Lime Putty Mortar – Buckshot

Lime Mortar Buckshot - the Wrong Way to Mix

Let me save you some time and trouble if you are considering specifying lime putty (ASTM C1489-01) for your next historic masonry restoration project. Forget about the standard way of mixing mortar with a gas-powered paddle mixer or drum type machine used in new masonry construction. These machines require the mortar ingredients to have a high rate of flow by adding enough water into the mixer to keep everything moving and mixing thoroughly. Not so with lime putty. This material is generally 50 percent water and 50 percent solid (looks like thick cream cheese) and requires a mixer that provides pressure or a kneading action to evenly incorporate the sand particles into the material.

Mixing lime putty and sand together works well when mixed by hand with a mortar hoe and shovel as you can place pressure into the mix by pressing down during the process. Ramming rods made from wood with handles also work well to beat the mortar into submission forcing the sand particles into the lime putty.

What is interesting about mixing lime putty mortar, if you have never had the pleasure to do so, is that it requires no additional water once properly mixed. There is enough water in the lime putty to create a good workable mixture that can be used for repointing. For years we have used a vertical shaft mixer that whips the material into form from the outside-in once all the ingredients are in the shaft mixer. So whatever you decide to do on that next historic masonry restoration project, if it involves lime putty, be ready for some good-old-fashion hand mixing or get ready for some buckshot of lime putty balls coated with sand!

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  1. #1 by Kevin Lee on September 15, 2011 - 10:40 pm

    I push the putty through a 1/4 inch sieve on to the sand. Then work it up by hoe.

  2. #2 by Wayne Robey on September 25, 2011 - 3:48 pm

    This is a how to use rather than how to make question:
    After reading about using lime sand mortar I mixed some by hoe in a plastic tub. I think I mixed it well, using lime putty made from type S masonry lime aged for 40 days and masonry sand. I think I used a bit more lime than minimum then used some for pointing, more for laying a few soft bricks and a soft brick to hard brick interface. The bricks were thoroughly damp and the finished work wraped with 2-3 layers course weave burlap which was kept damp for 6-7 days. After removing the burlap, the bricks are damp and the mortar is so soft that a fingernail is easily pressed into it. No evidence of carbonation. Since the pointing was done a day before laying additional brick, some mortar was left exposed to the air in a thin layer in the plastic mixing box. After one day, this dried mortar was too hard to remove with a stiff brush and water so carbonation did occur. What is the correct way to cure the brick? Should I let what I have dry with no effort to keep it damp? Should I restart with slightly hydraulic lime? I have a test sample of 2 bricks made at the same time as the above to experiment with.

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