CHAPTER XV

CEMENT AND CONCRETE PAINT TESTS

=Damp-proofing and Waterproofing.= The decoration and preservation of cement and concrete is a subject which is being given the careful consideration of many technologists on account of the wide usage of cement for structural purposes, and the necessity of properly guarding it against the destructive effects of moisture.

To obtain with various paints decorative effects, and, at the same time, provide a high degree of damp-proofing, is a process quite distinct from that of water-proofing cement and concrete superstructures. The use, in small percentage, of stearic acid solutions, aluminum stearate, marine animal soaps, and other lime-reacting materials, as a component of concrete while it is being mixed, has been in practice for some time, the resulting mixture being used largely upon base-work subjected to water under high pressure. Although some of the materials used for such purposes actually do give to the concrete a high power of water resistance, the degree of waterproofing to be obtained through the use of many such compounds varies to a wide extent, often interfering with the lime-silica reactions, and ultimately affecting the strength of the finished concrete.

=Decorative and Preservative Coatings.= The necessity of obtaining suitable paint coatings for cement and concrete surfaces suggested to the writer a series of tests on paints designed to prevent the destructive action of the lime which, by seepage and other physical action, is brought to the surface, causing saponification of some oil coatings, as well as destruction of color. The tests referred to were carried out during 1908, and although great advances have been made since that time in the preparation of concrete paints, the tests have, nevertheless, afforded information of a valuable nature as indicating the proper methods to follow in the painting of cement, as well as suitable materials to use in the manufacture of cement paints. The tests, moreover, show the comparative durability of a number of paints typical of those prominent in the market at the time the tests were started.

=Acid Reacting Compounds.= A series of acid reacting washes were included in the tests, having been designed as prime coaters for use previous to the application of oil paints. The application of many of these washes has the effect of neutralizing the lime within cement and concrete surfaces, and often precipitate insoluble lime compounds which aid in filling up the outer voids, thus presenting a surface more suitable to receive oil coatings. To the writer who has since made a careful study of the painting of concrete, it would seem advisable for painters to avoid, when possible, the use of these lime neutralizing washes, as some of them have more or less disintegrating and weakening influences upon concrete. Recent laboratory experiments, however, have indicated that zinc sulphate, an acid reacting material used for many years as a wash for concrete surfaces by Macnichol, actually has a strengthening effect upon cement and concrete surfaces. The more successful coatings of to-day, however, are those which may be placed directly upon the cement and concrete surfaces without the aid of such washes. Several fairly successful paints of this type have recently appeared in the market; some of them being made of acid rosins compounded with vegetable oils. Probably one of the first mixtures of this sort was the so-called suction varnish which the master painter has for years used as a prime coating on plastered walls previous to painting. These suction varnishes generally contain a high percentage of rosin, a material having an exceptionally high acid value and thus lending itself successfully to the neutralization of free lime. It has been claimed, however, by certain practical painters that the lime-rosin compounds formed when such paints are applied to the exterior of buildings, are of a brittle nature and subject to early failure. If this is true, it would seem advisable to use in a concrete paint an oil of a relatively unsaponifiable nature, which would withstand successfully the action of the lime, and, at the same time, prevent disruption of the coating and failure of the color used in the paint.

=Outline of Tests.= The tests referred to as carried out by the writer were made on a brick wall forty feet long, surface-coated with a four-inch coating of Portland cement mortar made of one part of Portland cement and three parts of sharp, clean sand. After the cement had hardened for three days, the solutions under test were applied.

In many of the tests outlined above, one-coat, as well as two-coat work, was used on different sections of the test surfaces. It was shown that the two-coat work gave far better results than with the one-coat work, and the writer would recommend for the painting of concrete at least two-coat work. Whenever paints containing Prussian blue or chrome green are applied to concrete surfaces, immediate whitening in the case of the blue, and yellowing in the case of the green, will take place, if any degree of action has been exerted by the lime within the concrete. For this reason, green is an especially delicate color to test and should be utilized for this purpose.

The materials used, and the results shown at an inspection made after two years' exposure, are given herewith.

=Test No. 1.= Concrete primed with a 25% solution of zinc sulphate crystals dissolved in water. A wide brush was used for the application, and the spreading rate was approximately 200 square feet per gallon. Second and third coated on the second day with No. 119 blue paint of the following composition:

NO. 119 BLUE PAINT

Sublimed white lead 50% Zinc oxide 35% Silica and barytes 12% Prussian blue 3%

Ground in linseed oil, turpentine and drier.

This panel, after three years' exposure, is in good condition. Slight checking observed.

=Test No. 2.= Concrete primed with a 20% solution of (alum) (aluminum sulphate). Second and third coated with No. 119 blue.

In similar condition to Test No. 1.

=Test No. 3.= Concrete primed with zinc sulphate followed by two coats of para red.

PARA RED FORMULA

Blanc fixe 60% Whiting 25% Zinc oxide 3% Paranitraniline lake 12%

Ground in linseed oil, turpentine and drier.

Panel in fair condition with exception of slight crazing. Characteristic dullness of color after exposure shown. Bright red color restored upon washing.

=Test No. 4.= Concrete primed with an 8% solution of stearic acid and rosin dissolved in benzine. Second and third coated with No. 119 blue.

This panel is not in as good condition as Tests Nos. 1 and 2, and would indicate the inferiority of the priming liquid used. Color failing in spots and checking observed.

=Test No. 5.= Concrete primed with mixture used in Test No. 4, and then given two coats of para red.

Test is in about the same condition as No. 4.

=Test No. 6.= Concrete primed with a 10% mixture of acid calcium phosphate, followed with two coats of No. 119 blue.

The acid phosphate solution evidently had a neutralizing effect upon the lime in the concrete, as the paint is in fair condition.

=Test No. 7.= Concrete primed with one coat of a soap emulsion of the following composition, then painted with two coats of No. 119 blue.

Water 85% Linseed oil 12% Alkali 3%

Very poor results obtained. Destruction of color and peeling resulted.

=Test No. 8.= Concrete primed with one coat of white paint of the following composition:

PRIMER

Zinc oxide 25% Silica 35% Corroded white lead 20% Gypsum 15% Whiting, etc. 5%

Ground in a vehicle of linseed oil and containing 35% of volatile hydrocarbon spirits and drier.

This coat was followed by one of the following composition, tinted blue:

Zinc oxide 60% Gypsum 20% Silica 20%

Ground in linseed oil with 12% of turpentine and drier.

Fair results shown during first year, but a breakdown occurred during the second year, and cracking and scaling resulted.

=Test No. 9.= This test was a duplicate of No. 8 with the addition of 5% of zinc sulphate solution emulsified into the primer.

Slightly superior to Test No. 8.

=Test No. 10.= Primed with a white paste paint thinned with turpentine. Second coated with same paint tinted blue.

FORMULA OF PASTE

Zinc oxide 40% Whiting 30% Silica 20% Alumina and gypsum 10%

Ground in 16% of linseed oil vehicle.

Scaling and peeling due to lack of binder and use of saponifiable oil resulted during the first six months' exposure. Entire destruction of coating at end of two years.

=Test No. 11.= Primed with a white mixture, and second coated with the same mixture tinted blue.

FORMULA OF MIXTURE

Whiting 30% Silica 30% Zinc oxide 40%

Stirred into a 5% solution of glue in water, until a fairly thick paste was obtained.

Much chalking was shown, and a bleaching of color. It is evident that this mixture would not serve to keep moisture out.

=Test No. 12 A.= Primed with a 5% solution of soluble nitrated cotton and paraffin dissolved in equal parts of amyl acetate and benzine. Second coated with No. 119 blue.

Not very good results were obtained, chalking and slight scaling resulting.

=Test No. 12 B.= Primed with a heavy varnish containing Chinese wood oil and kauri gum. Second coated with No. 119 blue.

Fair results obtained.

=Tests Nos. 13, 14, 15, and 16.= Primed with a solution made by dissolving 10 parts of sodium oxalate in 100 parts of water. Second and third coated with linseed oil paints in red, brown, blue, and green.

Very good results shown at end of test.

=Test No. 20, Special.= Primed and second coated with a green paint containing zinc oxide and barytes, ground in an oil having a low saponification value. Very slow drying was shown. Excellent results. No failure of color. Extremely glossy, waterproof surface presented.