Phase Composition of Hydrated DSP Cement Pastes

Abstract

Cement pastes densified with small particles (DSP) containing up to 48% silica fume by weight of cement, and hydrated to up to 180 d at room temperature, have been analyzed using TMS-GPC, TGA, and 29Si NMR to quantitatively estimate the amount of unreacted cement, Ca(OH),, and residual silica fume, respectively. Using a mass balance approach, the CaO/SiO, and H,O/SiO, molar ratios of the C-S-H in the samples were calculated. For samples containing silica fume, the values of CaO/SiO, lie between 0.9 and 1.3, depending on the degree of hydration and silica fume content, whereas for samples without silica fume they were 1.6. Silicate polymerization analysis using TMS-GPC suggests that the molecular structure of the C-S-H is similar to that formed in conventional hydration. No cross-linking species were found, but the fraction of higher polymers (above octamer) increases as the CaO/ SiO, ratio decreases.

Capillary Porosity Depercolation/Repercolation in Hydrating Cement Pastes Via Low‐Temperature Calorimetry Measurements and CEMHYD3D Modeling

“…As noted previously by Hansen, 19 it appears that low w/c cement pastes cured at higher temperatures have an inherently higher amount of chemically bound (or at least non‐evaporable) water than conventional w/c pastes. When the C–S–H is restricted to form in very confined spaces, it appears that while its physically bound water per unit hydration may decrease, its chemically bound water increases, as has also been observed for densified with small particles (DSP) w/cm =0.18 blended cement pastes hydrated at 23°C under “saturated” conditions by Lu et al 20 . Conversely, for w/c =0.33 pastes of tricalcium silicate, Odler and Skalny 21 have observed that the chemically bound water content per unit hydration (measured directly by X‐ray diffraction) actually decreases with hydration time for sealed hydration either at 25° or 50°C.…”

“… In agreement with previous results, 5 LOI measurements were found in this study to be a reliable indication of degree of hydration for w/c ≥0.35 cement pastes, consistent with measured developments of compressive strength and heat of hydration. However, for w/c =0.25 cement pastes and particularly for 40°C curing, the non‐evaporable water content per unit hydration appears to increase with time 19,20 . Under higher temperature curing conditions, LOI‐interpreted degrees of hydration should not be used for cement pastes with w/c ≤0.25. Sealed curing can result in a repercolation of the capillary porosity that was initially depercolated by hydration, because of chemical shrinkage and its accompanying self‐desiccation inducing autogenous stresses and strains on the cementitious gel‐like hydrated microstructure. While an increase in curing temperature from 20° to 40°C significantly accelerates cement hydration, it also produces a coarser capillary porosity system that takes longer to achieve complete depercolation in w/c =0.35 cement pastes (both in terms of required time and degree of hydration). In general, reasonable agreement was observed between the experimental measurements and CEMHYD3D predictions both for achieved degrees of hydration and for the volume fractions of connected capillary porosity. The volume fraction of connected capillary porosity created by autogenous damage under sealed curing could be effectively estimated, using the LTC measured peak height at −15°C, as was first “validated” for saturated curing conditions by comparison with the volumetric predictions of the CEMHYD3D model.…”

“…In agreement with previous results, 5 LOI measurements were found in this study to be a reliable indication of degree of hydration for w/c ≥0.35 cement pastes, consistent with measured developments of compressive strength and heat of hydration. However, for w/c =0.25 cement pastes and particularly for 40°C curing, the non‐evaporable water content per unit hydration appears to increase with time 19,20 . Under higher temperature curing conditions, LOI‐interpreted degrees of hydration should not be used for cement pastes with w/c ≤0.25.…”

Re-examination of cement hydration: sol–gel process

“…Based on the measurements from the trimethylsilylation (TMS) technique, Lu et al (1993) also concluded that dimer is the predominant silicate species in the calcium silicate hydrate phase of 'densified with small particles' (DSP) cement pastes, with linear pentamer being the next most abundant, along with some linear octamer. The presence of a small amount of trimer and tetramer has been attributed to side reactions.…”

Study of rubber-filled cementitious composites

“…This is in general agreement with the data reported in the literature for 28 days aerated and autoclaved cementitious specimens. 29,30 In the second set of experiments, the mixture was the same as the control, except that 1% of coupling agent solution was added to the mixture. Figure 4(c) shows the average stress-strain curve of coupling agent containing the cement paste system.…”