Masonry Magazine June 1963 Page. 17
Water Repellents for Limestone
Because of Indiana Limestone's durability, don't be misled about the necessity of water repellent. It is needed for several reasons. A coating of water repellent helps maintain a clean surface and the original beauty of the stone. Physical damage to Indiana limestone from water freezing is practically non-existent. But a water repellent keeps the mortar from disintegrating.
Considerable moisture is purposely built into a masonry wall during construction. It must come out and usually comes to the outside, due to heat from the sun. As the moisture is drawn out, it brings with it salts or staining matter from the back-up material. This staining material is deposited on the surface, and although it does no damage to the stone, is referred to as temporary objectionable efflorescence.
Water repellents cannot entirely eliminate this objectionable stain. However, they do reduce the problem because the moisture must transform into water vapor in order to pass through the treated surface. Therefore, an application of water repellent during mill fabrication will reduce temporary efflorescence as well as protect the stone from other stains in shipping, handling and storage.
At present, external treatments are one of three types: Stearates, silicone resins in a solvent, and sodium methyl siliconate in an aqueous solution. It is thought that the better treatments should reduce moisture penetration below the level likely to cause the appearance of dampness.
Thus, coating performance is rated as the capacity for reducing moisture penetration.
None of the three types of water repellents is the final answer to every masonry surface. The type used must deposit the basic material correctly in the pores of the masonry. It must not stain the masonry surface and must result in water repellency.
Stearates are usually metallic soaps and are limited in water repellency and durability. They are a coating and are of little benefit to Indiana limestone.
Silicone resins, in a solvent solution, cure by reaction between the silicone and the silicate portions of the substrate. In regular masonry or concrete, this silicate substrate is available. In limestone, however, only a carbonate lattice is available. The neutral, nonionic character of solvent type water repellents can do little to promote any bonding between carbonate and silicone to produce a permanent bond. Also, discoloration can result from some solvent carriers.
Sodium methyl siliconate, in an aqueous solution, cures by reaction with carbon dioxide in the air. By its basic and ionic character, it leaches out a few of the carbonate ions and substitutes silicone ions to give a chemical bond. Because limestone is of a hydrophilic character and the basic material or substrate will not absorb water, the surface of each particle of substrate is readily wetted with an aqueous solution and even distribution and penetration of the silicone is permitted.
Research indicates that an aqueous silicone solution is better for limestone. A mill application will protect the stone temporarily but a good coverage is difficult due to the excess moisture in the stone during fabrication. A field coat will fill in the voids to give complete protection to the stone and the mortar joints. However, the question always arises regarding the effect on the mortar bond to the stone when mill application is used because mill application includes all sides of the stone. Tests show that it does affect the bond, but not to any consequence. After all, the mortar joint is usually not used structurally (load-bearing)-only to seal the joint.
Mortar tests were made with silicone as an additive in the water. However, this did not make the mortar water-repellent in all cases only where the water in the mortar was worked to the surface in horizontal joints. In vertical joints, the water did not come to the surface. Consequently, adequate amounts of silicone were not carried to the surface to cause water repellency. Minute amounts indicate an increase in the workability of the mortar, but tests were not complete on strength. Here again, test procedures are lacking. Mortar tests are written on the ingredients, not on physical bond tests. So the modulus of rupture test was used by butting two pieces of stone together in mortar then applying pressure to the specimen at the midpoint.
Methods for Cleaning Limestone
It has been said that good construction and reasonable care used in the maintenance of masonry makes it practically ageless. Indiana Limestone has proved its durability by over one hundred years of use, with little attention being given to maintenance until recently.
Indiana Limestone, like any other material, will get dirty from exposure to the elements over a period of years. Dirt, soot, chemicals, etc., are washed out of the atmosphere by rain and the accumulation causes an unsightly appearance to the exterior of any building. In metal and glass buildings, the dirt is removed at great cost to the owner. In limestone buildings, the dirt has been allowed to accumulate and is seldom if ever cleaned.
Although Indiana Limestone can be left alone without losing its natural beauty and attractiveness, a cleaning will restore a look of new construction that is often worth the effort. And, the cleaning can be done effectively, economically and without harmful effects to the stone. There are several methods.
The wet aggregate method, using a combination of water and a soft, fine aggregate will do an excellent job of removing dirt and grime from limestone surfaces. Another good cleaning method is hydro-air, water shot from a special nozzle at very high pressure (800 to 1500 PSI).
If heavy concentrations of dirt or grease are not present on the stone, steam-cleaning will easily clean the surface without hurting the stone. However, it is dangerous to try to clean heavy grease or dirt with steam-cleaning combined with acid. The acid is very injurious to limestone.
Sandblasting limestone is not recommended, because although it will clean the limestone, it roughens the surface and allows dirt and grime to accumulate just that much faster.
Small areas may be cleaned with a belt sander if the surface is flat and the finish is smooth. On curved surfaces such as carvings and moldings, and for other finishes, the limestone can be cleaned with a 'Foamglas' solution. A liberal flushing with clear water will wash away the waste.
If dust and water are objectionable in cleaning limestone, an ammonium flouride liquid cleaner is available. The cleaner is applied with a stiff.