Reconstitution of historic lime mortar and renders
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.
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.
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
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.
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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