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Ahard look at polymers in cement mix

Modelling how superplasticizers can reduce water proportions in cement mixtures may help develop more efficient superplasticizers, as well as enhance concrete performance, shows the first comprehensive study conducted by A*STAR.

Superplasticizers are polymers that act as dispersants in cement mixtures. They hinder the aggregation of the cement particles, allowing dramatic reductions in the volume of water in the mixture without impacting on its flow and workability. While some water is needed to enable cement to harden to concrete through hydration, reducing the proportion of water in cement mixtures results in stronger products.

In collaboration with the global chemical company, Nippon Shokubai, Jianwei Zheng at the A*STAR Institute of High Performance Computing and colleagues used molecular dynamics simulations to model the adsorption of three novel superplasticizers – polycarboxylate ethers (PCEs) – onto the surface of magnesium oxide particles in a cement mixture.

PCE-based superplasticizers have negatively-charged carboxylic acid groups in their polymer backbone that adsorb electrostatically to particles in cement such as magnesium oxide. The long polyethylene glycol groups then act as spacers, preventing the cement particles from clumping. A number of earlier, smaller modeling studies suggested that the thickness of this adsorbed polymer layer directly correlates with the amount of dispersion seen. Zheng's team is the first to carry out a comprehensive modeling study to determine how the shape of the polymer influences layer construction and depth.

"We describe the correlation of molecular structures of PCE-type superplasticizers with polymer conformation as well as adsorption layer thickness in cement pore solution," explains Zheng. The team found that the thickness of the layer depends on how the polymers initially orientate themselves against the particle surface. Those that start perpendicular to the surface gradually form a tail with a loop on their end. These polymers eventually form the desired thicker layer. By contrast, those that start out parallel to the surface grow into a tail and result in a thinner layer.

The team plan to conduct further simulations with different polymeric structures to see if the layer depth can be increased further. "More efficient superplasticizers may be designed in near future," says Zheng. "The effect of superplasticizers on cement hydration will be under consideration in future models."