Printable mortar

A printable mortar is a fine-aggregate cement-based mix formulated so that it can be pumped through a delivery line, extruded through a nozzle and hold its own shape without formwork until it hardens. It is a mortar rather than a concrete in the ordinary sense, because the maximum aggregate size is limited by the nozzle opening and is typically only a few millimetres. The recipe has to satisfy two opposing requirements at once, extrudability in the line and buildability at the nozzle, which is why admixtures do much of the work.

In practice

Printable mortars are usually binder-rich, with a higher cement or binder content than a comparable cast concrete, because fines are what carry the pumping and the shape retention. That has a direct consequence: a printed element is not automatically lower in embodied carbon than a conventional one, and any comparison depends heavily on the mix and on the baseline chosen. Supplementary cementitious materials, limestone powder and calcined clays are the usual routes to reduce clinker content. Superplasticisers, viscosity-modifying admixtures and accelerators are dosed to place the mix in a narrow workable window, and the mix is normally validated by trial printing rather than by fresh-concrete tests alone.

Related terms

• Extrudability — the ability of the mix to leave the nozzle as a continuous filament.
• Buildability — the ability of the deposited filament to carry the layers above it.
• Thixotropy — the recovery of stiffness after shearing, which lets a mix flow then stand.