Accelerator

An accelerator is a chemical admixture that shortens the setting time of a cement-based mix or speeds up its early strength development. Accelerators are divided into set accelerators, which bring forward the transition from plastic to rigid, and hardening accelerators, which mainly raise strength in the first hours. Both alkali-free and alkaline formulations are used in construction; alkali-free products are common where reinforcement corrosion and efflorescence are a concern.

In practice

In 3D concrete printing an accelerator is often dosed late, close to or inside the print head, rather than in the main batch. That arrangement keeps the mix pumpable through the hose and stiffens it only after extrusion, which is the principle behind set-on-demand systems. Dosing is tuned against the print rate: too little and the lower layers deform under load, too much and the material sets in the line or the interlayer bond suffers because the previous layer is already too rigid to accept the next one. Mixing energy, temperature and the base binder all shift the effective dose, so trials on the actual machine are the only reliable calibration.

Related terms

• Set-on-demand — dosing a reactive admixture at the nozzle so stiffening starts after extrusion.
• Open time — the window in which a mix stays workable and can still bond to the next layer.
• Green strength — the load-bearing capacity of fresh material before hardening.