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Partial substitution of the clinker in the cement by a supplementarycementitious material (SCM) is one of the main solutions to reduce the carbonfootprint. Calcined kaolinite is a good candidate due to its availability andrelatively high reactivity compared to other SCMs. The main issue with thesecalcined 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 showthe role played by electrostatic interactions in the paste stiffening using asa model system pure calcined kaolinite paste prepared at various pH as well assalt types and concentrations. The study combines dynamic rheometrymeasurements in strain-sweep modes, surface charge characterization usingpotentiometric titration and electrophoretic measurements as well ascalculations of inter-particle interactions using Monte-Carlo (MC) simulationsin the framework of the primitive model. The calcined kaolinite is found topossess a negative permanent charge, presumably due to the Si(IV)/Al(III)substitution, and a titratable charge (as due to ionization of silanol andaluminol surface groups) with a point of zero proton charge at pH 4.65. Inconditions relevant for cement paste, the calcined clay bear a strong negativecharge ~ 300 mC/m2. The rheological measurements reveal that the pastestiffening is highly dependent on the pH, salt concentration and type asexpected for systems controlled by electrostatic interactions. The stiffnessincreases with the salt concentration at natural pH and is the largest insolutions buffered with Ca(OH)2, that is at high Ca2+ concentrations and pHwhere the negative charge of the calcined clay is the strongest. The MCsimulations of the inter-particle interactions are found to qualitativelyexplain the observed variation in the paste stiffness.

Paste Rheology and Surface Charge of Calcined Kaolinite