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RESEARCH PRODUCT

Impact of small organic molecules on the nucleation of calcium silicate hydrate

subject

description

Superplasticizers are nowadays commonly employed in concrete to improve its workability and mechanical performance. They generally have the side effect of retarding the hydration of Portland cement which results from two simultaneous reactions: the dissolution of cement grains and the precipitation of cement hydrates. The mechanisms responsible for this retardation and thus delay the setting time of cement remain however largely unexplained. The present work intended to tackle this puzzle from the perspective of cement hydrate formation and employing four small organic molecules as model for superplasticizers. In particular, the impact of these organics on the homogeneous nucleation of calcium silicate hydrate (C-S-H) was studied in detailed. The work further focused on the determination of the driving force of the nucleation (and dissolution) and thus on the determination and modeling of the ion speciation and organic complex formation with calcium, silicate and hydroxide aqueous species. The selected organics whether charged or neutral were all found to form a rich zoo of mono- and poly-nuclear complexes not only with calcium and hydroxide ions but also with silicate species. They were further successfully modeled with the speciation program PHREEQC. This allowed to compare and quantify the impact of the selected organic molecules on the nucleation rate of C-S-H. The nucleation rate was correlated to the complexation power of the organics. A modification of the C-S-H nucleation pathway induced by the organics was found for which the organic complexation with silicate and calcium ions seems to play a crucial role. More generally, the retardation power of the molecules on the hydration of alite, the main mineral phase of Portland cement, was found to correlate well with their impact on the nucleation rate of C-S-H.