The Importance of Clean Sand
Sand generally composes 80 percent of a stucco wall, and because it is the primary ingredient, it is vital that the quality of the sand be controlled. Imagine a bucket filled with tennis balls and different sized marbles. The marbles are filling in the spaces between the tennis balls. Cement paste, a desirable fine, is filling in the remaining spaces, while at the same time bonding to the tennis balls and the marbles. In a similar fashion, cement paste will hold together various sized aggregates of sand better than it will uniformly fine sands. The objective is to be rid of the spaces, or voids, and to create a denser, stronger wall. Cement paste is preferable to fine sand when it comes to filling in these voids because cement is the glue that holds the aggregate, or the sand, together.
Sand used in stucco should not have an excessive amount of fines or silt”. Fines or silt are objectionable and are undesirable, as they prevent the cement paste from binding together the particles of sand, thus reducing the strength of the cement mortar and making it more porous. In cement plaster, the weakest point in the compound is where the cement paste attaches to the aggregate; fine particles of the sand act to coat the larger particles of aggregate, thereby preventing the cement paste from sticking to the aggregate.
Fine sands may appear to contribute to high compressive strength, but this is deceptive because, at the same time, they are contributing to weakening the flexural strength. Sand with excessive fines is also more likely to cause plaster to develop shrinkage cracks because the finer particles require more water to be added to achieve a workable mix. The reason for this is that the finer particles have a greater amount of surface area than the larger particles. The greater the surface area, the more water that has to be added to the mix. However, when there is too much water in a mix, the excess liquid will evaporate as the stucco base coat dries, thus leaving voids behind. And some fines, such as fine clay, will actually soak up water, thus causing them to expand when wet and shrink when dry, adding to the porosity of the cement plaster.
In order to achieve a first-rate, dense stucco, certain guidelines need to be followed. Using clean, washed plaster sand that meets ASTM specification C-897 will contribute to a denser stucco base coat due simply to the fact that sand with fewer fines has less water in the mix. When there is less water to evaporate out of the mix, fewer voids will form. It has been demonstrated that floating the scratch and brown coats will also contribute to a denser base coat wall. The moist cure is also needed to achieve a hard, rigid cement.
Water will penetrate stucco through cracks, voids and air spaces in the stucco walls. The only way to avoid water penetration is to reduce the number of cracks. There are two, and only two reasons that stucco cracks. First of all, there are cracks caused by normal shrinkage of the mortar as the water evaporates from the wall.
Secondly, there are physical forces applying pressure to the stucco itself. These pressures may come from building movement resulting from plywood shear warping, green or wet lumber shrinkage, excessive loads on outside walls, or walls that are not tied in. The plasterer can assemble the best stucco wall system possible, but if the building structure is of poor quality, or the building is sitting on ground that is moving or sinking, the stucco wall will not perform to its fullest capability.A few plasterers like to use sands with low SEs, often as low as 33. They can mix sand-to-cement ratios as high as 9: 1, and are able to get the mortar to flow through the pump. This allows them to trowel, spread and float it more easily. We all know that sand is less expensive than cement, and that using high ratios of sand offers a quick way to reduce costs. While this provides a short-term method of increasing profits, over the long term the use of dirty sand with low SEs is bound to create problems.
Poor quality sand in the mix can be overcome by floating to densify the mix and moist curing to harden and fill in the voids in the stucco base coat. Though one might ask, “Is it worth the extra labor?” or “Can I really rely on the plasterers to float the brown coat?” It makes straightforward sense to use clean sand in correct ratios in the first place. Ultimately, plastering contractors will save themselves work and headaches, while building a fine reputation in the business.