Solid-state 27Al and 29Si NMR characterization of hydrates formed in calcium aluminate–silica fume mixtures

Geopolymer Composites—In Environmentally Friendly Aspects

In the present work, a new, low energy consumption sol–gel synthesis route has been developed for geopolymer composites. Instead of the typically published 0.1–1.0 Al/Si molar ratios, the efforts of this study concentrated on the realization of >2.5 Al/Si molar ratios in the composite systems. The higher Al molar ratio significantly improves the mechanical properties. A very important aim was also the recycling of industrial waste materials with attention to environmentally friendly requirements. The very dangerous, toxic red mud as a waste product of aluminum industrial fabrication was selected for reclamation. The structural investigation was carried out by 27Al MAS NMR, XRD, and thermal analysis. The structural examination has unambiguously proven the composite phases in both gel and solid systems. The characterization of composites was performed with mechanical strength and water solubility measurements.

Reversible Adsorption of Polycarboxylates on Silica Fume in High pH, High Ionic Strength Environments for Control of Concrete Fluidity

Polycarboxylate-based superplasticizers are essential for production of ultrahigh-performance concrete (UHPC), facilitating particle dispersion through electrostatic repulsion and steric hindrance. This study examines for the first time the effect of changes in pH, ionic strength, and charge on the adsorption/desorption behavior of a polycarboxylate-based superplasticizer on silica fume in aqueous chemistries common in low-CO₂ UHPC. Data from total organic carbon measurements, Fourier transform infrared and nuclear magnetic resonance spectroscopy, and zeta potential measurements reveal the silica surface chemistry and electrokinetic properties in simulated UHPC. Addition of divalent cations (Ca²⁺) results in polycarboxylate adsorption on silica fume via (i) adsorption of Ca²⁺ ions on the silica surface and a negative zeta potential of lower magnitude on the silica surface and (ii) reduction of polycarboxylate anionic charge density due to complexation with Ca²⁺ ions and counter-ion condensation. Addition of OH^- ions results in polycarboxylate desorption via deprotonation of silanol groups and a negative zeta potential of greater magnitude on the silica surface. Simultaneous addition of both Ca²⁺ and OH^- results in rapid polycarboxylate desorption via (i) formation of an electric double layer and negative zeta potential on the silica surface and (ii) an increase in polycarboxylate anionic charge density due to deprotonation of the carboxylate groups in the polymer backbone, complexation with Ca²⁺ ions, and counter-ion condensation. This provides an explanation for the remarkable fluidizing effect observed upon addition of small amounts (1.0 wt %) of a solid, powdered Ca source to fresh, low-CO₂, UHPC, which exhibits significantly higher fresh state pH (>13) than those based on Portland cement (pH 11).

Effects of Fe and Al ions during hydrogen sulphide (H2S)-induced corrosion of tetracalcium aluminoferrite (C4AF) and tricalcium aluminate (C3A)

With high concentration and toxic H₂S gas found in sewage facilities, it has ultra-high leakage risk and great environment impact, once the sealing material failure. In this study, hydrogen sulphide (H₂S)-induced corrosion effects of tetracalcium aluminoferrite (C₄AF) and tricalcium aluminate (C₃A) phases of cement were evaluated during the initial stages of reaction. Both phase changes and bond structures were examined by techniques including Fourier transform infrared spectroscopy (FTIR). The results indicate that corrosion occurred through a stepwise reaction promoted by Fe ions. The effects of Fe and Al ions were further quantitatively investigated by X-ray photoelectron spectroscopy (XPS) and ²⁷Al solid-state nuclear magnetic resonance (SSNMR). Compared with hydrated samples, the Fe ions were changed with Fe(II) ions increasing and Fe(III) ions reducing, and the AlO_x group was observed to be transformed form AlO₄ to AlO₆ after H₂S corrosion. Changes obtained will be benefit the further development of hazardous H₂S control material and risk management operations.

Elucidation of the Hydration Reaction of UHPC Using the PONKCS Method

This study explored the hydration reaction of ultra-high-performance concrete (UHPC) by using X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA) as analysis methods. The partial- or no-known crystal structure (PONKCS) method was adopted to quantify the two main amorphous phases of silica fume and C-S-H; such quantification is critical for understanding the hydration reaction of UHPC. The measured compressive strength was explained well by the degree of hydration found by the PONKCS method, particularly the amount of amorphous C-S-H. During heat treatment, the pozzolanic reaction was more intensified by efficiently consuming silica fume. After heat treatment, weak but continuous hydration was observed, in which the cement hydration reaction was dominant. Furthermore, the study discussed some limitations of using the PONKCS method for studying the complicated hydration assemblage of UHPC based on the results of TGA and NMR. Generally, the PONKCS method underestimated the content of silica fume in the early age of heat treatment. Furthermore, the structural evolution of C-S-H, confirmed by NMR, should be considered for more accurate quantification of C-S-H formed in UHPC. Nevertheless, PONKCS-based XRD could be useful for understanding and optimizing the material properties of UHPC undergoing heat treatment.

Heat-Induced Acceleration of Pozzolanic Reaction Under Restrained Conditions and Consequent Structural Modification

This study investigated the heat-induced acceleration of cement hydration and pozzolanic reaction focusing on mechanical performance and structural modification at the meso- and micro-scale. The pozzolanic reaction was implemented by substituting 20 wt.% of cement with silica fume, considered the typical dosage of silica fume in ultra-high performance concrete. By actively consuming a limited amount of water and outer-formed portlandite on the unreacted cement grains, it was confirmed that high-temperature curing greatly enhances the pozzolanic reaction when compared with cement hydration under the same environment. The rate of strength development from the dual reactions of cement hydration and pozzolanic reaction was increased. After the high-temperature curing, further strength development was negligible because of the limited space availability and preconsumption of water under a low water-to-cement environment. Since the pozzolanic reaction does not directly require the anhydrous cement, the reaction can be more easily accelerated under restrained conditions because it does not heavily rely on the diffusion of the limited amount of water. Therefore, it significantly increases the mean chain length of the C-S-H, the size of C-S-H globules with a higher surface fractal dimension. This finding will be helpful in understanding the complicated hydration mechanism of high-strength concrete or ultra-high performance concrete, which has a very low water-to-cement ratio.

A facile synthesis of layered double hydroxide based core@shell hybrid materials

A simple and scalable co-precipitation method to obtain zeolite Z13X@Mg₂Al–CO₃-LDH and Mg-MOF-74@Mg₂Al–CO₃-LDH has been reported.

Insights into the Microstructure of Hydrothermal Synthesized Nanoscale K2O-Al2O3-SiO2-H2O Particles

K-A-S-H (K₂O-Al₂O₃-SiO₂-H₂O) gel is a key phase that forms in most alkali-activated binders (eco-friendly binders which utilize a substantial amount of industrial by-product). An in-depth understanding of the microstructure and performance of this nano-sized key phase facilitates better application to alkali-activated binders. However, such studies remain little and undetailed. Therefore, our research aims to provide insights into the microstructure of K-A-S-H particles synthesized with accurate stoichiometric control by the hydrothermal method through thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BET surface area. The results show that for materials prepared at the curing temperature lower than 80 °C, the K-A-S-H products were completely amorphous. With increased curing temperature and time, the K-A-S-H products were transformed from the amorphous phase to the crystalline zeolite phase structure, with a reduction in the specific surface area. The TG results indicate that the crystalline phase contains more non-evaporated water or zeolite water for structural rearrangement. The degree of tetrahedral polymerization slightly decreased with an increase of the K₂O/SiO₂ ratio as the amount of non-bridged oxygen atoms increased, whereas it gradually increased with an increase of curing temperature and time, as suggested by the FTIR and NMR results. Various K₂O/SiO₂ ratios resulted in the formation of zeolite K-H and K-G zeolite, both of which exhibited highly polymerized three-dimensional network structures. However, there was no significant effect of the SiO₂/Al₂O₃ ratio on the structure of K-A-S-H products. Overall, these results provide insight into understanding the chemical stability of K-A-S-H.

Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement

We report a high-field in-situ solid-state NMR study of the hydration of CaAl₂O₄ (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state ²⁷Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS ²⁷Al NMR spectra, was used to monitor the ²⁷Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of ²⁷Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.