Set Retarders & Accelerators
Tune the hydration window between hemihydrate slurry and the cut-off knife.
Control over the hemihydrate → dihydrate reaction is the single largest lever in board-line stability. Retarders and accelerators place the initial set precisely between forming and the knife. This page describes the chemistries we can specify and source; performance on a given line depends on stucco source, water chemistry and mixer geometry and must be derived from plant-specific trials.
Retarders act by either complexing calcium ions in solution (citric acid, tartaric acid, sodium gluconate) or by adsorbing onto growing dihydrate crystal faces (hydrolyzed keratin and collagen proteins, the BMA family). Accelerators are typically finely ground gypsum (BMA-type ground gypsum accelerator) sometimes combined with potassium sulfate, which lowers dihydrate solubility and shortens the induction period. See Wirsching, Ullmann's Encyclopedia of Industrial Chemistry, 'Calcium Sulfate'.
On a continuous board line, a small shift in the initial set time can move the board arrival state at the knife between crumbling, sticking, or kiln entry while still soft. Choice of retarder chemistry interacts with the calcination route (kettle vs flash) and with dispersant chemistry; matching them is a line-specific trial exercise.
- §Wirsching, 'Calcium Sulfate' — Ullmann's Encyclopedia of Industrial Chemistry — Wiley-VCH
- §EN 13279-1 — Gypsum binders and gypsum plasters — CEN