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Partial substitution of the clinker in the cement by a supplementary cementitious material (SCM) is one of the main solutions to reduce the carbon footprint. Calcined kaolinite is a good candidate due to its availability and relatively high reactivity compared to other SCMs. The main issue with these calcined clay types of cements is the high-water demand at low clinker factors, a problem which remains not well understood. In this proceeding, we will show the role played by electrostatic interactions in the paste stiffening using as a model system pure calcined kaolinite paste prepared at various pH as well as salt types and concentrations. The study combines dynamic rheometry measurements in strain-sweep modes, surface charge characterization using potentiometric titration and electrophoretic measurements as well as calculations of inter-particle interactions using Monte-Carlo (MC) simulations in the framework of the primitive model. The calcined kaolinite is found to possess a negative permanent charge, presumably due to the Si(IV)/Al(III) substitution, and a titratable charge (as due to ionization of silanol and aluminol surface groups) with a point of zero proton charge at pH 4.65. In conditions relevant for cement paste, the calcined clay bear a strong negative charge ~ 300 mC/m2. The rheological measurements reveal that the paste stiffening is highly dependent on the pH, salt concentration and type as expected for systems controlled by electrostatic interactions. The stiffness increases with the salt concentration at natural pH and is the largest in solutions buffered with Ca(OH)2, that is at high Ca2+ concentrations and pH where the negative charge of the calcined clay is the strongest. The MC simulations of the inter-particle interactions are found to qualitatively explain the observed variation in the paste stiffness., Comment: Conference Proceedings

Wet chemistry C-S-H precipitation experiments were performed under controlled conditions of solution supersaturation in the presence and absence of gluconate and three hexitol molecules. Characterization of the precipitates with SAXS and cryo-TEM experiments confirmed the presence of a multi-step nucleation pathway. Induction times for the formation of the amorphous C-S-H spheroids were determined from light transmittance. Analysis of those data with the classical nucleation theory revealed a significant increase of the kinetic prefactor in the same order as the complexation constants of calcium and silicate with each of the organics. Finally, two distinct precipitation regimes of the C-S-H amorphous precursor were identified: i) a nucleation regime at low saturation indexes (SI) and ii) a spinodal nucleation regime at high SI where the free energy barrier to the phase transition is found to be of the order of the kinetic energy or less., Comment: Accepted in Cement and Concrete Research. 30 pages plus supplementary materials. arXiv admin note: substantial text overlap with arXiv:2111.02743

This study investigates the effect of gluconate, a carboxylate ion, and three uncharged hexitols, D-sorbitol, D-mannitol and D-galactitol on the solubility of C-S-H. Thermodynamic modeling was used to determine the kind and amount of Ca-organic-silicate-OH complexes that potentially form in the conditions studied. All the organics form complexes with calcium and hydroxide, In addition, heteropolynuclear organics complexes with calcium, hydroxide and silicate are observed at high pH values and high calcium concentrations with the exception of mannitol. The strength of complexation with silicate decreases from gluconate > sorbitol > galactitol. The adsorption of the selected organics on portlandite and C-S-H systems follows the order gluconate >> sorbitol > mannitol = galactitol. For C-S-H system buffered with Ca(OH)2, a typical Langmuir isotherm was found and adsorption on C-S-H increased with the Ca/Si ratio as well as with the increase of the organic concentration., Comment: preprint (25 pages, 8 figures) and supp. info. (6 pages, 2 tables, 1 figure)

Calcium silicate hydrates (C-S-H), the main components of hydrated cement, are known for being responsible of its cohesion. In this respect, they act as "glue" for the other solid components of mortar or concrete and thus strongly influence the mechanical properties of the outcoming material. Given the scarce knowledge about the influence of polymers on the growth and nucleation of C-S-H, this work focuses on the investigation of ongoing processes in the pre- and postnucleation stage of C-S-H precipitation. With the help of titration in combination with ion selective electrodes, effects of additives on the formation of C-S-H can be monitored and quantified resulting in new insights into the C-S-H formation in the presence of additives. Thus, this work can be regarded as a step towards a facilitated design of tailor-made C-S-H, which might result in superior cementitious materials on the long term., Comment: preprint (33 pages, 7 figures) and supp. info (32 pages, 51 figures, 4 tables)

The present paper investigates the influence of gluconate and hexitol additives on the precipitation mechanism and kinetics of C-S-H. To this end, wet chemistry C-S-H precipitation experiments were performed under controlled conditions of solution supersaturation, under varying silicate concentration, while the transmittance of the solution was followed. This allowed determining induction times for the formation of C-S-H precursors in the presence and absence of gluconate and three hexitol molecules. Characterization of the precipitates was performed via small angle X-ray scattering and cryo-transmission electron microscopy experiments, which allowed the identification of a multi-step nucleation pathway also in the presence of the organics. Analysis of the induction time data in the framework of the classical nucleation theory revealed a significant increase of the kinetic pre-factor, which is associated to physical rates of cluster collision and aggregation during the nucleation process. Values of the kinetic pre-factor increase in the same order as the complexation constants of calcium and silicate with the each of the organics. This points to a retarding mechanism of crystallization related to steric hindrance of the aggregation of the early formed clusters via adsorption of the organics at their surfaces.

The current paper investigates the impact of gluconate, D-sorbitol, D-mannitol and D-galactitol on calcium speciation at high pH values by i) solubility measurements of portlandite (Ca(OH)2) and ii) potentiometric titration measurements of calcium salt solutions. Thermodynamic modeling was used to fit the chemical activities of Ca2+ and OH- ions and thus to determine the strength and kind of the different Ca-organic-hydroxide complexes. The strength of complex formation with Ca decreases in the order gluconate >> sorbitol > mannitol > galactitol, which follows the same order as sorption on portlandite. Heteropolynuclear gluconate complexes with calcium and hydroxide dominate the Ca-speciation in the presence of portlandite, while for sorbitol ternary CaSorbOH+ complexes were dominant under alkaline conditions. We expect that these results will help in better understanding the influence of gluconate and hexitols on the hydration of alite and Portland cement., Comment: Preprint accepted in Cement and Concrete Research

Monte-Carlo simulations and small-angle x-ray scattering experiments were used to determine the phase diagram of aqueous dispersions of titratable nano-colloids with a moderate size polydispersity over a broad range of monovalent salt concentrations, 0.5 mM $\leq c_s \leq$ 50 mM and volume fractions, $\phi$. Under slow and progressive increase in $\phi$, the dispersions freeze into a face-centered-cubic (fcc) solid followed unexpectedly by the formation of a body centered cubic (bcc) phase before to melt in a glass forming liquid. The simulations are found to predict very well these observations. They suggest that the stabilization of the bcc solid at the expense of the fcc phase at high $\phi$ and $c_s$ results from the interaction (charge) polydispersity and vibrational entropy., Comment: 5 pages, 4 figures, Accepted in PRL

A good understanding and determination of colloidal interactions is paramount to comprehend and model the thermodynamic and structural properties of colloidal suspensions. In concentrated aqueous suspensions of colloids with a titratable surface charge, this determination is, however, complicated by the density dependence of the effective pair potential due to both the many-body interactions and the charge regulation of the colloids. In addition, colloids generally present a size distribution which results in a virtually infinite combination of colloid pairs. In this paper we develop two methods and describe the corresponding algorithms to solve this problem for arbitrary size distributions. An implementation in Nim is also provided.The methods, inspired by the seminal work of Torres et al., are based on a generalization of the cell and renormalized jellium models to polydisperse suspensions of spherical colloids with a charge regulating boundary condition. The latter is described by the one-pK-Stern model. The predictions of the models are confronted to the equations of state of various commercially available silica dispersions. The renormalized Yukawa parameters (effective charges and screening lengths) are also calculated. The importance of size and charge polydispersity as well as the validity of these two models are discussed in light of the results.

We report small-angle x-ray scattering (SAXS) experiments on aqueous dispersions of colloidal silica with a broad monomodal size distribution (polydispersity 18%, size 8 nm). Over a range of volume fractions the silica particles segregate to build first one, then two distinct sets of colloidal crystals. These dispersions thus demonstrate fractional crystallization and multiple-phase (bcc, Laves AB$_2$, liquid) coexistence. Their remarkable ability to build complex crystal structures from a polydisperse population originates from the intermediate-range nature of interparticle forces, and suggests routes for designing self-assembling colloidal crystals from the bottom-up., Comment: 5 pages, 3 figures and 2 tables + supplemental analysis (5 pages, 1 figure, 3 tables)

Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titration and charge reversal (CR) experiments on silica surfaces by Dove et al. and van der Heyden it et al, we show that ion-ion correlations quantitatively explain why divalent counterions strongly promote surface charge which, in turn, eventually causes a charge reversal (CR). Titration and CR results from simulations and experiments are in excellent agreement without any fitting parameters. This is the first unambiguous evidence that ion-ion correlations are instrumental in the creation of highly charged surfaces and responsible for their CR. Finally, we show that charge correlations result in "anomalous" charge regulation in strongly coupled conditions in qualitative desagreement with its classical treatment., Comment: 4 pages, 4 figures, submitted to PRL