Effects of superplasticising admixtures used with blended cements: part one

Superplasticisers are key ingredients in modern concretes. Most of the superplasticisers are based on polycarboxylate ether. They improve the workability of concrete and have a positive impact on the concrete properties such as compressive strength and durability. The effect of these admixtures is strongly influenced by the specific surface of the cement constituents and readily soluble cement components.

Polycarboxylate ethers (PCEs) are copolymers of mostly polyether side chains grafted to a polycarboxylic main chain. Typically, the main chain is negatively charged in the alkaline cement paste’s pore solution. Via the negative charges, PCEs adsorb on positive surfaces competing with sulfate ions. The uncharged side chains of the sorbed PCEs tangle into the pore solution and disperse the particles by steric hindrance.

Practical experience has shown that in unfavourable conditions the same type and quantity of superplasticiser in combination with different cements can lead to incompatibility reactions. This can result in an undesired consistency loss, over-fluidification of the concrete, sedimentation, retardation of cement hydration or delayed strength development.

The material parameters of blended cements influencing the mode of action of PCEs have therefore been determined. The investigated cements contained the same clinker and sulfate carrier and a systematic variation of the type and proportion of the additional main constituent.

Cements with limestone or granulated blastfurnace slag

Increasing proportions of limestone or granulated blastfurnace slag (GBFS) in the cement changed the ionic composition of the cement paste’s pore solution significantly. In particular, the alkali and sulfate ion concentrations decreased virtually linearly due to substitution of the clinker (readily soluble alkali sulfates) by initially insoluble lime stone or amorphous GBFS particles. Due to the reduced alkalinity of the pore solution, the solubility of calcium ions and therefore its concentration increased. In particular, the reduced sulfate concentration and the increased calcium content shifted the zeta potential of the cement paste, as a measure of the particle’s surface charge, from weak negative to slight positive values. As a result of the reduced sulfate concentrations and the more positively charged surfaces, in particular a less negatively charged PCE for ready-mixed concrete adsorbed to a greater extent, compared to Portland cement.

With an increasing proportion of limestone or GBFS in the cement, the corresponding cement paste could be plasticised more effectively with smaller quantities of PCEs due to the reduced amount of initial hydration phases with their large specific surfaces. The quantity of PCEs needed for the maximum plastification of cement paste (saturation dosage) fell with increasing clinker substitution. At the same substitution level, the saturation dosage increased with the BET specific surface of the cement constituents. This applied for the already mentioned PCE for ready-mixed concrete (PCE 11) as well as for a more negatively charged PCE for precast concrete (PCE 22). Added quantities of PCEs beyond the saturation dosage always led to sedimentation and/or retardation.

In combination with Portland cement, both PCEs exhibited considerable specific effects. PCE 11 maintained the consistency of the cement paste for a long time, whereas PCE 22 caused prompt stiffening. When the PCEs were used with blastfurnace cement CEM III/B, their specific effects had virtually disappeared. PCE 11 sorbed to a greater rate and the cement paste exhibited a distinct consistency loss because the pore solution had considerably lower sulfate ion content and the zeta potential was more positive. The clinker substitution and therefore the essentially reduced reactive surface resulted in a decreased sorption of PCE 22. Therefore, this superplasticiser provided a prolonged dispersing effect.

This is part 1 of a 2-part article that was originally published in Newsletter 3/2015 of the European Cement Research Academy and is reproduced by kind permission of ECRA.

Read the article online at: https://www.worldcement.com/special-reports/17092015/effects-superplasticising-admixtures-used-blended-cements-610/