Jean-Claude Mutin

Dynamic mode rheology of cement and tricalcium silicate pastes from mixing to setting

Dynamic mode rheometry was used to study the evolution of the structure of cement and pure tricalcium silicate pastes from mixing up to setting and even after setting, together with the nature of the forces responsible for the mechanical properties of the pastes. A special mixer-type tool was used to study rheology during the very first minutes following the end of mixing, which are out of reach with classical tools. Both kinds of pastes have the same behavior. It was found that the main evolution of the structure of the pastes occurs during the very first minutes following the end of mixing, while there is no change in interparticular forces up to setting and even a few hours later. Setting, as defined by the Vicat needle, is not related to any particular change of the nature of the forces or structural transition within the paste.

Studies on mechanism and physico-chemical parameters at the origin of the cement setting. I. The fundamental processes involved during the cement setting

Abstract The mechanical evolution and the structure of the cement paste have been analysed in relation with the chemical evolution of the system. The setting process can be described as following two fundamental steps: the coagulation of cement grains during the first minutes following the mixing, and the rigidification of the coagulated structure which arises simultaneously with the acceleration of the hydrates formation During the coagulation step, the structure formed is proved to be mechanically reversible. The rigidification of the coagulated structure is provided by the hydrates formation in the contact zone. The increase of the paste cohesion at this stage is proportional to the quantity of precipitated hydrates, the structure thus rigidified becoming mechanically irreversible.

Hydration of calcium sulfate hemihydrate (CaSO4·H2O) into gypsum (CaSO4·2H2O). The influence of the sodium poly(acrylate)/surface interaction and molecular weight

Abstract The retarding influence of sodium poly(acrylate) (PANa) on the hydration of calcium sulfate hemihydrate (CaSO 4 · 1 2 H 2 O) was investigated. This study reports the influence of sodium poly(acrylate) on hemihydrate dissolution, on homogenous and heterogeneous gypsum (CaSO 4 ·2H 2 O) nucleation as well as on gypsum growth. It is shown that adsorption of PANa does not hinder the dissolution of hemihydrate in the present experimental conditions. The specific interaction of PANa with gypsum can explain the oriented growth of gypsum crystal. The gypsum growth is slowed down but cannot be blocked by the adsorption of PANa. On the other hand, PANa can block the heterogeneous and homogenous gypsum nucleation. As soon as a critical surface density of PANa onto the hemihydrate surface is reached, the heterogeneous gypsum nucleation is prevented and hemihydrate hydration is indefinitely blocked. The interaction between PANa and the hemihydrate surface is of prime importance to control hydration. Also, the influence of the molecular weight of PANa on homogenous nucleation has been investigated. The precipitation of calcium polyacrylate can explain the differences between the two molecular weights used (2100 and 20xa0000). This work leads to the conclusion that heterogeneous nucleation is the key process that controls hydration of a system in which hemihydrate dissolution, gypsum nucleation and growth are all occurring at the same time in a continuous manner.

Reactivity of gypsum faces according to the relative humidity by scanning force microscopy

Abstract This article reports the experimental observation of the stability of the different faces of calcium sulphate dihydrate (gypsum CaSO 4 .2H 2 O) according to the relative humidity. Scanning Force Microscopy experiments were carried out with a view to discerning the topography of the surfaces, the chemical compositional domains, and in an attempt to evaluate the friction and viscoelastic properties of the surface. Our results indicate that the (010) face of gypsum is hydrophilic and very reactive contrary to the less hydrophilic (120) and (101) faces which remain stable depending on the relative humidity. It is clear from our results that a precipitation like process can be induced by the SFM tip. The dissolution of this precipitate depends on the amount of water in the meniscus between the probe and the surface. For the purpose of comparison, the cleaved surfaces of natural anhydrite (CaSO 4 ), calcite (CaCO 3 ), and muscovite mica were also observed. The Lateral Force Microscopy images provided the undisputed evidence that the process of dehydration of the (010) face actually occurs with a change of the elastic constant of the crystal.

Physico-chemical parameters determining hydration and particle interactions during the setting of silicate cements

Abstract Hydration of tricalcium silicate (Ca 3 SiO 5 ), the pure phase used as a model of the portland cements, is the chemical process leading to the formation of hydrates, while setting is a definite time event corresponding to the change of the paste from the soft to the hard state. Setting results from interactions between anhydrous or very partially hydrated particles. The analysis of these interactions leads to the identification of two fundamental steps: the coagulation of cement grains during the first minutes following the mixing and the rigidification of the coagulated structure which arises simultaneously with the acceleration of the calcium silicate hydrates (Cue5f8Sue5f8H) formation. The chemical evolution of the system, which controls the lime concentration in solution, determines the nature of particle interactions and the physical evolution of the suspension or paste: there exists a critical range of lime concentration required to initiate the coagulation of cement particles and the ability of Cue5f8Sue5f8H to make rigid the coagulated structure depends on the solubility of these particles.

Studies on mechanism and physico-chemical parameters at the origin of the cement setting II. Physico-chemical parameters determining the coagulation process

The physico-chemical parameters determining the coagulation of cements grains previously identified as the first fundamental process of cement setting have been investigated in diluted suspensions using an adapted granulometric method. The analysis of the influence of the ionic concentration in solution on the coagulation reveals that calcium concentration is the parameter which determines the particle interactions. There exists a minimum critical concentration of calcium ions in solution which is required to occur the coagulation of cement particles and a dispersive effect appears for very high concentrations. The results are discussed in relation with DLVO theory and specific interactions.

Correlating surface forces with surface reactivity of gypsum crystals by atomic force microscopy. Comparison with rheological properties of plaster

Abstract Atomic Force Microscopy (AFM) was used to image the surface reactivity as well as to characterize quantitatively the surface forces between two gypsum (CaSO 4 , 2H 2 O) crystals. Measurements on different crystal faces, which vary in morphology, structure, hydrophilicity, surface charge, were performed in both air and ionic solutions. In ionic solutions, varying the experimental parameters, the ionic nature and the concentration as well as the duration of the contact leads to the conclusion that the adhesion occurs whatever the orientation of faces. Nevertheless, the magnitude of the adhesion of a physical nature (Van der Waals and ionic correlation) depends on the surface charge density of each face while its increase is related to the growth of the contact area between the two crystals. In air, the systematic determination of surface potential with respect to the orientation of crystalline faces, the relative humidity and the duration and the area of contact between gypsum crystals suggests three types of physical forces: the Van der Waals forces, the electrostatic and capillary forces. In addition, the reactivity study displayed a reactive process of matter transfer, which is added to the surface forces to ensure the adhesion. Correlation between AFM results related to the micrometer scale and rheological measurements, performed on plaster (CaSO 4 , 0.5H 2 O) suspensions (pastes) and on macroscopic hardened plaster beams, is also demonstrated.

Microstructural modifications resulting from the dehydration of gypsum

Various faces of dehydrated gypsum-cleaved blocks and single crystals have been investigated using optical microscopy, SEM, and TEM. The orientation of the cracks with respect to the initial gypsum structure and the Miller indices of all possible crack planes have been determined. These crack planes have been found to be few and to correspond to planes of high atomic density in the initial and final structures, therefore characterised by a low surface tension. These observations allow us to propose cracking criteria based on general concepts of crystal growth. The cracks dimensions measured on different faces of the pseudomorph are distributed over several orders of magnitude and seems to be self-similar.

Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resulting pseudomorphs

Abstract Endothermic decomposition reactions (solid 1→solid 2+gas) generally affect only part of the inter-atomic bonds of the solid 1 structure. In consequence, the morphology and external dimensions of particles remain unchanged (the particles of solid 2 are referred to as pseudomorphs of those of solid 1). The gas release normally leads to a decrease of the molar volume of the precursor solid, resulting in cracking and formation of intra-particle porosity. This work deals with such a reaction: the dehydration of gypsum into sub-hydrated and anhydrous phases. Ultrathin (010) gypsum cleaved plates have been dehydrated under controlled conditions and studied by electron microdiffraction. All reactions have been shown to preserve four different sets of three-dimensional orientation relations between all structures. These topotactic orientation relations allow us to clarify the associated atomic rearrangements. The reactions are described in a simplified net common to all structures, which allows to predict the microstructure of the resulting pseudomorphs.

Mapping the influence of stress on the surface elasticity with an atomic force microscope

This letter reports an original experimental observation of the lateral stress effects on pure surface. The surface elasticity has been separated from topographical informations at the microscopic and atomic levels. The stress applied to the sample leads to modify the stiffness of the surface and affect the force modulation images. Measurements also revealed that the spring constant and the quality factor of cantilevers play an important role in the contrast mechanism of the elasticity images.