Viscosity-modifying admixture (VMA)

A viscosity-modifying admixture, abbreviated VMA, is an admixture that raises the viscosity and cohesion of the paste phase of a cement-based mix. Most VMAs are water-soluble polymers, such as cellulose ethers, welan or diutan gum, or synthetic polyacrylamides. They bind free water and entangle in the pore solution, which holds fine particles in suspension without demanding more binder.

In practice

In printing, a VMA is what stops a highly dispersed, superplasticised mortar from bleeding in the hose or slumping as soon as it leaves the nozzle. The pair is usually tuned together: the superplasticiser lowers yield stress for pumping, the VMA restores cohesion and the rate at which structure rebuilds at rest. Cellulose ethers also retain water, which helps a thin extruded filament stay workable long enough to bond to the layer below, but the same effect retards setting and must be balanced against the accelerator dose. Because VMA response is strongly non-linear, small dosage steps and repeated flow tests on the actual mix are more useful than supplier tables.

Related terms

• Superplasticiser — disperses cement particles and lowers yield stress, usually dosed alongside a VMA.
• Thixotropy — the reversible stiffening at rest that a VMA helps to build.
• Extrudability — the ability of a mix to pass through the nozzle as a continuous, stable filament.