0000000000079912
AUTHOR
Sandrine Gauffinet
Investigation of magnesium phosphate cement hydration in diluted suspension and its retardation by boric acid
Abstract Magnesium phosphate cements (MPCs) are used for rapid repair works, but they may also offer prospects for the stabilization/solidification of deleterious waste. MPCs contain calcined magnesium oxide and a water-soluble acid phosphate, such as potassium dihydrogen phosphate (KH 2 PO 4 ). The main precipitated hydrate is then K-struvite (MgKPO 4 ·6H 2 O). This work aims at giving new insight into the processes involved in its formation. Since cement hydration is very rapid, the second objective is to understand how boric acid, a common admixture for field application, retards cement hydration. A multi-stage process is evidenced in diluted suspension: MgHPO 4 ·7H 2 O likely precipitat…
Observation directe de la croissance d'hydrosilicate de calcium sur des surfaces d'alité et de silice par microscopie à force atomique
Direct observation of the growth of calcium silicate hydrates, the tricalcium silicate hydration products, at the solid-solution interface were performed by atomic force microscopy. The covering of the surface of alite or silica by a three-dimensional oriented aggregation of nano particles of calcium silicate hydrate is always observed whatever the sample. All observations and quantifications made on calcium silicate growth at the submicronic level are in agreement with the data deduced from the study of the system evolution at the macroscopic level.
Porous structure and mechanical strength of cement-lime pastes during setting
Abstract The acceleration of a cement paste setting as a result of lime addition may be shown from isothermal calorimetry measurements. We investigated the underlying mechanisms through two techniques that provide information on porous structure (using Nuclear Magnetic Resonance) and mechanical properties (elastic modulus measured by rheometry). The correlation of the two sets of results on a cement paste clearly reveals the successive steps of setting, and particularly highlights the so-called induction period. We show that this induction period disappears in the presence of lime, leading to an acceleration of the setting. We also show that beyond some critical concentration of added lime …
Structure and Yielding of Colloidal Silica Gels Varying the Range of Interparticle Interactions.
The relationship between interaction range, structure, fluid-gel transition, and viscoelastic properties of silica dispersions at intermediate volume fraction, Φv ≈ 0.1 and in alkaline conditions, pH = 9 was investigated. For this purpose, rheological, physicochemical, and structural (synchrotron-SAXS) analyses were combined. The range of interaction and the aggregation state of the dispersions were tuned by adding either divalent counterions (Ca(2+)) or polycounterions (PDDA). With increasing calcium chloride concentration, a progressive aggregation was observed which precludes a fluid-gel transition at above 75 mM of calcium chloride. In this case, the aggregation mechanism is driven by s…
Study of nucleation and growth processes of ettringite in diluted conditions
Abstract Ettringite is a key compound of cementitious materials, formed during hydration that influences the rheological behavior of the cement paste. Despite its importance, ettringite formation has been poorly studied. In this work, nucleation and growth processes of ettringite were separately investigated. Ettringite nucleation was described using the Classical Nucleation Theory and an interfacial crystal-solution energy was determined. After the initial primary homogeneous nucleation, when the surface of already formed crystals was still low, the precipitation rate was shown to depend almost exclusively on the solution supersaturation. When the surface of crystals in suspension was gett…
Effect of blastfurnace slag addition to Portland cement for cationic exchange resins encapsulation
In the nuclear industry, cement-based materials are extensively used to encapsulate spent ion exchange resins (IERs) before their final disposal in a repository. It is well known that the cement has to be carefully selected to prevent any deleterious expansion of the solidified waste form, but the reasons for this possible expansion are not clearly established. This work aims at filling the gap. The swelling pressure of IERs is first investigated as a function of ions exchange and ionic strength. It is shown that pressures of a few tenths of MPa can be produced by decreases in the ionic strength of the bulk solution, or by ion exchanges (2Na + instead of Ca 2+ , Na + instead of K + ). Then,…
Investigation of the swelling behavior of cationic exchange resins saturated with Na+ ions in a C3S paste
Ion exchange resins (IERs) are widely used by the nuclear industry to decontaminate radioactive effluents. Spent products are usually encapsulated in cementitious materials. However, the solidified waste form can exhibit strong expansion, possibly leading to cracking, if the appropriate binder is not used. In this work, the interactions between cationic resins in the Na+ form and tricalcium silicate are investigated during the early stages of hydration in order to gain a better understanding of the expansion process. It is shown that the IERs exhibit a transient swelling of small magnitude due to the decrease in the osmotic pressure of the external solution. This expansion, which occurs jus…
Solidification of ion exchange resins saturated with Na+ ions: Comparison of matrices based on Portland and blast furnace slag cement
International audience; Ion exchange resins (IERs) are widely used by the nuclear industry to decontaminate radioactive effluents. After use, they are usually stabilized and solidified by encapsulation in cementitious materials. However, for certain combinations of cement and resins, the solidified waste forms can exhibit strong expansion, possibly leading to cracking of the matrix. In this work, the behaviour of cationic resins in the Na$^+$ form is investigated in Portland cement (CEM I) or blast furnace slag cement (CEM III/C) pastes at early age in order to have a better understanding of the swelling process. The results show that during the hydration of the CEM I paste, the resins exhi…