ph of a 20 by weight plaster suspension in exchanged water was 8 2 the plaster was introduced into a plough type mixer designated lodige followed if necessary by mixing therein the waterproofing agent the water required to set the plaster was then added to the mixer by means of an atomized spray following attainment of a homogeneous mixture the wet powder was poured into a mold and the compacting pressure necessary to obtain the desired density was applied a plurality of specimens were prepared and subjected to the tests described hereinabove the test conditions and the results obtained are compiled in table iii table iii__________________________________________________________________________ water hours on a bedtest excess proofing control of moist sandnumberplaster water agent density p c f s c s r c f h c h__________________________________________________________________________1 55 2 0 1 65 190 50 189 21 lt 5 lt 152 55 8 0 1 63 210 73 148 22 lt 5 lt 153 55 2 0 5 1 61 80 50 100 20 8 154 55 2 1 1 61 80 48 105 20 8 205 55 2 1 5 1 59 84 46 95 20 6 256 55 8 0 5 1 59 110 40 120 18 lt 5 lt 157 55 8 1 0 1 59 93 38 100 18 lt 5 lt 158 55 8 1 5 1 58 141 39 60 18 lt 5 lt 15__________________________________________________________________________ a series of tests was performed according to the operating procedure described in example 4 but utilizing a phosphogypsum and a phosphoplaster derived from moroccan phosphate rock the gypsum utilized was a phosphogypsum derived from moroccan phosphate.