Insight into the Decalcification Mechanism of Calcium Silicate Hydrate under a Water–Heat–Salt Environment

17O and 29Si MAS NMR study of β– C2S hydration and the structure of calcium-silicate hydrates

The secondary aluminum ash is the black slag left after the primary aluminum ash is extracted from the metal aluminum. To address the environmental pollution and resource waste caused by the accumulation and landfill of aluminum ash, this study fabricated non-fired bricks by using secondary aluminum ash as the principal raw material, which was supplemented by cement, slaked lime, gypsum and engineering sand. The effects of mix proportions of various admixtures on the mechanical properties of non-fired bricks were investigated, and on this basis, the hydration mechanism was analyzed. The results showed that the mix proportions were 68.3% aluminum ash, 11.4% cement, 6.4% slaked lime, 4.2% gypsum and 9.7% engineering sand. The compressive strength of the fabricated bricks reached 22.19 MPa, and their quality indicators were in line with the MU20 requirements for Non-fired Rubbish Gangue Bricks. Evident hydration reaction occurred inside the non-fired bricks, with main products being calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH) and ettringite (AFt). Besides, a dense structure was formed, which enhanced the brick strength.

This study systematically investigates the effects of slag from the argon–oxygen decarburization (AOD) process, fly ash, and recycled aggregate (RA) replacement ratios on the mechanical properties of mortar samples and AOD slag–fly ash recycled concrete. The sustainable reuse of industrial by-products and construction waste is significant for reducing environmental impact and resource consumption during pavement construction. Experimental results demonstrate that when AOD slag and fly ash are used in combination, they undergo synergistic hydration reactions, producing calcium hydroxide (CH), calcium silicate hydrate (C-S-H) gel, and ettringite (AFt), resulting in superior strength compared to the individual use of either material. This research reveals that concrete strength decreases significantly when the recycled aggregate replacement ratio exceeds 50%; therefore, RA = 50% was selected as the optimal replacement ratio for subsequent studies. On this basis, when the combined replacement ratio of AOD slag and fly ash is 10–20%, concrete performance reaches its optimum level: maximum compressive strength is 33.9 MPa, which is 8.57% and 36.2% higher than using fly ash or AOD slag alone, respectively; maximum flexural strength is 4.6 MPa, which is 6.08% and 14.44% higher than using fly ash or AOD slag alone, respectively; and peak axial compressive and splitting tensile strengths are 24.9 MPa and 3.4 MPa, respectively. These findings demonstrate that the synergistic use of AOD slag, fly ash, and recycled aggregates can produce concrete that meets pavement application requirements, while effectively promoting the resource utilization of industrial by-products and construction waste, aligning with circular economy principles.

The mechanisms of hydration are described in the context of concrete mix design. Properties of hydration products are described with emphasis being placed on those of calcium silicate hydrates. The evolution in the volumes of solids present and in their compositions with hydration is discussed. Relationships between cement composition, mix design proportions and property development in concrete as influenced by hydration are reviewed.

Case study on coal-based solid waste utilization in backfill paste: Hydration mechanism and performance analysis

Ab initio metadynamics simulations on the formation of calcium silicate aqua complexes prior to the nuleation of calcium silicate hydrate

Calcium silicate nanostructured materials are promising drug carriers owing to their excellent biocompatibility, good bioactivity and high drug-loading capacity. In recent years, studies have been carried out on the synthesis of calcium silicate hydrate (CSH) nanostructured materials with desirable sizes and morphologies and their applications in drug delivery, where very interesting results and important insights have been documented. This editorial is not intended to offer a comprehensive review on the research on CSH nanostructured materials as drug carriers; rather, it presents representative examples: i) mesoporous microspheres; ii) ultrathin nanosheets; iii) iron oxide/CSH core/shell nanocomposites; and iv) CSH/block copolymer nanocomposites, and important results obtained in the study of CSH drug delivery systems for ibuprofen (IBU) as a model drug. These results show that the nanostructured CSH materials with specially designed architectures as IBU carriers have ultrahigh drug-loading capacity and sustainable drug release properties; thus, they are promising drug carriers for IBU. In addition, a new drug release kinetics has been found in the nanostructured CSH drug delivery systems. Most recently, new insight has been gained by tracking the behavior of these drug delivery systems on the molecular level using synchrotron-based X-ray spectroscopy.

In this paper, calcium silicate hydrate (CSH) gel and calcium silicate hydrate doped with asphalt emulsion (EACSH) were synthesised using Na2SiO3•9H2O, Ca (NO3)2•4H2O, NaOH, deionised water and anionic asphalt emulsion. N2 isothermal adsorption, X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the effect of asphalt emulsion on morphology and porosity of CSH. The Frenkel–Halsey–Hill (FHH) model was used to analyse the surface fractal characteristics. The results showed that the asphalt emulsion had little effects on the change of specific surface areas and the distribution of the micropores for CSH gel, which may be related to the absence of the intercalation reaction and the “opening” effect. The asphalt emulsion could promote the formation of mesoporous hydrated calcium silicate and decreased the number of mesopores in CSH gel but had little effects on the macroporous and microporous structure of CSH. The calculation results of the FHH model showed that the fractal characteristics between CSH and EACSH were different. The emulsifier may adsorb CSH gels result in globular accumulation state.

Simultaneous removal of nitrogen and phosphorus by magnesium-modified calcium silicate core-shell material in water

A simple and low-cost solution synthesis is reported for low-crystalline 1.4 nm tobermorite-like calcium silicate hydrate (CSH) ultrathin nanosheets with a thickness of ~2.8 nm and with a large specific surface area (SSA), via a reaction-rate-controlled precipitation process. The BET SSA of the CSH ultrathin nanosheets can reach as high as 505 m(2) g(-1) . The CSH ultrathin nanosheets have little cytotoxicity and can be converted to anhydrous calcium silicate (ACS) ultrathin nanosheets with a well preserved morphology via a heat treatment process. The crystallinity of CSH ultrathin nanosheets can be improved by solvothermal treatment in water/ethanol binary solvents or a single solvent of water, producing well-crystalline 1.1 nm tobermorite-like CSH nanobelts or nanosheets. CSH ultrathin nanosheets acting as building blocks can self-assemble into layered nanostructures via three different routes. The CSH ultrathin nanosheets are investigated as promising adsorbents for protein (hemoglobin, Hb), drug (ibuprofen, IBU), and metal ions (Cr(3+) , Ni(2+) , Cu(2+) , Zn(2+) , Cd(2+) , Pb(2+) ). The highest adsorbed percentages of Hb and IBU are found to be 83% and 94%, respectively. The highest adsorption capacities of Hb and IBU are found to be as high as 878 milligram Hb per gram CSH and 2.2 gram IBU per gram CSH, respectively. The ppm level metal ions can be totally adsorbed from aqueous solution in just a few minutes. Thus, the CSH ultrathin nanosheets are a promising candidate as excellent adsorbents in the biomedical field and for waste water treatment. Several empirical laws are summarized based on the adsorption profiles of Hb and IBU using CSH ultrathin nanosheets as the adsorbent. Furthermore, the ACS ultrathin nanosheets as adsorbents for Hb protein and IBU drug are investigated.

To comprehensively learn the dynamic decalcification process of C-S-H, changes of calcium-to-silicon molar ratio (Ca/Si ratio) of C-S-H powders with NH4NO3 solution concentration and with decalcification time as well as decalcification homogeneity are investigated. C-S-H particles are decalcified by NH4NO3 solution of different concentration at different time. Ca/Si ratios on the outer surface and inside the C-S-H particles were measured by Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). X-ray diffraction (XRD) tests were conducted to study the crystalline structure of C-S-H. It is found that the calcium at the edge of C-S-H particles is preferentially removed, and the decalcification gradually progress inside as the leaching period is extended. Instead of immersing C-S-H powders in the standard 6 mol/L NH4NO3 solution for a predetermined time, it is recommended that C-S-H powders are immersed in NH4NO3 solution of different concentration for extended time periods to obtain homogeneously decalcified C-S-H particles.

EXAFS study of U(VI) uptake by calcium silicate hydrates

We studied the Cm(III) retention by calcium silicate hydrate (C-S-H), portlandite (Ca(OH)2) and their alteration products calcite, vaterite, and aragonite in high ionic strength carbonate-containing solutions representing specific formation waters. For this, we synthesized C-S-H gels with calcium to silicon (C/S) ratios of 1.0 and 2.0 in the absence and presence of Cm(III), resulting in Cm(III)-free and Cm(III) doped C-S-H gel, respectively. For phase identification purposes we applied X-ray diffraction (XRD) while for the identification of the Cm(III)/C-S-H binding mode we applied site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS). The stability of Cm(III) doped phases under repository-relevant conditions was evaluated by studying the time-dependent release of Cm(III) from the Cm(III) doped C-S-H gel into leaching solutions containing 0.02 M NaHCO3 or 2.5 M NaCl/0.02 M NaHCO3 over 60 d. Speciation changes of Cm(III) due to leaching were followed with TRLFS while C-S-H structure alterations and secondary phase formation were monitored with XRD. From the results it could be concluded that Cm(III) is not mobilized by aqueous carbonate but either remains incorporated in the C-S-H structure and portlandite or becomes partially re-immobilized into secondary CaCO3 phases. The presence of NaCl led to an accelerated conversion of metastable secondary CaCO3 phases into calcite.

Preparation of C-S-H gels by mechanochemistry and its influences on properties of super-retarded cement-based materials with sucrose

Na and Cl immobilization by size controlled calcium silicate hydrate nanometer pores