A Study on Strength of Wet Gypsum

Abstract

While studying the relationship between strength of wet gypsum and its apparent specific gravity. I tried to set forth the relationship in mathematical formulae, such as I obtained before for dry gypsum.The formulae are as follow:log σx2/σx02=n2log ρ1/ρ0 or log σx2=n2 log ρ1+log σx02/ρ0n2when σx2 and ρ1: compressive strength of wet gypsum and apparent specific gravity of dry gypsum.n2: experimental constant of wet gypsum.σx02 and ρ0: compressive strength of wet gypsum and apparent specific gravity of dry gypsum. Dry gypsum is supposed to be in an ideal condition, i.e. the quantity of water used is just the theoretical amount required to bring about the chemical change.In order to ascertain the correctness of the formulae, experiments were conducted. Specimens were tested 3, 4 and 24 hours after the setting. These specimens were wet. The theoretical formulae indicate a straight line. The experimental logσx2-logρ1 curves were almost similar to the theoretical line. Also in the case of dry gypsum, experiments endorsed the theoretical anticipation, as I described it in another report of mine. It is clear from the curves, that “n”, the experimental constant in the case of dry gypsum, is smaller than “n2”.When gypsum is wet, or when there is water in excess of the amount required for crystallization, its strength is about half that of dry gypsum.This phenomenon is explained as follows.When gypsum is wet, excess water exists among crystallized particles of gypsum. The quantity of this water does not affect the strength.Crystallized particles of gypsum contact each other so that cohesive and frictional force works through these contacts. However, when there is water between two particles, the cohesive and frictional force is weakened because the water separates the two particles or, in other words, the distance between them increases a little. When gypsum is wet, it is easily broken if pressure is applied on it. Cleavage occurs along the part weakened by water, as described above.When a specimen of dry gypsum is destroyed, the cleavage runs irregularly along the most vulnerable part of the specimen, but when the specimen is wet, it is cleft along the wet part where the cohesive and frictional force between particles is weakened.