ИСТИНА

Rheological behaviour of calcium carbonates is intensely studied because of their importance as hydrocarbon reservoirs, materials for historical monuments, etc. Aqueous environments are known to provoke considerable changes in their properties. The most efficient creep mechanism is called “intergranular pressure solution (IPS)”: the stress-induced excessive chemical potential provides a driving force for material dissolution within grain contacts, diffusion through the intergranular liquid and re-precipitation elsewhere. Dissolution and growth of calcite are well documented to depend on the presence of various impurities [1]. Therefore the kinetics of IPS may also be expected to be sensitive to the chemical composition of aqueous solutions. In this work, IPS of calcite powders in the presence of chelating agents has been studied, and an attempt is made to interpret the results. Prewetted calcite powder was tested in a cylindrical steel matrix at a constant uniaxial stress. Creep rate significantly decreases with increasing HEDP or NTA concentration in the solution. Both additives are known to slow the dissolution of calcite and growth of its crystals. We have measured the decrease in dissolution rate of calcite single crystals as a function of HEDP concentration using the ICP-AES method. A good agreement has been found between the fraction of occupied sites on the calcite surface estimated from creep and dissolution slowing down. Similarly, the adsorption isotherm of tritium labeled NTA on calcite powder correlates with its IPS rate in NTA solutions. These data indicate that dissolution rate, rather than diffusion one, is the controlling factor during calcite dissolution-precipitation creep, at least in the conditions range used in this work. [1] A. Xyla, J. Mikroyannidis, P. Koutsoukos // J. Colloid Interface Science. V. 153. P. 537. 1992.