Molecular structure and early age behaviour of metakaolin geopolymers : insights from nanoscale modelling

Abstract

Growing demand for sustainable building materials is driving research in alternative cements, but their applicability in the construction field is still limited by an insufficient understanding of their durability. Geopolymers belong to this group of materials, since their production involves less carbon dioxide emissions than traditional cement. Geopolymers are aluminosilicate raw materials (e.g. metakaolin) activated with an alkali solution. Their binding phase commonly consists in sodium-alumino-silicate hydrate (N-A-S-H), which provides strength at the macroscale. Hence, understanding how mechanical properties emerge during the formation of N-A-S-H is crucial to control the macroscale performance of geopolymers. This dissertation discusses the results obtained with an integrated approach combining nanoscale modelling and experiments to clarify chemo-mechanical behaviour of N-A-S-H, especially at early age. This work delivers two main contributions. First, it presents a new molecular model of N-A-S-H, obtained by atomistic simulations. The uniqueness of this model lie in its ability to capture both amorphous and crystalline features displayed by literature data, whereas the other existing models only focus on either amorphous or crystalline structures, hence not fully explaining experimental observations. Second, this work quantifies for the first time the volumetric changes of geopolymers in the first stages of their formation, addressing the current lack of literature data on the so-called ”chemical shrinkage” of geopolymers. The results in this work, actually, indicate that geopolymers undergo chemical expansion, and not shrinkage, and a theoretical model is proposed to explain this uncommon behaviour. Finally, preliminary considerations on understanding ageing in geopolymers are presented. In particular, a mesoscale model based on aggregated nanoparticles is discussed, together with long term experiments on drying shrinkage and creep behaviour. Overall, this dissertation provides insights into the development of mechanical properties in geopolymers at early age, addressing some gaps whose limited understanding is a current barrier for the standardisation and commercialisation of geopolymers.