Acceleration by retardation

The «acceleration by retardation» concept is to make an admixture composition that will accelerate the setting of Portland cement by retarding two other processes; dissolution of gypsum and retardation of C3A hydration by another admixture than gypsum. Small dosages of pyrocatechol (0.02—0.03 % of cement mass) leads to flash set of cement, most likely by inhibiting dissolution of calcium sulphate and thereby rapid hydration of C3A. These small dosages of pyrocatechol seem not to affect C3S hydration. It has been indicated by isothermal calorimetry and physically observed on excess paste that the «flash set» induced by pyrocatechol can be counteracted by other retarders in smaller dosages than gypsum (e.g. sodium gluconate), leading to a faster setting (i.e. set acceleration) than the reference. However, these retarders delays onset of C3S hydration and may therefore also delay strength evolution.
If the assumed mechanism of pyrocatechol is correct, the inhibited gypsum may react on a later stage and lead to a kind of delayed Ettringite formation that could generate expansion and cracking. This possibility was checked out by casting concrete with and without similar admixture blends, and monitoring their linear and volumetric expansions. However, no abnormal expansion has been observed until present (2 months).

Author: H. Justnes

Section: Science and production

Keywords: acceleration, expansion, flash set, gypsum, isothermal calorimetry, retardation