  Partial substitution of the clinker in the cement by a supplementary
cementitious material (SCM) is one of the main solutions to reduce the carbon
footprint. Calcined kaolinite is a good candidate due to its availability and
relatively high reactivity compared to other SCMs. The main issue with these
calcined clay types of cements is the high-water demand at low clinker factors,
a problem which remains not well understood. In this proceeding, we will show
the role played by electrostatic interactions in the paste stiffening using as
a model system pure calcined kaolinite paste prepared at various pH as well as
salt types and concentrations. The study combines dynamic rheometry
measurements in strain-sweep modes, surface charge characterization using
potentiometric titration and electrophoretic measurements as well as
calculations of inter-particle interactions using Monte-Carlo (MC) simulations
in the framework of the primitive model. The calcined kaolinite is found to
possess a negative permanent charge, presumably due to the Si(IV)/Al(III)
substitution, and a titratable charge (as due to ionization of silanol and
aluminol surface groups) with a point of zero proton charge at pH 4.65. In
conditions relevant for cement paste, the calcined clay bear a strong negative
charge ~ 300 mC/m2. The rheological measurements reveal that the paste
stiffening is highly dependent on the pH, salt concentration and type as
expected for systems controlled by electrostatic interactions. The stiffness
increases with the salt concentration at natural pH and is the largest in
solutions buffered with Ca(OH)2, that is at high Ca2+ concentrations and pH
where the negative charge of the calcined clay is the strongest. The MC
simulations of the inter-particle interactions are found to qualitatively
explain the observed variation in the paste stiffness.
