Why superplasticizers fear clay

Polycarboxylate superplasticizers adsorb onto cement particles through carboxyl groups and disperse them by steric hindrance from polyether side chains. Montmorillonite's interlayer space can swallow those side chains, taking the whole molecule with it.

Intercalated molecules no longer disperse anything and also trap free water, producing acceptable initial flow followed by rapid slump loss. Each 1% increase in clay content can require over 10% more active dosage.

There are three approaches: longer or denser side chains to reduce intercalation; stronger anchoring groups such as phosphate or sulfonate to compete with clay; and dedicated clay-tolerance additives.

Phosphate groups adsorb strongly and are small enough to avoid being trapped, preserving part of the effective dosage at moderate addition levels. Clay type matters: montmorillonite is most sensitive.

Always validate with the customer's aggregate — paste flow shows the trend, but mortar and concrete slump retention is the real criterion.