•Traces of CH in the C4A3$-C2S system were characterized using XRD, TG, FTIR, and pH.•The RQPA method was used to determine the compositions of the C4A3$-C2S hydration.•The hydration process and ...evolution of the C4A3$-C2S system were investigated.
A C4A3$-C2S (ye'elimite-belite) clinker system with different C2S and C4A3$ contents is synthesized from analytical reagents and a C4A3$-C2S hydration system is established with a water-cement ratio of 0.4. The traces of calcium hydroxide or portlandite (CH) in the C4A3$-C2S hydration system were characterized using X-ray diffraction (XRD), thermo-gravimetric (TG) analysis, Fourier transform infrared (FTIR) spectroscopy and pH tests. The Rietveld quantitative phase analysis (RQPA) method was used to determine the compositions of the C4A3$-C2S hydration products. The hydration process and microstructure evolution of the system were also investigated. The hydration of the C4A3$-C2S involves two key process: one the consumption of AH3 phase and another the generation and stabilization of CH phase. The occurrence of the CH traces in C4A3$-C2S hydration system can be described in terms of the changes in the C2S content and the curing age. When the C2S content is less than 55 wt%, the newly generated AH3 preferentially reacts with the C2S to generate C2ASH8. When the content of C2S is between 55 wt% and 85 wt%, the C2S begins direct hydration but the generated CH is immediately consumed by the C2ASH8. When the C2S content is higher than 85 wt%, the CH generated by the hydration of C2S becomes stable in the C4A3$-C2S hydration system.
Magnesium potassium phosphate cements (MKPCs) are prepared using calcined magnesia (MgO) and an acidic solution of potassium dihydrogen phosphate (KH2PO4). Their fast setting and high heat of ...hydration can be problematic when large volumes of materials are produced. Boric acid (B(OH)3) is thus commonly added as a set retarder. This work investigates MKPC hydration in paste (water-to-cement ratio w/c = 1) and its retardation by B(OH)3. The precipitation of K-struvite (MgKPO4·6H2O) is preceded by that of phosphorösslerite (MgHPO4·7H2O) and Mg2KH(PO4)2·15H2O. Cattiite (Mg3(PO4)2·22H2O), an end-product in diluted suspension (w/c = 100), is not observed. B(OH)3 slows down the formation of hydrates in two ways: (i) by stabilizing in solution the cations that outbalance the negative charges of the polyborates formed at pH above 6, and (ii) through the precipitation of an amorphous mineral containing borate and orthophosphate. The first process occurs both in diluted suspensions and pastes, the second is specific to pastes.
Progress in understanding hydration is summarized. Evidence supports the geochemistry dissolution theory as an explanation for the induction period, in preference to the inhibiting layer theory. The ...growth of C–S–H is the principal factor controlling the main heat evolution peak. Electron microscopy indicates that C–S–H “needles” grow from the surface of grains. At the peak, the surface is covered, but deceleration cannot be attributed to diffusion control. The shoulder peak comes from renewed reaction of C3A after depletion of sulfate in solution, but release of sulfate absorbed on C–S–H means that ettringite continues to form. After several days space becomes the major factor controlling hydration.
The use of new analytical technique is improving our knowledge of the action of superplasticizers and leading to the design of molecules for different applications. Atomistic modeling is becoming a topic of increasing interest. Recent publications in this area are reviewed.
Hydration Mimicry by Membrane Ion Channels Chaudhari, Mangesh I; Vanegas, Juan M; Pratt, L.R ...
Annual review of physical chemistry,
04/2020, Letnik:
71, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Ions transiting biomembranes might pass readily from water through ion-specific membrane proteins if these protein channels provide environments similar to the aqueous solution hydration environment. ...Indeed, bulk aqueous solution is an important reference condition for the ion permeation process. Assessment of this hydration mimicry concept depends on understanding the hydration structure and free energies of metal ions in water in order to provide a comparison for the membrane channel environment. To refine these considerations, we review local hydration structures of ions in bulk water and the molecular quasi-chemical theory that provides hydration free energies. In doing so, we note some current views of ion binding to membrane channels and suggest new physical chemical calculations and experiments that might further clarify the hydration mimicry concept.
•Isothermal hydration heat and heat kinetics of cement paste containing mechanically-activated fly ash are analyzed.•Compared with as-received fly ash, mechanically activated fly ash has promoted the ...reaction rate of the fly ash-cement system.•Three different mathematical methods have been adopted to calculate the ultimate hydration heat Qmax of the system.
Fly ash is a widely used supplementary cementitious material in the world. Mechanical activation, as a potential approach, can realize maximum bulk utilization of fly ash. In this study, the isothermal hydration heat flows of cement paste containing fly ash with or without mechanical activation are tested and modelled by Avrami equation, reaction order model and three-parameter exponential function at different stages. The results indicate that addition of 20% as-received fly ash retards the cement hydration at the nucleation and growth stage, because the aluminate ions in the pore solution might decrease heterogeneous nucleation sites of C-S-H and suppress its growth. Moreover, as-received fly ash impairs the reaction rate of the whole system at final tail stage owing to its low heat release. However, compared with as-received fly ash, mechanically activated fly ash has promoted cement hydration at the very early age due to the enhanced filler effect. More surface area supplied by mechanically activated fly ash promotes C-S-H growth and partially offsets the retarding effect of aluminate ions. The reaction rate of the whole cementitious system at the deceleration stage is also promoted by the improvement of pozzolanic reactivity after mechanical activation. Furthermore, three different mathematical calculation approaches have been adopted to calculate the ultimate hydration heat Qmax of the cementitious systems. In spite of different values for Qmax, the consensus can be achieved that mechanical activation towards fly ash can promote the reaction degree of the system.
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•Different dispersive graphene oxide (GO) nanosheets were obtained by ultrasound technique.•GO nanosheets can construct the bar-like crystals into flower-like and polyhedral ...crystals.•Flower-like and polyhedral crystals can improve flexural strength and compressive strength.•Formation process of cement hydration crystals are proposed.
The effects of graphene oxide (GO) nanosheets with different sizes and dosages on cement hydration crystal shapes and the mechanical strength of cement paste are investigated. It is found that flower-like crystals are more prominent at low GO dosage (<0.03%) and polyhedral or lamellar crystals at high dosage (>0.03%). The flexural and compressive strengths of cement paste with smaller GO nanosheets are clearly improved. Formation processes for hydration crystals are proposed based on scanning electron microscopy images. GO nanosheets can promote assembly of flower-like and polyhedral structures from rod-like crystals, with the ultimate formation of a dense structure.
This study reports on the effect of graphene oxide (GO) on the hydration of Portland cement (PC) and industrial clinker. GO accelerates PC hydration, whereas it temporarily retards that of clinker. ...This difference reflects a twofold behaviour of GO in cement pastes. Retardation is due to the interaction of GO with the surface of hydrating grains, while acceleration results from a seeding effect. Gypsum causes this difference. GO is shown to have little effect on the strength of hardened pastes, and this merely relates to the change of hydration degree, as opposed to reinforcing effect formerly assumed. Overall, GO is not particularly active as a nucleation surface, as it aggregates and behaves in a similar way to inert fillers (e.g. quartz). Polycarboxylate-ether copolymer could make GO an active seed in cement pastes, as it prevents GO from aggregating. Nevertheless, this was found to occur only in alite pastes but not PC pastes.
The rapid drop in internal temperature of mass concrete can readily lead to temperature cracks. Hydration heat inhibitors reduce the risk of concrete cracking by reducing the temperature during the ...hydration heating phase of cement-based material but may reduce the early strength of the cement-based material. Therefore, in this paper, the influence of commercially available hydration temperature rise inhibitors on concrete temperature rise is studied from the aspects of macroscopic performance and microstructure characteristics, and their mechanism of action is analyzed. A fixed mix ratio of 64% cement, 20% fly ash, 8% mineral powder and 8% magnesium oxide was used. The variable was different admixtures of hydration temperature rise inhibitors at 0%, 0.5%, 1.0% and 1.5% of the total cement-based materials. The results showed that the hydration temperature rise inhibitors significantly reduced the early compressive strength of concrete at 3 d, and the greater the amount of hydration temperature rise inhibitors, the more obvious the decrease in concrete strength. With the increase in age, the influence of hydration temperature rise inhibitor on the compressive strength of concrete gradually decreased, and the decrease in compressive strength at 7 d was less than that at 3 d. At 28 d, the compressive strength of the hydration temperature rise inhibitor was about 90% in the blank group. XRD and TG confirmed that hydration temperature rise inhibitors delay early hydration of cement. SEM showed that hydration temperature rise inhibitors delayed the hydration of Mg(OH)
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