Masonry Magazine January 2006 Page. 28
LIME MORTAR
The most commonly used pozzolans are: brick dust made by crushing and sieving soft, under-burnt bricks; volcanic ash, called trass; or calcined china clay, sold under the name of Polestar or Metastar. These are specialized restoration products that rarely get used on modern projects. (The name 'pozzolan' came from the Romans' use of volcanic dust from the region of Pozzuoli in Southern Italy for the purpose of introducing hydraulic qualities to an otherwise non-hydraulic lime-sand mortar mix.)
Another option for a faster setting mortar is to use hydraulic lime, which is made by burning limestone and the impurities found within it, such as clay. Hydraulic lime undergoes partial hardening by reaction with water much like a "gauged" mortar, which is a lime-based mortar that contains Portland cement to make it artificially hydraulic. This setting property of hydraulic lime derives from the presence of the impurities that, upon slaking, react with the lime in the presence of water, forming solid crystals. Proportions of these impurities occurring in the limestone and combined during the burning affect the strength of the lime and the speed with which it will harden. A low proportion of impurities will produce a feebly hydraulic lime, while a higher proportion will yield a moderately or eminently hydraulic lime.
When using a straight lime and sand mix that contains pozzolans, or when using a hydraulic lime mix, the carbonation process can still take place. However, when working with gauged mortars, the lime provides plasticity, but the Portland cement reportedly prevents the carbonation from occurring. The Portland cement provides the strength, basically at that point.
It is also important not to apply a lime-sand mix, without the addition of any Portland cement, within a month of freezing point temperatures.
Finally, abundantly spray the joint prior to applying the mortar. It is also very important to cover the wall with a tarp or wet burlap for several days, and to keep the wall wet. I usually keep the tarp on the wall for five days and try to spray the wall three to seven times a day during this period.
The caution with using a pure lime and sand mix is that one must really follow set, known guidelines. Time will do the rest.
Mario Cantin, president of Invisible Tuckpointing Ltd., located in Ontario, Canada and Lewiston, N.Y., offers brick mortar replacement with a perfect color match and teaches others how to earn a six-figure income doing the same. You can contact him at (416) 948-2591.
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26 Masonry
January 2006
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