Reconstruction of Historic Lime Mortar and Renders FAQ Friday

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Q: I want to restore my old stone house and have heard there are ways to reuse the mortar already in my building during the repointing process. Can you explain how to do that and what are the positive and negative impacts?

A: Reusing old historic mortar is possible in more ways than one. The first methodology would be to leave sound mortar alone and keep using it for as long as possible, only doing “patch” pointing in areas that need to be fixed. In cases where a building owner desires to repoint the whole building you can follow the backyard guidelines below.

Of course if one were to follow these guidelines then there must be a conservation mindset in the back of their head. Knowing this, a mortar analysis would be highly recommended to help conclude an appropriate mortar mix design.

Reconstitution of historic lime mortar and renders

The purpose of this procedure is to separate the lime from the historic aggregates to reproduce a mortar joint or render while retaining as much of a buildings historic fabric as possible without the use of acids.

Beginning with the removal of any loose, cracked or friable render or mortar in the gentlest means possible, put debris aside.

Using a concrete barrel mixer place all saved debris in the mixer with large heavy stones or steel balls, about the size of a bowling ball. Make sure that the chosen crushing medium is harder than the debris to minimize contamination of the crushed aggregate by foreign matter. The mixer should be filled only to the point where the crushing medium will effectively fall from the apex of the mixing rotation and crush the debris to what would be the consistency of a pre-blended dry bagged mortar. This process should take anywhere from 20 – 60 minutes depending the quantity of the material loaded in the mixer (less is faster) and the desired fineness of the aggregates. Be sure not to over mix. Over mixing will result in the deformation of the aggregate.

Once the debris has been crushed to the desired particle size, place in 5-gallon pails or wheelbarrow for the separation process.

Place the end of a hose running water beneath the crushed aggregate. While the container is filling with water be sure to slowly agitate the wet aggregate mix with a hoe or trowel. Be sure to allow the fine lime particles to spill over the sides of container to be discarded. Make sure that the larger aggregate stays settled to bottom of the container for reclamation. This process should take approximately 20-60 minutes depending on amount of crushed debris and its binder to aggregate ratio. The process is completed when the water runs fairly clear and the remaining fines in the mixture can be easily made into a ball that resembles saturated beach sand.

Upon completion of filtering the lime from the aggregate allow to dry in the open air and strong sun by spreading it out on as thinly as possible over a tarp.

Once the aggregates are dry, the large particles of broken stone, any other foreign matter and large unbroken debris can be dry screened out using the desired sieve size. Typically anything retained on a #4 sieve can be discarded.

Now that the aggregate is dried and sieved, new mortar formulations can commence. Depending on the amount of work to be done with the reclaimed aggregate and the amount that was actually reclaimed, determination of how much other similar aggregate is to be gauged in to the new repointing or rendering mix.

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