Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Constitution, Configuration and Conformation: Molecular engineering of tougher cement

Modern cements set and attain strength in faster time frames than their ancient counterparts while sacrificing long term toughness. Aluminium is proven to play an integral role in toughening both ancient and contemporary clinkers but the mechanisms by which this is achieved are still unresolved at the atomic scale. This thesis focuses on investigation of the role Aluminium plays when substituted for Silicon in cementitious networks using a computational approach to model the short range order of C-(A)-S-H (cement's main binding phase). The configuration of cement chains, as well as the conformation and orientation of molecular components are treated using computational modelling to simulate a gel phase based on first principles. This model aligns with and supplements the structure of C-S-H, which has predominantly been explored in terms of its constitution using experimental techniques. C-S-H Al content is increased and modulated in relation to local phenomena such as pore and chain distribution. Using a combination of electronic structure calculations and ab initio molecular dynamics porous vs non-porous C-S-H clusters of varied morphologies are modelled and the influence Aluminium has on cements at the molecular level is refined. Key novel results pinpoint Aluminium as aiding and maintaining chain growth, providing structural 'hinge points' through its flexible coordinate geometry and curtailing shrinkage in porous systems. A positive relationship between Al avoidance and increased chain growth is also present. Dynamic simulations revealed a coalescing of apparent layers, manifesting in spherical topologies for C-S-H particles and their pores. The significance of these results points towards local mechanisms of Aluminium toughening, by lengthening and increasing angular flexibility of C-(A)-S-H chains and facilitating larger hydrated pores. These findings may be used to manipulate and guide cement design to incorporate Aluminium such that in a less brittle concrete with higher longevity.