The formation of supramolecular inclusion compounds (pseudopolyrotaxanes) produced by cyclodextrins and polymers can be monitored through turbidimetry. In this paper, we studied the kinetics of the threading of linear poly(ethylene glycol) chains of different molecular weights and of a four-arm star-like polymer as a function of temperature. The main thermodynamic parameters are extracted. The aggregation and precipitation of pseudopolyrotaxanes are described in terms of the Avrami-Erofe'ev model, which provides relevant information on the mechanism of these processes. SAXS and TGA experiments confirm the structure and hydration of the final products obtained from the different polymers. A new hypothesis for the interaction between pseudopolyrotaxanes that leads to aggregation and precipitation, based on the spatial dielectric anisotropy, is proposed.
Magnesium-based cement is one of the most interesting eco-sustainable alternatives to standard cementitious binders. The reasons for the interest towards this material are twofold: (i) its production process, using magnesium silicates, brine or seawater, dramatically reduces CO2 emissions with respect to Portland cement production, and (ii) it is very well suited to applications in radioactive waste encapsulation. In spite of its potential, assessment of the structural properties of its binder phase (magnesium silicate hydrate or M-S-H) is far from complete, especially because of its amorphous character. In this work, a comprehensive structural characterization of M-S-H was obtained using a multi-technique approach, including a detailed solid-state NMR investigation and, in particular, for the first time, quantitative (29)Si solid-state NMR data. M-S-H was prepared through room-temperature hydration of highly reactive MgO and silica fume and was monitored for 28 days. The results clearly evidenced the presence in M-S-H of "chrysotile-like" and "talc-like" sub-nanometric domains, which are approximately in a 1 : 1 molar ratio after long-time hydration. Both these kinds of domains have a high degree of condensation, corresponding to the presence of a small amount of silanols in the tetrahedral sheets. The decisive improvement obtained in the knowledge of M-S-H structure paves the way for tailoring the macroscopic properties of eco-sustainable cements by means of a bottom-up approach.
A recently developed method based on Differential Scanning Calorimetry (DSC) has been used to study the hydration kinetics of tri-calcium silicate (C 3 S) in the presence of superplasticizers. We studied some of the most common additives employed in the cement industry, i.e., a sulfonated naphthalene-formaldehyde polycondensate, a polycarboxylate and a polyacrylate. The overall hydration kinetics of the superplasticized pastes have been obtained and compared to that of C 3 S cured in pure water. Superplasticizers do not change the kinetic laws involved in the hydration processes, the first according to an Avrami-Erofeev nucleation and growth law, and the second a three-dimensional diffusion equation. However, the induction times of the tri-calcium silicate paste are affected by the presence of additives. The polycarboxylate and the polyacrylate additives reduce the temperature dependence of the so-called "dormant period". The activation energy of the nucleation and growth stage is about doubled in the presence of superplasticizers, and is the largest for the polyacrylate additive, that is the most effective additive. DSC analysis shows that a larger amount of water reacts with C 3 S during the acceleration period in the presence of polycarboxylic and the polyacrylic additives, meaning that the water is more available, i.e., the paste is more fluid. SEM images show that the acrylic additive produces a change in the morphology of the formed hydrated calcium silicate gel, from a fiber-like structure to a sheet-like structure. The quantitative determination on how additives affect the curing process of cement is reported for the first time, allowing a determination of a scale of additives efficacy based on activation energies of the nucleation process.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.