Occasionally, it is beneficial that cement plaster which has been removed from a wall or soffit be tested in a laboratory. One test that may be performed is a determination of the degree of hydration of the applied and cured cement plaster. That determination may be helpful in evaluating suitability of cement plaster in the event of a report of softness, delamination, or excessive cracking in the plaster membrane. Often times it is not necessary that a laboratory test be made, if an experienced and knowledgeable person has examined the project and has performed several field tests on site. The degree or extent of hydration of the portland cement paste in plaster or mortar is a close approximation of the extent to which the specimen of cement plaster was cured. A portion of the procedure described later elates to an in-house test employed by a cement manufacturer whom I once represented. A portion of the procedure is based upon standard tests such as an ASTM procedure. The balance of this paper represents my own contribution.
Procedure
A minimum of twenty square inches of portland cement plaster, representative of the installation, should be removed from the wall as a sample. This specimen must represent the full thickness of the plaster membrane. The entire thickness may be used for testing, or only the two base coats (scratch and brown), or only the finish coat (color coat), depending upon the apparent problem observed on the job. The portion or portions of the removed sample to be tested are to be ground to a minus eight mesh.
One gram of the finely ground sample must be weighed on an "as is" basis and the loss on ignition determined in accordance with ASTM C-114 test procedure.
In preparation for the determination of the degree of hydration, 100 grams of the precrushed plaster in question should then be ground to fineness of minus 30 mesh. This ground increment is to be dried to constant weight at 115 degrees Celsius, which will require approximately 12 to 16 hours exposure to that temperature.
A second increment of the ground sample must be weighed for determination of carbon dioxide (C02) in the specimen. The determination of carbon dioxide may be accomplished by means of a laboratory instrument known as an alkalimeter.
After determination of the percentage of carbon dioxide and loss on ignition, the figures representing percentage of carbon dioxide are to be subtracted from the value representing the percentage of loss on ignition. The difference between the two sets of figures is considered to be water of hydration, which is the principal information desired in this case.
Calculations necessary to determine the degree of hydration must involve arithmetical adjustment based on the percentage of cement determined to be present in the plaster.
The following information may be helpful as reference material. The ignition loss minus the percentage of carbon dioxide, on a 100 cement content basis, is approximately 26 for a completely hydrated specimen of neat Type I-II portland cement. The ignition loss minus the percentage of carbon dioxide, on a 100 basis, is typically very slightly over 22 for a completely hydrated specimen of neat Calaveras plastic cement.
Within the cement company whom I represented for many years, we determined, over a period of many years of testing specimens of plaster, that a suitably cured sample of portland cement plaster would be within the approximate range of 70 to 80 hydration. Generally, samples of mortar removed from a project and tested in the laboratory tend to test approximately within that described range, or to be definitely outside that range.
An additional test of plaster that I consider to be informative and valuable is the determination of the ratio of sand to cement in the wall or ceiling in question. ASIM C-85 is the recommended procedure to be followed in order to measure the ratio of sand to cement. It is necessary to make the above calculations in the course of this test. This reported ratio, in effect, informs us of the approximate number of cubic feet of sand used with each one-cubic- foot bag of cement.
To accomplish a meaningful measurement of the ratio of sand to cement in a specimen of plaster, it is necessary that a suitable sample of the cement and sand that was employed in the mix, or comparable samples of each material, be provided the laboratory to serve as references or standards. The laboratory should have the described samples of cement and sand, or an accurate chemical analysis of them, in order that allowance may be made for the calcium and silica content in those two ingredients.
A meaningful determination of the ratio of sand to cement in a specimen cannot be accomplished if lime or any lime or silica-containing material had been added to the batch of plaster in the mixer. The presence of lime or silica from any source other than the cement and sand incorporated in the batch of plaster would negate the validity of the described test. The actual ratio of sand to cement in a specimen to be tested can only be determined if the analyzed amount of lime and silica in the sample of plaster was contributed only by the cement and sand which composed the batch.
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