Competitive adsorption between superplasticizer and retarder molecules on mineral binder surface

In a self-levelling mortar based on the ternary binder system ordinary Portland cement (OPC), calcium aluminate cement (CAC) and anhydrite, a polycarboxylate-based superplasticizer (PC) showed no plasticizing effect in combination with citrate retarder while good flowability was observed with tartrate. The mechanism behind the incompatibility between PC and citrate was investigated by means of adsorption and zeta potential measurements. Also, anionic charge densities of the admixtures were compared. Adsorption measurements revealed that, in presence of citrate, PC adsorption drops dramatically to less than 10% of dosage added, implying a complete loss of fluidity in the paste. In presence of tartrate, however, PC adsorption remains high enough to still provide good flowability. In contrast, adsorption of casein biopolymer is not much affected by addition and type of retarder. Thus it provides high fluidity with both retarders. Comparison of specific anionic charge density of Ca2+ retarder complexes and PC reveals a direct correlation between their adsorption behaviour and anionic charge density. Admixtures with higher anionic charge density show higher affinity to the binder surface and thus adsorb preferredly. When several admixtures are present, molecules with lower anionic charge density will adsorb only if, after adsorption of the admixture with higher anionic charge, a cationic surface charge and enough adsorption area still exists. The incompatibility problem between PC and citrate in the self-levelling mortar formulation was solved by increasing the anionic charge density of the PC molecule. Similar to casein, adsorption of this modified PC is not much influenced by retarder molecules.