  Suspensions of calcite in water are employed in many industrial fields such
as paper filling, pharmaceutics, heritage conservation or building
construction, where the rheological properties of the paste need to be
controlled. We measure the impact of simple ions such as calcium, sodium or
hydroxide on the elasticity of a nanocalcite paste, which behaves as a
colloidal gel. We confront our macroscopic measurements to DLVO interaction
potentials, based on chemical speciations and measurements of the zeta
potential. By changing the ion type and concentration, we go beyond the small
repulsion regime and span two orders of magnitude in shear modulus. Upon
addition of calcium hydroxide, we observe a minimum in shear modulus,
correlated to a maximum in the DLVO energy barrier, due to two competing
effects: Calcium adsorption onto calcite surface rises the zeta potential and
consequently the electrostatic repulsion, while increasing salt concentration
induces stronger electrostatic screening. We also demonstrate that the addition
of sodium hydroxide completely screens the surface charge and leads to a more
rigid paste. A second important result is that carbonation of the calcite
suspensions by the atmospheric CO 2 leads to a convergent high elasticity of
the colloidal gels, whatever their initial value, also well rationalized by
DLVO theory and resulting from a decrease in zeta potential and in surface
charge density.
