Soft Plaster

In this brief article, Mr. Geary first discusses the ideal plaster hydration and curing process, then lists some potential mistakes that would lead to soft plaster. He lists possible remedies, mostly involving wetting and finishing the curing process, including a simple test for evaluating the cement plaster curing quality. He also describes how to collect cement plaster samples to send to laboratories for more extensive testing. He stresses that hydration is what gives cement plaster its strength, and that proper hydration with each coat is necessary.




Calaveras Div., The Flintkote Co.

It is the nature of cement plaster to be hard and strong when applied properly. Firm plaster results when the portland cement portion is hydrated sufficiently. Hydration is the chemical reaction that starts to take place between portland cement and water when they are first brought together, and this process continues over a long period of time. After a period of time on the wall (normally less than an hour in warm weather), as hydration continues, the plaster becomes firm and cannot be removed by the trowel. This is called "taking up" in the trade. The plaster continues to gain strength during and after take-up through continuing hydration. The drying of cement plaster SHOULD NOT BE MISTAKEN FOR GAIN IN STRENGTH.

What may cause soft plaster? Some reasons or contributing factors include:

    Failure to provide enough moist curing when adverse drying conditions cause rapid loss of mix water from the plaster.

    Failure to apply the scratch coat thickly enough, in which case the plaster dries too quickly and does not gain full potential strength.

The addition of too much sand per bag of cement. (Normally this isn't the case because excess sand causes difficult spreading and resultant higher labor costs.)

Freezing of plaster on the wall prior to full hydration of the cement paste in the mix. (The cause and prevention of problem walls resulting from freezing is discussed in detail in my article "Plastering in Cold Weather," in April, 1968, issue of "Plastering Industries.")

What can be done when the plaster is soft? Two steps are recommended. Assume initially that the wall may not have been cured sufficiently and therefore will require more water. Several wettings throughout the day, and one (the most important) in the evening are advisable so that water will be retained in the wall overnight and provide maximum benefit to the plaster. There are cases, such as interior cement plaster jobs not subject to drafts of air, where no additional wetting of cement plaster is required in order for good strength to develop.

Normally, additional and properly timed curing with water will improve stucco unless too much sand has been added or other adverse factors are influencing the strength, and even then there may be some benefit from additional wetting. A good field test to determine whether the stucco was cured long enough or whether more water on the wall will help, is to remove a sample from the wall and break a small portion of it. The portion doesn't have to be any larger than a square inch.

Place the small part of the sample in a glass of water and allow the sample to soak overnight. The next day remove the soaked specimen and compare its hardness with the original sample removed from the wall, by breaking and scratching both parts. If there is a significant difference in strength of the soaked specimen compared with the unsoaked plaster, it is obvious that the wall will be helped by more water curing. If there isn't any noticeable improvement in the soaked section, it may be too late to soak the wall or the softness was not caused by premature drying, in which case analysis of the stucco is in order.

If the latter is the case, save the balance of the sample removed from the wall in a clean paper bag or envelope until the cause of the difficulty has been determined. Send a sample of the soft plaster, accompanied by a sample of the sand, cement, lime or other components in the mix to a laboratory for determination of the ratio of cement to sand and the degree of hydration.

Another possible field test prior to clean-up around the job is to pick up a bit of dropped plaster from the ground, hopefully one that has been in contact with water or moist earth. Scratch the plastered wall with the piece of plaster picked from the ground. If the piece from the ground is harder than the plaster on the wall, the difference is in the extent of curing, and the solution is to wet the walls some more. Sometimes drying conditions of the surrounding soil may be such that plaster that has fallen to the ground MAY NOT BE well cured, in which case this test is nullified.

To obtain an adequate sample of plaster from the wall, hold a clean sheet of paper, plywood or metal below the area of the wall to be sampled. Knock out enough plaster to produce an area of removed stucco about 3 or 4 inches square (whole or in pieces). Catch most of the chunks and dusts. Obtain the full thickness of plaster, including scratch, and brown coat material. Measure the thickness of the plaster at this time to determine if inadequate thickness was a factor. If the stucco is too weak to permit removal of anything other than fine material, that fine sample will be satisfactory for test. It doesn't have to be in chunks or one piece. However, the FULL THICKNESS of plaster has to be represented in the specimen.

Label the source of the sample such as the address of the job and directional exposure of the wall from which the specimen was taken, and the date of removal for the laboratory, which also must be provided with some of the sand, cement, lime or other ingredients, if used, that were added to the mix. Therefore, save a bag each of cement and lime and a clean container of sand from the job. Label each sample with brand or trade name, type of material and name of the job. A cement or concrete laboratory can determine the ratio of cement to sand and the degree of hydration in plaster that has been taken from a wall if the lab has the materials and information described above.

If the plaster is weak because of not having been cured well, prevention of future problems of this nature is simple. Fog spray the walls, double back with the brown coat or do both.

The most important phase of curing cement plaster is the wetting of the scratch coat, for a very good reason. Because of the relative thinness of the first coat (primarily because it is ONLY ONE COAT), there isn't enough substance to hold moisture for curing. After the brown coat has been applied over the scratch coat, there is more plaster to retain water for a longer time, hence more effective curing.

It is important to know that cement plaster gains its strength ONLY from the hydration process, and hydration, described simply, is the chemical combination of locking together of the cement, water and sand. If the cement isn't exposed to enough water, complete hydration can't take place, therefore adequate strength won't develop in the plaster.



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