Abstract: A mastic composition formed of a substantially homogeneous imporous mixture of a predetermined volume of low-density expanded closed-cell synthetic resin beads and a slurry prepared from calcined gypsum and water wherein the interstitial volume of the expanded beads is not substantially less than the volume of slurry. The substantially homogeneous mixture is self-adhering as a thick layer on an inverted smooth surface and yet separable therefrom as an integral layer, the mastic composition setting to a hard, strong, dimensionally stable, heat- and sound-retardant material. Methods of mixing mastic compositions, and composite structural panels utilizing mastic compositions as well as the methods and apparatus for forming such panels are also disclosed.

Abstract: A mastic composition formed of a substantially homogeneous imporous mixture of a predetermined volume of low-density expanded closed-cell synthetic resin beads and a slurry prepared from calcined gypsum and water wherein the interstitial volume of the expanded beads is not substantially less than the volume of slurry. The substantially homogeneous mixture is self-adhering as a thick layer on an inverted smooth surface and yet separable therefrom as an integral layer, the mastic composition setting to a hard, strong, dimensionally stable, heat- and sound-retardant material. Methods of mixing mastic compositions, and composite structural panels utilizing mastic compositions as well as the methods and apparatus for forming such panels are also disclosed.