Portland Cement Plastering

Portland-cement based plaster can be applied to both interior and exterior surfaces, and is typically applied in two or three coats as specified in ASTM C926. The standard specifies plaster mixes that utilize portland cement or blended cement, plastic cement, and/or masonry cement. This Common Product describes a portland cement based plaster consisting of a base-coat mix that may be used for two- or three-coat applications, and a finish coat. Mixes containing blended, plastic, or masonry cements were beyond the scope of this Common Product. Base coat concentrations are based on a Type C plaster mix. Since manufacturers commonly supply mixes that may be used in both the scratch coat (1st coat) and brown coat (2nd coat) of three coat applications, this Common Product assumes that the same mix is used for both layers of the base coat. The finish coat is based on a Type F plaster mix. Weight percentages are estimated based on nominal thicknesses for vertical applications applied over metal lath, specified in ASTM C926. 

Portland-cement based plaster can be purchased pre-mixed, and manufacturers commonly indicate that the mixes contain proprietary additives. These additives are not well disclosed. In addition, some admixes are available for purchase that include bonding agents, pump aids, and fibers. A number of products offer acrylic admixes designed to accelerate hydration or improve properties like bond, tensile, and flexural strength. Mixes may also contain glass or plastic fibers to prevent cracking. These additives and admixes will add additional hazards not considered by this Common Product. 

This Common Product does not describe, in detail, system accessories such as metal lath, sheathing, water-resistive barriers, crack reduction systems, foam architectural details, or coatings. General information on some of these components is, however, included in the All Contents tab.