The most promising solution towards cementitious materials with a lower carbon footprint is the partial substitution of the clinker by supplementary cementitious materials (SCMs) such as fly ash, blast furnace slag, limestone and calcined clays. The production of these materials does not imply the formation of CO2, but its implementation into cement pastes tends to lower the early-age strength of the latter. However, it has recently been demonstrated that the addition of zinc can enhance the mechanical strength of a clinker and therefore allow for a higher substitution of SCMs. The main hydration product of Portland Cement is calcium-silicate-hydrate (C-S-H), which constitutes 60% of its volume and is responsible for the main peak in the heat release curve during cement hydration (main hydration peak). Therefore, it is of utmost importance to study the effect of zinc in isolated and synthetic C-S-H systems.

This work introduces an extensive study through 29Si solid-state nuclear magnetic resonance (NMR), density functional theory (DFT) simulations and molecular dynamics (MD) brick modelling to resolve the atomic structure of zinc-modified synthetic C-S-H. MD simulations are a method for analysing the movement and final positions of atoms in a structure in a fixed period of time. DFT is a computational quantum mechanical modelling used to investigate the electronic structure of the previously obtained bricks to determine the expected chemical shifts from the Si species. The possible structures obtained from MD and their expected chemical shifts obtained through DFT are then compared to NMR experimental data. To prepare the samples for this NMR analysis, aqueous sodium metasilicate, calcium nitrate and zinc nitrate solutions are reacted under controlled pH, temperature and atmospheric conditions. This precipitations process yields pure single-phase C-S-H in the presence of zinc with a target Ca/Si ratio of 1.75. Dynamic nuclear polarization (DNP) enhanced multi {1H}29Si cross-polarization (multiCP) magic-angle spinning (MAS) NMR is used to measure the populations of the different Si species in the zinc-modified C-S-H structure. In addition, we use Incredible Natural Abundance Double Quantum Transfer Experiment (INADEQUATE) to determine the connectivities between silicate species which are present in the samples. Based on atomistic modeling, DFT simulations and experimental evidence it is found that there are two main new Si species, Q(1,Zn) and Q(2p,Zn), present.

The next step towards a complete description of C-S-H and zinc-modified C-S-H is understanding the nucleation and growth mechanisms of both materials. Here, a comprehensive C-S-H model is also developed which includes a complete set of thermodynamic and kinetic equations from classical nucleation theory and is fitted to high-quality data of all stages of precipitation: pre-titration, pre-precipitation, nucleation, growth, and equilibration. In addition, the model is extended to describe zinc-modified C-S-H and has potential to be extended to other C-S-H-derived materials. Although the model needs to be refined, it is a big step towards understanding the thermodynamic and kinetic processes involved in the nucleation and growth of C-S-H.