Montmorillonite intercalates polyether side chains and traps the superplasticizer molecule inside its interlayer space, so the effective concentration in solution collapses. The result is acceptable initial flow with rapid slump loss.
Compensating by raising dosage is unreliable: cost rises, and overdosing causes bleeding and retardation.
Phosphate groups complex calcium on the cement particle surface more strongly than carboxyl groups, and their small size means they are less easily captured by clay interlayers.
After copolymerization the backbone carries both carboxyl groups (dispersion) and phosphate groups (strong adsorption, clay tolerance), preserving part of the effective adsorption in muddy systems.
Clay type matters: montmorillonite is most sensitive, kaolinite and illite much less so. Always validate with the customer's actual aggregate.