A TEM study on the very early crystallization of C-S-H in the presence of polycarboxylate superplasticizers: Transformation from initial C-S-H globules to nanofoils

Polycarboxylate superplasticizers (PCEs) present common admixtures to adjust the rheology of concrete. Previous studies revealed that they can also influence the crystallization of cement hydrates, such as e.g. ettringite. In this study, we investigated the impact of methacrylate ester-based PCEs on the nucleation and crystal growth of C-S-H precipitated from aqueous solutions of Ca(NO3)2 and Na2SiO3. TEM imaging revealed that in the absence of PCEs following a non-classical nucleation mechanism C-S-H initially precipitates as metastable, globular nanoparticles (droplets, d ~ 20-60 nm) which within less than one hour convert to the characteristic nanofoils of early C-S-H. In the presence of PCEs, the initial globuli exhibit a core-shell morphology, presumably with PCE as shell. The shell was found to delay the conversion to nanofoils for several hours by stabilizing the metastable globules. The PCE polymer exhibiting higher anionicity delayed the conversion to nanofoils more than the PCE possessing fewer carboxylate groups.