More than 50 years ago, contractors realized using factory pre-blended concrete rather than field mixing concrete on job sites was more profitable and delivered better results for pouring slabs. About two decades ago, contractors came to the same conclusion about using factory pre-blended mortar rather than field mixing mortar for laying brick or block. While the direct cost for factory pre-blended materials is slightly higher than field mixing, the mixes provide greater consistency and efficiency on job sites. In addition, using factory pre-blended products reduces labor costs and minimizes material waste. These benefits are magnified when 3,000-pound bulk bags of factory pre-blended material are mixed using a silo system. Today, contractors can enjoy these same performance and cost-saving with factory pre-blended stucco for wall and ceiling jobs.
Field mixing stucco requires a concentrate powder that contains Portland cement, lime and other additives, but sand is the backbone to any good stucco mix, and that starts by selecting a sand that meets ASTM C 897 standards. When field mixing, shoveling the exact same amount of sand into a mixer throughout the day is critical. You also have to account for sand bulking, which is added moisture that can affect the volume and weight of a sand measurement. Most sand piles also contain about 5 percent moisture, although that can vary due to environmental conditions like excessive rains or extended dry periods. Not to mention that sand piles can contain impurities like salt or iron that can contribute to efflorescence or streaking. Finally, you have to consider the cost of wasted sand and/or the expense of disposing of leftover sand when field mixing.