Con~ary to other porous materials such as smadstones, bricks or porous glass, the inter-atomic bonding continuity of cement-based materials is fax from obvious. When scrutin~ed at very microscopic level, continuity of the ioniccovalent bonding in the solid phase is almost everywhere intemapted by water molecules or liquid water films of variable thickness. Yet, concrete and cement pastes are able to withstand stresses of the same magnitude as rocks. The purpose of this paper to explore the possible reasons for such a high cohesion in terms of inter-particle forces using general argtmaents and molecular simulation computations including ab initio quantum chemical methods applied to C-S-H. As it will be discussed, molecular simulation studies provide strong arguments for predicting that short-and mediumrange attractive electrostatic forces are the essential components of the cohesion of C-S-H with, at short distance (sub-nm), a significant iono-covalent contribution involving strongly localized calcium ions and water molecules and, at larger distance (a few nm), ionic correlation forces involving hydrated and mobile calcium ions in liquid water films. Only a marginal contribution is expected from van der Waals attraction whereas capillary forces might contribute at a level comparable to that of correlation tbrces in unsaturated conditions. The parallel with clay-based earthen construction materials is part of the clue of this rationale. RESUME Contrairement gz d~utres matdriaw; por~ux comme les gr~s, les briques ou eertains verres, les mat~riau~ cimentaires ne possbdent t'x~ un schdma simple de liaisons int~atomiques. s continuit~ du rd~eau de liaisons iono-covalentes y est
The following study investigates the effect of brine ionic composition and salinity (a mixture we call dynamic water) on calcite surface wettability alteration using atomic force microscopy (AFM) and surface compositional analysis using X-ray photoelectron spectroscopy (XPS) in addition to contact angle measurements. Three ions are particularly varied (Mg 2+ , Ca 2+ , and SO 4 2−) to examine their role for surface alteration. Dissolution, adsorption, and precipitation are among the processes competing at the interface in the calcite/crude oil/brine system. Dissolution takes place according to a specific path with reiteration of etched pits deepening, steps retreat, and material removal from the surface. Aging calcite in crude oil causes adsorption and precipitation phenomena, especially asphaltene and carboxylic acids. The AFM topographic images of calcite saturated in deionized water and NaCl brine show that calcite dissolution occurs rapidly but maintains its crystal rhombohedral shape. Surface wettability measurements of aged calcite surfaces showed that sulfate has a dominant effect on altering surface wettability to strongly water-wet followed by diluted seawater. Nonetheless, when magnesium ions were removed from the brine, surface wettability alteration was weak, which indicated the need for such an ion in the brine. AFM topographic images show clear indication of surface morphology changes in all studied surfaces, although less features were observed when treated with SWME*0Mg (brine without any magnesium ions). All other surfaces showed some magnesium surface adsorption, while in the case of SWME*4S (brine with 4 times sulfate concentration compared to seawater), round-shape and vermicular features were also observed. An increase in the magnesium/ calcium ionic ratio in the brine will enhance surface dissolution and favor the presence of nucleation pits and growth of round-shape features. An increase in the sulfate/calcium ionic ratio alters the calcite surface by enhancing the formation of platelets and round-shape features. The effect of determining ions (most effective surface alteration ion) on altering calcite wettability is not a single ion because there are affinities between sulfate, calcium, and magnesium that will change surface morphology at certain ratios. Surface compositional analysis using XPS indicates the presence of sulfur on each treated calcite surface, but that by itself does not explain the wettability alteration results. However, the data provide clear evidence that magnesium ions need to be present in the brine mixture.
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