Structural Changes to the Calcium–Silicate–Hydrate Gel Phase of Hydrated Cement with Age, Drying, and Resaturation

Abstract

The effects of drying to various relative humidity (RH) levels on the internal structure of hydrated cement paste were investigated using small‐angle neutron scattering (SANS). Specimens of young and mature portland cement paste were analyzed in the initial saturated state, in the dried state, and then again after resaturation, allowing reversible and irreversible effects to be separated. While the observed changes on drying are mainly physical in nature, the ability of the microstructure to resist permanent structural rearrangement increased over time as the hydration and aging reactions progressed. Permanent changes to the nanometer‐to‐micrometer scale microstructure induced by drying were quantified by applying a fractal model to the SANS data for resaturated pastes. At RH levels above ≈54%, capillary stresses compact the nanometer‐level pore structure of the calcium–silicate–hydrate (C–S–H) gel phase, increasing the gel density by a mechanism related to that governing the classical “constant rate period” for pure gels. Owing to the restraining effects of the other solid phases in cement paste, this decrease in the volume of the C–S–H gel also increases the intensity of surface fractal scattering that arises from the deposition of hydration product onto the surface of the reacting cement particles. At all RH levels, but particularly below 54%, drying decreases the measured total internal surface area of the specimens. This is attributed to a loss of surface area at particle contacts as the average separation distance between adjacent C–S–H gel nanoparticles decreases on drying.

Influence of Nucleation Seeding on the Hydration Mechanisms of Tricalcium Silicate and Cement

“…Details of the experimental setup and analysis method are given elsewhere. 39 Specimens for SEM micrographs were impregnated with epoxy, gently ground, and then polished with progressively finer diamond pastes, terminating with a fineness of 0.25 µm. After being sputter-coated with carbon, they were analyzed in back-scattered electron (BSE) mode in a JEOL 5800 SEM.…”

A Numerical Model for Concrete Strength Change under Neutron and Gamma-ray Irradiation

“…The drying and heating associated with gamma-ray irradiation affects the properties of hardened cement paste (Maruyama et al 2014a), because C-S-H formed from Portland cement has a colloidal feature, and the morphology of C-S-H is affected by the drying and heating process (Pihlajavaara 1974;Litvan 1976, Parrott et al 1980Parrott and Young 1981;Cong and Kirkpatrick 1995;Thomas and Jennings 2003;Jennings 2008;Thomas et al 2008;Maruyama et al 2014a;. Therefore, gamma heating largely impacts the physical property of hardened cement paste.…”

The Colloid/Nanogranular Nature of Cement Paste and Properties

“…These aligned nanobricks entrap tiny pores similar in size to IGP size. Upon rewetting these pores fill and disjoining pressure pushes adjacent surfaces apart so that the nanobricks return to the more open pattern of the original saturated structure, as is seen by neutrons [12]. The pores change in size during resaturation, however, the smallest pores fill with water that becomes pressurized and the new equilibrium, which involves the slow diffusion of this water leads to a reduction of the smallest pores, and associated slow weight loss.…”

“…Recent SANS results [12] indicate that the packing becomes more compact, but that very little change occurs in the nanobricks (although their water content and density may change slightly). A granular model where the grains rearrange under the influence of capillary forces best reconciles these observations.…”