Two-Step Nucleation Process of Calcium Silicate Hydrate, the Nanobrick of Cement

Abstract

Despite a millennial history and the ubiquitous presence of cement in everyday life, the molecular processes underlying its hydration behavior, like the formation of calcium–silicate–hydrate (C–S–H), the binding phase of concrete, are mostly unexplored. Using time-resolved potentiometry and turbidimetry combined with dynamic light scattering, small-angle X-ray scattering, and cryo-TEM, we demonstrate C–S–H formation to proceed via a complex two-step pathway. In the first step, amorphous and dispersed spheroids are formed, whose composition is depleted in calcium compared to C–S–H and charge compensated with sodium. In the second step, these amorphous spheroids crystallize to tobermorite-type C–S–H. The crystallization is accompanied by a sodium/calcium cation exchange and aggregation. Understanding the formation of C–S–H via amorphous liquid precursors may allow for a better understanding of the topography of the nucleation in cement paste and thus the percolation of hydration products leading to the mechanical setting as well as the retarding effect of known chemical species like aluminum ions and polycarboxylate ethers.

Modeling Dissolution–Precipitation Kinetics of Alkali-Activated Metakaolin

“…65,66 Recently, results of small angle X-ray scattering (SAXS) experiments have shown that the nucleation of calcium-silicate hydrates (C–S–H) in Portland cement occurs by aggregation of prenucleation amorphous entities. 67 Similar results were obtained for metal–organic frameworks topologically affine to zeolites. 68 SAXS experiments performed on alkali-activated metakaolin showed analogous results with the formation of 2 nm clusters, defined as oligomers by the authors, by the aggregation of smaller particles, 69 suggesting an analogy between condensation of oligomers and nucleation.…”

New insights into the role of MWCNT in cement hydration

“…These ionic clusters are unstable and may redissolve into solution before reaching a critical cluster [40]. These stable clusters then form nuclei, which gradually enter the growth process [13]. When MWCNT, cement, and water were mixed, the solution quickly contains multiple ions.…”

Advances in characterizing and understanding the microstructure of cementitious materials

“…Recent studies showed evidences of a non-classical precipitation of C-S-H with a transition from globular or spheroidal nanoparticles, defined as amorphous precursors, to the more characteristic C-S-H nanofoils (Figure 14) [153,154]. The presence of PCEs delays this conversion, probably by forming a layer around the precursors that stabilizes the metastable particles and prevent them from converting to the more stable foil-like structure [154].…”

“…Explaining this at the nanoscale remains one of the most important challenge in admixtures technology. Combining advanced techniques, such as NMR, SAXS or cryo-TEM, that have been useful in the development of new insight for the C-S-H precipitation [153,154], has the potential to increase the understanding of the action of PCEs on ettringite nucleation and growth. This challenge is however not trivial because ettringite precipitation is fast and, if not well controlled, can be altered by the precipitation of other phases, such as AFm or calcium sulfates.…”