology, all silanes greatly improved the ability of pastes to withstand load above the critical deformation. This results in increased bending strength by up to 35% compared to neat cement.