Wednesday, November 5, 2003

Get the right mix for flagstone, brick

By TIM CARTER
By Tribune Media Services

Q. I am installing flagstone and some paving brick on top of a concrete patio. A book I read suggests mixing sand, cement and hydrated lime in different proportions to make the mortar mix. What is hydrated lime, and is it necessary to add it? Lastly, as cold weather is around the corner, when should I stop working so as to prevent freeze damage to fresh mortar? — R.A., Blacksburg, Va.

A. Using basic and readily available building materials, as you describe above, you can create a decorative patio surface that will last for many years.

The hydrated lime you read about is a dry, fluffy powder made by adding a very specific amount of water to regular lime and mixing the two ingredients in large mechanical mixing machines. The small amount of water activates the calcium oxide in the lime but does not turn it into a paste or putty. Mortars that contained traditional lime were often subject to popping, pitting or disintegration if they were not carefully proportioned and mixed. Hydrated lime takes the guesswork out of mixing mortar and as such is very user-friendly. You can purchase hydrated lime at full-service building supply businesses that sell cement, mortar, plaster and other basic building materials.

Hydrated lime is not a necessary mortar ingredient. You can make mortar with just Portland cement, sand and water. The mortar that I used to permanently attach my brick and stone to my patios did not contain any hydrated lime. But adding hydrated lime to the mortar mix can be beneficial. The plasticity or workability of the mix is better. When hydrated lime is added to the mix, the sand and the cement do not separate. The final mortar is also more waterproof. Shrinkage cracking can often be eliminated or minimized when hydrated lime is added to the mixture.

Replacing 10 percent to 15 percent of the total volume of cement with hydrated lime usually produces optimum results. Keep in mind that the Portland cement is the glue that holds the flagstone and brick to the concrete patio. Years ago, many recipes I looked at suggested a ratio of three parts sand to one part cement for cement mortars. I altered that recipe and always mixed my ingredients 3 parts sand to 1.5 parts cement. After surviving 25 years of brutal winter freezing temperatures, my patios look like they did the day they were finished. I am convinced that my recipe is one that will work for you as well.

If you decide to use the hydrated lime, you can use this recipe for the mortar: Three 5-gallon buckets of dry sand, 6.38 gallons of Portland cement and 1.12 gallons of hydrated lime. Blend these ingredients together well before adding any water to the mix. Add water slowly until you get a mixture that resembles regular bricklayers mortar. Only mix as much mortar as you can use in one or two hours. Do not add water to the mortar if it starts to get stiff.

Cool weather is your friend, but cold weather is indeed your enemy as you attempt to finish this job. If the mortar had its choice of weather conditions, it would undoubtedly tell us to install it on days when the air temperature was in the mid-50s F with overcast skies. A nighttime low temperature of 40 F would be ideal. Believe it or not, sunny, hot, dry or breezy weather are not great conditions for working with mortar and concrete.

I would not install any flagstone or brick if the air temperature is forecast to drop below 28 F in the 48 hours after you install them. You can help yourself immensely by preheating the materials you will be working with. If at all possible, keep the sand, cement and hydrated lime indoors at room temperature and only bring these ingredients outdoors to mix them. Use very warm or hot water to mix the mortar. Try to heat the brick or flagstone as well, although this may be far more difficult to accomplish.

After you lay the flagstone or brick, cover them as soon as possible with waterproof insulating blankets. These can be rented at tool rental businesses. The mortar actually creates a small amount of heat as it cures and hardens, and if retained within the masonry instead of released to outer space, it helps the mortar get stronger faster. Keep the masonry covered with the blankets for at least 48 hours if possible.

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