Effect of counterions on comb-like polycarboxylate conformation in aqueous solutions

Advances in Organic Rheology-Modifiers (Chemical Admixtures) and Their Effects on the Rheological Properties of Cement-Based Materials

Organic rheology modifiers, especially superplasticizers and viscosity-modifying admixtures (VMAs), have become key components for the workability optimization of modern concrete. The development of these admixtures is crucial to the further performance improvement of modern concrete under different casting and service conditions. Many of the former reviews have summarized research advances in respect of these admixtures from chemical and material perspectives, focusing on the effects of structure and the performance. In this paper, from a rheological perspective, an overview is provided of the microscale behavior of polycarboxylate (PCE) superplasticizers and VMAs (e.g., adsorption, conformation, and bridging) in terms of the evolution of the microstructure of the paste, the effect of chemical structure on the yield stress, the apparent viscosity and thixotropy of cement-based materials, and the structure design of these admixtures. Most importantly, in addition to a general discussion with assumptions (monolayer adsorption of a "flat" conformation, with each molecule on a single particle; statistical polymer composition), special conditions (e.g., preferential adsorption, depletion effects, hydration modification effects, and the polydispersity of the polymer composition) are discussed. Newly developed admixtures, realized through regulation of the microscale behavior, and by the modification of adsorption, topological structure, and molecular frame, are introduced.

Molecular Modeling in Anion Exchange Membrane Research: A Brief Review of Recent Applications

Anion Exchange Membrane (AEM) fuel cells have attracted growing interest, due to their encouraging advantages, including high power density and relatively low cost. AEM is a polymer matrix, which conducts hydroxide (OH-) ions, prevents physical contact of electrodes, and has positively charged head groups (mainly quaternary ammonium (QA) groups), covalently bound to the polymer backbone. The chemical instability of the quaternary ammonium (QA)-based head groups, at alkaline pH and elevated temperature, is a significant threshold in AEMFC technology. This review work aims to introduce recent studies on the chemical stability of various QA-based head groups and transportation of OH- ions in AEMFC, via modeling and simulation techniques, at different scales. It starts by introducing the fundamental theories behind AEM-based fuel-cell technology. In the main body of this review, we present selected computational studies that deal with the effects of various parameters on AEMs, via a variety of multi-length and multi-time-scale modeling and simulation methods. Such methods include electronic structure calculations via the quantum Density Functional Theory (DFT), ab initio, classical all-atom Molecular Dynamics (MD) simulations, and coarse-grained MD simulations. The explored processing and structural parameters include temperature, hydration levels, several QA-based head groups, various types of QA-based head groups and backbones, etc. Nowadays, many methods and software packages for molecular and materials modeling are available. Applications of such methods may help to understand the transportation mechanisms of OH- ions, the chemical stability of functional head groups, and many other relevant properties, leading to a performance-based molecular and structure design as well as, ultimately, improved AEM-based fuel cell performances. This contribution aims to introduce those molecular modeling methods and their recent applications to the AEM-based fuel cells research community.

Effect of Sodium Hexametaphosphate and Trisodium Phosphate on Dispersion of Polycarboxylate Superplasticizer

Enhancement in dispersion of polycarboxylate superplasticizer (PCE) could be obtained by incorporating retarders in normal concrete. The generally believed reason was that the consumption of free water and polymer at the beginning was reduced by retarding cement hydration. This theory could not convincingly explain why sodium hexametaphosphate (SHMP) was able to promote the dispersion capacity of PCE, while trisodium phosphate (TSP) could not, despite that both TSP and SHMP could obviously retard the cement hydration. The adsorption behavior of PCE and phosphate was investigated and the mechanism was analyzed in order to gain deeper understanding. The results showed that TSP and SHMP delayed the cement hydration, impeded adsorption process of PCE, and increased thickness of adsorption layer. It was interesting that TSP reduced the dispersion, but SHMP enhanced. The reason for this contradiction was due to the difference in composition of adsorption layer. In the PCE-TSP system, this layer was composed of the precipitates (formed by TSP and Ca²⁺) and the invalided PCE (caused by these precipitates in the immediate vicinity of the cement grains); the invalided PCE was due to the decrease of PCE dispersion. In the PCE-SHMP system, “Inner-phosphate (multi-layers) + Outer-PCE (single layer)” structure was formed to make the PCE work more effective, hence enhancing the dispersion. These results were expected to be useful for the design of highly efficient dispersants.

Binding of calcium cations with three different types of oxygen-based functional groups of superplasticizers studied by atomistic simulations

This work investigated interactions between calcium cations (Ca²⁺) and three common types of oxygen-based functional groups of concrete superplasticizers using density functional theory (DFT) calculations and all-atom molecular dynamics (MD) simulations. The three common types of oxygen-based functional groups were modeled as three hypothetical, low-molecular-weight organic molecules, each containing a methyl-terminated oxyethylene dimer and an adsorbing head of two oxygen-based functional groups, and are referred to as carboxylate, sulfonate, and phosphate groups, respectively, following the usual terminology in the field of concrete admixtures. Our DFT results show that the binding strength of the three groups with calcium cations follows (from high to low) phosphate>carboxylate>sulfonate, and both the electrophilic attack and the chemical reactivity of the three groups contribute significantly to the binding strength. The MD simulation results indicate that the adsorption of the three small molecules on the calcite (1 0 4) surface in aqueous solution shares a similar pattern in the sense that just two oxygen atoms of two adjacent anchor groups adsorb on the calcium atoms on the top layer of the crystal. The adsorption strength among the three types of functional groups follows the same order as the binding strength obtained from DFT calculations; both results corroborate a similar rule-of-thumb established by experiments. Furthermore, interactions of the three types of groups with water molecules suggest that strong hydrogen-bonding interactions exist in those systems. Graphical abstract Binding of calcium cations with three different types of oxygen-based functional groups of superplasticizersᅟ.