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.
Calcium Silicate Hydrates (C–S–H) are the major hydration products of portland cement paste. The accurate description of acid–base reactions at the surface of C–S–H particles is essential for both understanding the ion sorption equilibrium in cement and prediction of mechanical properties of the hardened cement paste. Ab initio molecular dynamics simulations at the density functional level of theory were applied to calculate intrinsic acidity constants (pKa's) of the relevant ≡SiOH and ≡CaOH2 groups on the C–S–H surfaces using a thermodynamic integration technique. Ion sorption equilibrium in C–S–H was modeled applying ab initio calculated pKa's in titrating Grand Canonical Monte Carlo simulations using a coarse-grained model for C–S–H/solution interface in the framework of the Primitive Model for electrolytes. The modeling results were compared with available data from electrophoretic measurements. The model predictions were found to satisfactorily reproduce available experimental data.
Growth models of charged nanoplatelets are investigated with Monte Carlo simulations and simple theory. In a first model, 2-dimensional simulations in the canonical ensemble are used to demonstrate that the growth of a single weakly charged platelet could be limited by its own internal repulsion. The short range attractive interaction in the crystal is modeled with a square well potential while the electrostatic interactions are described with a screened Coulomb potential. The qualitative behavior of this case can also be described by simply balancing the attractive crystal energy with the screened Coulomb repulsion between the crystal sites. This repulsion is a free energy term dominated by counterion entropy and of course reduced by added salt. For a strongly coupled system, that is with high charge density and divalent counterions as in calcium silicate hydrate, the main product of cement hydration, the screened Coulomb approximation becomes inadequate and the growth behavior has to be described with the full primitive model. In this case, the energetic interactions become relatively more important and the entropy of the system plays a minor role. As a consequence, the electrostatic interactions gradually become less of a hindrance for aggregation and in extreme cases electrostatics actually promote the growth. This is manifested as an increased aggregation with, for example, increasing surface charge density. In the presence of divalent calcium ions and at the high negative surface charge density typical for calcium silicate hydrate, electrostatic interactions are not a hindrance for an infinite growth of the particles. By combining experimental and simulated data we can show that the limited sized platelets found in cement paste is due to a very fast nucleation rate compared to the growth rate.
ABSTRACTA new electrokinetic setup was developed for assessing the active layer ζ-potential of tubular membranes based on tangential streaming potential and electrical resistance measurements. Although the flow was not wholly laminar (because of the large hydraulic diameter of channels), the electrokinetics theory could be used to convert the streaming potential data into ζ-potentials because the electrical double layer lay within a laminar sublayer near the channel walls. Electrical resistance data allowed for the account of the conduction phenomenon through the membrane porous body. The new device was tested over a range of pH with a tubular ceramic membrane composed of three channels with a titania active layer. The isoelectric point was found to be in good agreement with that determined from salt retention data. The ζ-potential value determined at pH = 3.5 using the present device was compared with that obtained on a flat membrane made of the same material using the traditional microslit electrokinetic setup. A good agreement between the two measurements was observed. It was shown that neglecting the electric conduction phenomenon through the membrane porous body leads to a low underestimation of the ζ-potential (less than ~20%). This is related to the large size of channels. The contribution of the membrane porous body was found to be independent of the pH of solution. This suggests that the support layer of the membrane would make a decisive contribution to the electric conductivity of membrane porous body.Key Words:: Tangential streaming potentialZeta potentialTurbulent flowElectric conductanceSurface conductionTubular membrane
