Superplasticiser

A superplasticiser is a high-range water-reducing admixture that disperses cement particles so a mix stays workable at a low water to binder ratio. Modern products are mostly polycarboxylate ethers, which adsorb onto cement grains and separate them by a combination of electrostatic repulsion and steric hindrance. The practical effect is a large drop in yield stress at constant water content, or the same flow with considerably less water.

In practice

Printable mortars are usually binder-rich and fine-grained, so without a superplasticiser they would need far more water than their strength and shrinkage targets allow. The admixture is therefore used to buy pumpability, while a viscosity-modifying admixture and thixotropy restore the stiffness needed for buildability after extrusion. Dosage is narrow: a small overdose can cause bleeding, segregation and a long delay in setting, and the response differs between cements and between batches of the same cement. Polycarboxylates are also sensitive to the clay content of sands, which is a common cause of unexplained loss of flow when a local aggregate source changes.

Related terms

• Viscosity-modifying admixture — thickens the paste phase to counter segregation caused by high dispersion.
• Yield stress — the shear stress a fresh mix must exceed before it starts to flow.
• Printable mortar — a fine mix formulated to be pumped, extruded and stacked.