Chemical admixtures and control of the hemihydrate/gypsum conversion process: Fid-Bau Portal

Chemical admixtures and control of the hemihydrate/gypsum conversion process

Jolicoeur, C. / Yaqini, A. / Michon, M. / Sharman, J. / Page, M. / Simard, M.A.

The gypsum wallboard manufacturing industry relies heavily on chemical additives (admixtures) to optimize process throughput (paste workability, hydration rate, setting/drying rate), and to ensure adequate physical properties of the hardened gypsum (density, strength, water resistance). Most gypsum (G) board processes now typically include dispersants (water reducers), set modifiers (accelerators, retarders), surfactants (air-entraining agents, or foaming agents) and co-surfactants (foam stabilizers). While each additive is introduced for a specific function, many of them will also influence the hemihydrate (H) hydration and the H-to-G conversion process. Combinations of admixtures can thus interact in complex, and often unpredictable, ways to modify the conversion process rate and the properties of the resulting gypsum. The present paper examines how common dispersants and air-entraining surfactants influence the hydration rate of hemihydrate and the precipitation of gypsum in dilute slurries. The solutions processes are monitored using electrical conductivity and the extent of H-to-G conversion is followed by measurement of the evolved heat. A comparative study involving individual additives and several dispersant/surfactant pairs yields evidence of a strong synergy between some of the additives. The joint inhibitory (retarding) action of several pairs of admixtures on gypsum precipitation is considerably more pronounced than the sum of individual admixture effects. Interpretation of these phenomena and their impact on gypsum process application are briefly discussed