Impact of different types of polycarboxylate superplasticisers on spontaneous crystallisation of ettringite

The spontaneous crystallisation of ettringite from combined calcium hydroxide (Ca(OH)₂) and aluminium sulfate (Al₂(SO₄)₃) solutions was investigated in the presence and absence of three different kinds of polycarboxylate superplasticisers. The crystallisation period was limited to 10 s only to study the initial crystals, which provide the anchoring sites for most admixtures in cement. It was found that, in the absence of polycarboxylate polymers, only few, but large (l ∼ 3·5 μm), hexagonal, prismatic ettringite crystals exhibiting numerous defects on their surfaces were formed. However, when polycarboxylate polymers were present during the crystallisation, then the crystal sizes decreased to ∼ 0·5–2·1 μm. Generally, crystal sizes were found to decrease with increasing specific anionic charge of the same kind of polycarboxylate polymer. Apparently, polymers exhibiting a higher affinity to the crystal surfaces of ettringite inhibit its growth more strongly and produce smaller crystals. Furthermore, the specific chemistry (base monomers) of the polycarboxylate also has an impact on the size of ettringite crystals.