The properties of unmodified, acrylic latex-modified and partially epoxy-impregnated gypsum composites are investigated. A hyperbolic function of the form ax\(l + bx)(n) is derived empirically to describe the water/gypsum ratio dependence of the mechanical properties of gypsum. This function shows good agreement with the test results The water/gypsum ratios, which give the maximum mechanical properties, are dependent on the material parameters n and b in proposed function, the values of which lie between 17.8 % and 18.14 %. The setting times of acrylic latex (methacrylic acid esters and styrene)-modified gypsum composites increase with an increase in latex content in the mix. These composites show a clear improvement in flexural strength. However, their compressive strengths remain almost in the same range of the unmodified gypsum. Epoxy impregnation does not lead to a significant increase in splitting tensile strength. The durability of the composites is examined by determining their behaviour in water after different periods. Seven days of immersion in 20degreesC water causes a reduction of about 70% in the mechanical strengths of latex-modified gypsum, whereas epoxy (diglycidyl ether of bisphenol A epoxy resin cured with an alkylenediamine curing agent)-impregnated gypsum composites with a surface coating of epoxy retain 100% of their original strengths even afler 7 days exposure to water. (C) 2001 Elsevier Science Ltd. All rights reserved.