Q&A / 

Mortar Mix for Flagstone and Paving Brick

DEAR TIM: 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? Rick A., Blacksburg, VA

DEAR RICK: Here is a subject near and dear to my own heart. I can't begin to tell you how many paving brick and pieces of flagstone I have set in cement mortar over the years, but it has been more than I care to remember. The great news is that each and every one is still solidly set and shows no sign of delaminating from the concrete base below. 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 bags of lime and cement look almost identical but believe me, they are as different as cats and dogs.

The bags of lime and cement look almost identical but believe me, they are as different as cats and dogs.

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 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 the 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 will improve. 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 to 15% 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 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: 3 five 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-50sF with overcast skies. A nighttime low temperature of 40F 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 28F for 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 it outdoors to mix it. 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.

As the air temperature drops so does the temperature of materials stored outdoors and the actual concrete to which the flagstone and brick will be attached. These cold temps radically affect the chemical reaction of hydration that starts the moment water is added to the powdered cement and hydrated lime. Hydration is the reaction that turns the powdered cement and lime into actual microscopic cement paste crystals that interlock the sand particles together.

To resist the expansion forces of freezing temperatures, a certain amount of crystals need to form. The way to accomplish this is to fool the mortar into thinking it is 55F or higher. You do just that by preheating all of the ingredients and by storing all of that heat as you do each night when you sleep under blankets or a comforter.


5 Responses to Mortar Mix for Flagstone and Paving Brick

  1. I don't have a concrete base for my flagstone, I do have 8 inches of rock base...Can I use the same mixture above?

  2. Also wondering about my base.. We were planning on doing a sand base. Would the above plan still work? Or do I need to do something different?

  3. Dear Tim,
    I'm considering using a dark grout om my flagstone terrace. The stone is lovingly collected stone that has been taken up from old installations-from the 1930's to1960's. It's weathered and doesn't look like someone that just got a new hair cut. What are your thoughts on most attractive grouting. 1000 square plus job.

  4. I've recently had my cement porch and step covered with flagstone. It looks really good except that there are places where I think the cement mortar is too wide as well as too white looking. Does it darken over time and how long does it take AND is there anything I can put on it to make it appear darker, even more buff colored? Thanks.

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