Abstract: Additive compositions, made up essentially of a mixture of a salt of a very high molecular weight condensation product of naphthalenesulfonic acid with formaldehyde, containing not less than 35% by weight of the condensation products with a molecular weight greater than 5000, together with a glucopheptonic acid salt, are mixed with hydraulic cement to reduce the water which needs to be added, and to provide improved strength.

Abstract: A plaster composition is disclosed having a long setting time, which composition comprises a gypsum-based plaster and an effective amount of a setting retarding agent comprising a polymer having a molecular weight of less than 1000 of the formula: ##STR1## wherein R.sub.1 is hydrogen or methyl,R.sub.2 is COOH, COOR.sub.4, wherein R.sub.4 is C.sub.1 to C.sub.4 alkyl, CONH.sub.2 or CN,R.sub.3 is hydrogen or methyl,X is a sodium, potassium, lithium, calcium, magnesium, or ammonium ion, andx and y designate the molar ratio of the two monomers forming the setting retarding agent of the formula I.

Abstract: Gypsum and anhydrite plaster is improved by adding at least one alkylene diphosphonic acid or water soluble salt thereof, represented by the following formula: ##STR1## wherein X is hydrogen or lower alkyl of 1-4 carbons, Y is hydrogen, hydroxy or lower alkyl of 1-4 carbons, and n is 1-10. The plaster is found to have an increased finishing time, while the quantity of water necessary is reduced and the mechanical strength of the finished plaster is improved.