Masonry Magazine January 2007 Page. 43
Masonry admixtures have many advantageous properties, from activating or retarding the mortar's setting time, to repelling water and controlling efflorescence; increasing weather resistance, to protecting mortar color. But they have to be used correctly to work effectively.
Masonry experts emphasize the importance of following recommended guidelines when using admixtures. In particular, they cite ASTM C 1384, the Standard Specification for Admixtures for Masonry Mortars, which provides criteria for evaluating admixtures for use in masonry mortars.
Manufacturers' specifications also are important, said Jeff Leonard, vice president of bulk materials for The QUIKRETE Companies in Atlanta. "Masons have to follow all manufacturers' directions when using admixtures," he said. "They have to be very cognizant of those directions."
Neal Jablonski, technical service specialist for masonry products for Cambridge, Mass.-based Grace Construction Company, agreed, saying mason contractors should also check with manufacturers' suppliers for their recommendations. "Admixture suppliers ought to be consulted for proper mixing procedures," he said.
Using Admixtures in Extreme Weather
ADMIXTURES OFTEN are added to mortar in hot or cold temperatures to keep the mortar workable and to adjust the setting time. In cold weather, accelerators are used to speed up the rate of the mortar's development. Accelerators don't eliminate the need to protect the mortar from freezing, but they may reduce the amount of time needed to protect the mortar from low temperatures.
When using an accelerator, crews need to follow the cold weather practice of using hot water in the mixture, Leonard said. The hot water activates the accelerator; without it, the admixture may not work correctly.
Jablonski said that, in addition to using hot water with accelerators, mason contractors sometimes heat the sand and cement for the mixture.
To the opposite end of the spectrum, retarders slow the rate of hydration in warm weather, keeping the mortar workable for a longer period of time. Leonard said he's seen masons using ice with the mixture in hot weather to aid the retarder.
He advised mixing smaller batches of mortar when the temperature climbs to keep it from setting too soon. "Mortar will tend to 'flash set' in hot weather," Leonard said. "You want to mix small batches. It's easier to control a smaller amount."
A severe change in temperature at the job site can impact the admixture, Leonard explained. Crews need to be aware of the amount of admixture they're using and the specifications for that day's temperature.
"If they're adding an accelerant, and consistently adding it for a few days, and then they get a warm day, it may change the characteristics of the accelerant," he said. "They have to be cognizant of that."
Temperatures also are a factor in storing admixtures. Their degradation is increased when exposed to heat and direct sunlight, while temperatures below 32 degrees Fahrenheit may cause the admixture chemicals to crystallize.
"Some admixtures added to the mortar in the field are liquid," Leonard said. "When [the temperature falls] below freezing, you have to take extra care so it doesn't freeze. You can't just leave it out at the job site to use the next day."