Acrylic Resin-Modified Portland Cement Plaster

In this article, Mr. Geary explains the rationale of incorporating acrylic resin in portland cement plaster mix, describes how this is to be done, and explains how doing so must modify the curing process and protocol. Acrylic resin is said to improve stucco adhesion, flexibility, moisture resistance, and various measures of strength and durability. However, it must be mixed strictly in accordance with manufacturers' specifications, explains the author, with reasoned warnings of potential untoward consequences if these are not followed. Hydration by moist curing the stucco must be delayed to allow for the acrylic to harden. The author closes with a helpful list of dos and don'ts.





The addition of acrylic resin to cement plaster can be beneficial when mixed and used in the proportion and manner prescribed by the manufacturer and when conditions have been established for proper curing. The benefit obtained from addition of acrylic resin accrues from increased flexibility of the cement plaster membrane, an increase in tensile strength, abrasion and impact resistance and improved resistance to absorption of moisture. It is generally accepted that mixing acrylic resin in cement plaster does not increase compressive strength. Portland cement plaster has excellent compressive strength, without alteration, under normal conditions.

Acrylic resin added to portland cement plaster is beneficial in the development of improved bond of the modified coat of plaster to the substrate. The manufacturer of acrylic resin has informed me that the addition of acrylic to a bond, slush or slurry coat permits application of the bonding coat to a wider range of substrates than in the case when the mix is composed only of Portland cement, sand and water. These substrates include aged and new concrete and portland cement plaster, wood, metal, masonry and polymeric foam. This widened range of potential substrates can be beneficial to the architect and contractor.

The manufacturer of acrylic resin has stated that inclusion of the emulsion in plaster will cause an increase in the amount of entrained air resulting from mechanical beating of plaster in the typical plaster mixer. Vendors of acrylic resin normally incorporate a defoamer in the emulsion to reduce air entrainment to a minimum so as to maximize mortar strength properties. Entrapment of air in plaster within reasonable limits is considered beneficial in the plastering industry and is normal procedure. This is a good feature for plastering because a controlled amount of entrained air reduces the amount of mix water required, as well as aiding workability.

Acrylic resin can be and is successfully mixed with plastic cement or a blend of plastic and regular cements to produce excellent modified cement plaster. I have encountered cement plaster into which acrylic resin has been mixed at a ratio, in the mixing fluid, of one volume of acrylic to three volumes of clean water which was very hard and strong, with excellent resistance to breakage under strong blows of a prospector's pick. One may successfully incorporate acrylic resin in plaster made of plastic cement if proper controls are maintained. Typically, these controls would include avoidance of enrichment of the mix with acrylic resin beyond the ratio specified by the manufacturer (normally no more than 10-20 of acrylic polymer solids, by weight, based on cement content), limiting mixing time to approximately five minutes to control" entrapment of air and, after application, effectively curing the wall AFTER the resin has converted to a hard film. Sub-standard strength in a membrane into which acrylic or other resin had been incorporated could relate to over-mixing or failure to cure properly. In summation, the only reasonable requirement should be that the completed wall has sufficient strength and toughness to perform the functions for which the wall is intended.

The percentage of entrained air which can be incorporated because of inclusion of acrylic resin in the batch can vary. Air entrainment is directly related to the amount of acrylic resin in the mixing liquid as well as the length of time and extent of agitation of the batch rotating in the mixer. Batches containing acrylic resin should not be allowed to mix fifteen or twenty minutes, as sometimes happens with unmodified batches of plaster. Approximately five minutes is adequate blending time.

The acrylic resin should be blended with water in a clean container in the proportion specified by the manufacturer, prior to addition to the mixer. The blend of mixed acrylic resin and water is to be used exactly like clear mixing water, after having been blended in the proper proportions.

The improved strength and adhesion obtained from acrylic resin in cement plaster develops as the resin dries and forms a film within the cement- sand matrix. This occurs as mix water evaporates from the outer surface of the plaster membrane and the resin emulsion converts from a milky-white liquid to a clear or nearly clear film. The emulsion converts to a film while the cement paste in plaster is developing its final set, which in the case of Calaveras Plastic and Type I-II cements takes approximately four hours. The time required to convert acrylic resin from an emulsion to a dry, hard film in the plaster could be affected by temperature of the air, thickness of the plaster, temperature of the substrate, force of wind blowing across the face of the wall, relative humidity of the air and other factors.

Air curing alone, at the outset, is normally recommended for acrylic modified cements. During the time that acrylic resin is present in the liquid phase in plaster, the plaster should NOT be sprayed with curing water. This admonition is to prevent washing resin emulsion out of the mix. However, after the resin has converted from the emulsion phase to a hardened film, curing water may and sometimes MUST be sprayed on the surface of the cement plaster wall. Additional curing water, applied after conversion of the resin to a hard film, is beneficial to the wall by developing greater strength and hardness within the cement plaster by means of continued hydration of the cement.

An overnight period of cure should be more than sufficient in most cases for acrylic resin to set and harden, under normal conditions. The principal exception would be conditions which would retard the cure of non-modified Portland cement plaster or which would retard evaporation of water from plaster. Effective factors which might retard cure and/or evaporation of mix water are low temperature, high humidity and greater thickness of the plaster membrane.

Unless weather conditions are such that a cementitious product would not need moist curing, water should be sprayed on the resin-modified plaster wall on the day following application of the plaster, in accordance with standard moist curing procedure. No harm will be done to the wall by moist curing after the resin has hardened.


Do not add an excess of acrylic resin to plaster batches in any case, but especially to batches containing air-entrained cement.

Do not add a separate air-entraining agent to the batch in the mixer.

Blend the acrylic resin and water prior to introduction into the mixer.

Do not spray water over newly-applied plaster which contains acrylic resin until the emulsion has converted from liquid to a solid film.

If moist curing of ANY cement plaster seems advisable, curing water is to be sprayed on walls which contain acrylic resin in cement plaster on the day following application of the plaster.



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