Mechanisms of cement hydration

Degree of hydration and gel/space ratio of high-volume fly ash/cement systems

Impact of admixtures on the hydration kinetics of Portland cement

To understand the effects of colloidal nanoSiO2 (CNS) on cement hydration and gel properties in the early and later age, hydration heat, calcium morphology, hydroxide content, non-evaporable water (NEW) content and nanoscale mechanical properties were measured. Some comparison studies were conducted on silica fume (SF) paste, as well. Results revealed that the accelerating effect of CNS on hydration in the early age is achieved by the acceleration of cement dissolution and hydrate nucleation on reacted nanoSiO2 particles. Although cement hydration can be greatly accelerated by CNS in the early age, its later age hydration is hindered. The NEW content of CNS-added paste experiences a higher rate of increase initially, but gradually becomes smaller than that of the control paste due to changes in the gel structure, making NEW content an unsuitable method for monitoring the hydration of CNS-added paste. However, nanoindentation results revealed that CNS modifies the gel structure to increase the high-stiffness C–S–H gel content.

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Modification effects of colloidal nanoSiO2 on cement hydration and its gel property

A digital image-based model of the microstructure of cement paste, coupled with exact transport algorithms, is used to study the diffusivity of Portland cement paste. The principal variables considered are water∶cement ratio, degree of cement hydration and capillary porosity. Computational methods are described and diffusivity results are presented, which are found to agree with the available experimental measurements within experimental error. Model cement pastes prepared with different water∶cement ratios, and having different degrees of hydration, are found to have diffusivities that lie on a single master curve when plotted as a function of capillary porosity. Concepts from percolation theory are used to explain quantitatively the dependence of diffusivity on capillary porosity. The effect of silica fume addition on diffusivity is also examined.

Computer simulation of the diffusivity of cement-based materials

Reactions of colloidal silica fumes with calcium hydroxide or hydrating tricalcium silicate (C3S) have been studied using calorimetry, chemical analyses, and scanning electron microscopy. Silica fume reacts immediately with calcium hydroxide forming a colloidal calcium silicate hydrate (C-S-H) similar to that formed by the hydration of C3S. When excess silica is present it reacts with C-S-H already formed to produce a new, highly polymerized C-S-H, having a very low C/S ratio (1.0). Silica fume accelerates the hydration of C3S, reduces the amount of calcium hydroxide formed by reacting with it, and slightly lowers the C-S-H ratio of the C-S-H formed by hydration. When large amounts of silica fume are present the formation of calcium hydroxide may be entirely suppressed and a highly polymerized C-S-H is formed. Silica fume is considered a good model for reactive pozzolans used in concrete.

The hydration of tricalcium silicate in the presence of colloidal silica

Double membrane gasholder for biogas storage: Odor pollution risk and permeation characteristics

Retention of organic matter on the surface of illite particle under the influence of different cations: A molecular dynamics simulation study

In this study, three mesoporous-activated crab shell biochars were prepared by carbonation and chemical activation with KOH (K−CSB), H3PO4 (P−CSB), and KMnO4 (M−CSB) to evaluate their tetracycline (TC) adsorption capacities. Characterization by SEM and a porosity analysis revealed that the K−CSB, P−CSB, and M−CSB possessed a puffy, mesoporous structure, with K−CSB exhibiting a larger specific surface area (1738 m2/g). FT-IR analysis revealed that abundant, surface ox-containing functional groups possessed by K−CSB, P−CSB, and M−CSB, such as −OH, C−O, and C=O, enhanced adsorption for TC, thereby enhancing their adsorption efficiency for TC. The maximum TC adsorption capacities of the K−CSB, P−CSB, and M−CSB were 380.92, 331.53, and 281.38 mg/g, respectively. The adsorption isotherms and kinetics data of the three TC adsorbents fit the Langmuir and pseudo-second-order model. The adsorption mechanism involved aperture filling, hydrogen bonding, electrostatic action, π-π EDA action, and complexation. As a low-cost and highly effective adsorbent for antibiotic wastewater treatment, activated crab shell biochar has enormous application potential.

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Zhao-Qi Wu

Papers

The hydration of tricalcium silicate in the presence of colloidal silica

Double membrane gasholder for biogas storage: Odor pollution risk and permeation characteristics

Retention of organic matter on the surface of illite particle under the influence of different cations: A molecular dynamics simulation study

Mesoporous Activated Biochar from Crab Shell with Enhanced Adsorption Performance for Tetracycline