Other significant observation is the high yield stress values for the
mixture calcite–bentonite when the pH is increased to 10 by using
soda ash (Fig. 10). This may indicate that the Ca2+ cations released by
calcite and combined with Na+ ions from soda ash contribute to the
increase in viscosity. It had been reported that for montmorillonites in
the presence of Na+ and Ca2+ ions in the exchangeable sites, a maximum
viscosity is obtained when Ca2+ ions occupy about 40% of these
sites (Bleifuss, 1973). This balance of cations in montmorillonite can
be affected by the presence of extra cations in the surroundings as it
was studied by Bradshaw et al. (2013). They explained that Na+ ions
were exchanged by divalent cations present in the environment and
that one of the sources was the dissolution of calcite. The exchange of
Na+ for bivalent cations is thermodynamically favourable, and there
are examples that confirm complete replacement of Na+ by Ca2+ and
Mg2+ (Bradshaw et al., 2013; Meer and Benson, 2007).