Materials should comply with building codes and job specifications. Packaged materials should be labeled properly, indicating the manufacturer, brand name, and recommendations for use. Packaged materials that might be damaged by moisture should be protected. Proprietary or specialty plaster should be mixed in accordance with the manufacturer’s recommendations.
The cement may consist of any of the following types: portland cement, conforming to ASTM C 150 (gray or white), type as required. Blended cement, conforming to ASTM C 595, type as required. Sulfate-resistant cements, masonry cements, or mineral admixtures should be used where sulfate soil or ground water conditions exist. Low-alkali cements conforming to ASTM C 150 or blended cements conforming to ASTM C 595 should be used with potentially reactive aggregates. Alternatively, suitable combinations of cement and mineral admixtures may be necessary. Masonry cement conforming to ASTM C 91, Types N, S, and M. Plastic cement conforming to the requirements of UBC Standard Chapter 47 or ASTM C 926. Air-entraining cements may be used where available.
Use Type S, special hydrated lime conforming to 524R-4 ACI COMMITTEE REPORT ASTM C 206 or C 207. Air-entraining limes may also be used where available.
The aggregates may be either natural or manufactured sand conforming to ASTM C 897 or lightweight perlite or vermiculite aggregate conforming to ASTM C 35, Table No. 1. Conventional portland cement plaster should not be applied to base coats containing perlite or vermiculite. Portland cement plaster containing perlite or vermiculite aggregates has low resistance to effects of freezing and thawing. When ASTM C 897 aggregates are not available, ASTM C 144 aggregates may be substituted. The use of substandard aggregates may increase the water demand, resulting in a weaker mix with poor durability and a greater tendency toward cracking. Aggregates that are frozen should not be used.
Potable water is generally acceptable. The water used in mixing and for curing portland cement plaster should be clean and free from injurious amounts of oil, acid, alkali, organic matter, salts, or other deleterious substances. Such substances may impair the setting and hardening characteristics of the plaster, or stain or discolor the surface.
The following admixtures may be added provided they are accepted in the project specifications.
6.1 Air-entraining admixtures
Air-entraining admixtures conforming to ASTM C 260 reduce water demand, absorption, or water penetration while improving workability and resistance to freezing and thawing. Air-entraining admixtures should be pretested when used with air-entraining cements or limes. Householdtype detergents should not be substituted for appropriate air-entraining admixtures.
6.2 Calcium chloride
Calcium chloride should conform to ASTM D 98. Caution should be exercised in the use of calcium chloride as an accelerator. The flake form should be put into solution prior to being added to the plaster mixture. Calcium chloride or accelerating admixtures containing significant amounts of calcium chloride should not be used when portland cement plaster will come into contact with metal lath, anodized aluminum, galvanized steel, or zinc accessory products. Chloride ions may accelerate the corrosion of such metals, causing expansion within the portland cement plaster resulting in cracking. Noncorrosive accelerating admixtures meeting ASTM C 494 are recommended when needed.
Water-reducing admixtures and water-reducing/set retarding or accelerating admixtures conforming to ASTM C 494 may be used to reduce the water-cement ratio of portland cement plaster. Manufacturer’s recommendations should be observed in the use of chemical admixtures. Very small changes in quantities may have a significant effect on the properties of portland cement plaster.
6.4 Water repellent admixtures
Stearate emulsions, in amounts not to exceed 2 percent by weight of cement, may be used to improve water repellency and decrease absorption. Some of these emulsions may also entrain air. The use of stearates may reduce bond between coats and may degrade with time.
Coloring agents should be of uniform color, free of lumps, and conform to ASTM C 979. To avoid strength reductions, the pigment content should not exceed 10 percent of the weight of the cement. Only mineral pigments should be used as coloring agents. The use of lamp black or carbon is not recommended.
6.6 Other admixtures and additives
Bentonite or other clays, diatomaceous earth, pozzolans, and the plasticizers are used in plaster to improve workability. Fly ash and other mineral co-mixtures may be added to plaster to improve sulfate resistance. Bonding agents and other additives are used to improve other characteristics of plaster. Manufacturer’s recommendations should be observed.
When accepted by the project specifications, fibers conforming to ASTM C 1116 may be used. If glass fibers are used, it is important that they are alkali resistant. The use of these fibers generally improves cohesiveness, crack resistance, impact resistance, and resistance to water penetration. Fibers should be added to the mix in the manner and amount recommended by the manufacturer.
8-Bonding agents Bonding agents permit direct application of plasters to clean, structurally sound surfaces such as concrete, brick, and concrete masonry units. There are two types of bonding agents: surface applied and integrally mixed.
8.1 Surface applied bonding agents
Surface applied bonding agents are single-component, ready-to-use liquids, which are applied to the surface by brush, roller, or spray. Surface-applied bonding agents should conform to the requirements of ASTM C 631 for interior plaster or C 932 for exterior plaster. Refer to the manufacturer’s recommendations for specific application directions.
8.2 Integral bonding agents
Integral bonding agents are acrylic, styrene-butadiene, or latex polymers, generally diluted with water at the jobsite, and added to the plaster mix to promote bond.