Composition and density of nanoscale calcium–silicate–hydrate in cement

Abstract

Although Portland cement concrete is the world's most widely used manufactured material, basic questions persist regarding its internal structure and water content, and their effect on concrete behaviour. Here, for the first time without recourse to drying methods, we measure the composition and solid density of the principal binding reaction product of cement hydration, calcium-silicate-hydrate (C-S-H) gel, one of the most complex of all gels. We also quantify a nanoscale calcium hydroxide phase that coexists with C-S-H gel. By combining small-angle neutron and X-ray scattering data, and by exploiting the hydrogen/deuterium neutron isotope effect both in water and methanol, we determine the mean formula and mass density of the nanoscale C-S-H gel particles in hydrating cement. We show that the formula, (CaO)1.7(SiO2)(H2O)1.80, and density, 2.604 Mg m(-3), differ from previous values for C-S-H gel, associated with specific drying conditions. Whereas previous studies have classified water within C-S-H gel by how tightly it is bound, in this study we classify water by its location-with implications for defining the chemically active (C-S-H) surface area within cement, and for predicting concrete properties.

Microstructural changes of globules in calcium–silicate–hydrate gels with and without additives determined by small-angle neutron and X-ray scattering

“…Also, we only fitted the Q-range of 0.02 Å À1 < Q < 0.70 Å À1 and neglected the surface fractal effect in the Q-range chosen for the SAXS and SANS data analysis. This is because the surface fractal feature is mainly present within the Q-range below 0.01 Å À1 [12,13]. Finally, because the change of the n during the fitting process had little effect on the inter-particle structure factor S(Q) in the range of Q > 0.02 Å, we fixed n at 670 Å in accordance with what is reported in Refs.…”

“…The layer thickness of a globule is not significantly affected by the PCEs added. This is a clear proof of the lack of any intercalation phenomena in the CÀSÀH/ a n is fixed as 670 Å [11,12] and v saxs is fixed as 0.064, assuming the density of the calcium silicate layer to be the same as CÀSÀH particle in D-drying condition. The width parameter Z of the number of layers with Schultz distribution is collapsed onto the lowest boundary 0.01 set by the fitting process, indicating the wide distribution of n. Here, we allow L SAXS and L SANS to be different within a few angstroms to consider the difference of the instrument resolutions.…”

“…Finally, because the change of the n during the fitting process had little effect on the inter-particle structure factor S(Q) in the range of Q > 0.02 Å, we fixed n at 670 Å in accordance with what is reported in Refs. [11,12]. Fig.…”

“…We fixed v SAXS to be 0.064 for both CÀSÀH gels with and without additives, assuming the density of the calcium silicate layer to be the same as that of the CÀSÀH particle in D-drying condition [12]. This is reasonable because the scattering length densities of PCEs are very close to water's SLD.…”

“…Very recently, we used small-angle neutron scattering (SANS) to study the microstructure of synthesized pure calcium-silicate-hydrate (I) gel (CÀSÀH (I)) [11]. The analysis was based on Jennings' colloidal model-II (CM-II) [12,13]. In CM-II, CÀSÀH gel is described as a fractal-like object formed by the packing of colloidal particles called ''globules''.…”