I am helping my boss apply molded concrete textured stone on his
family room wall. We have spread the base mortar and it is dry.
Today we started applying the stone. We are using a portand cemet mix
with clean fine sand.
I don't know, but I believe that mortar is like creme brule (i.e.,
very sensitive). It demands crystal clean water and just the right
amount of mixing. It should be used fresh and spillage shoud never be
thrown back on the butter board for reuse. The boss (perhaps he would
skin a flea for its hide and tallow) is like a busy bee scoping up
spillage and mixing it back in our fresh mortar.
I know it is his mortar and his wall, but I would apprecriate some
corraboration that his miserly habits may result in unstuck stone
falling on his beautiful Italian tile floors. Thank you.
The only reason not to reuse spilled material is if it is old. Once it
starts to set, it won't be as strong in bond or compressive strength.
Use a polymer admix, like an acrylic emulsion. Well worth the extra cost
in bond strength for wall applications.
Adding old to new mortar changes the setting time, adhesion, and
working characteristics. This may not matter on many jobs, but on
critical interior work it can make a significant difference. A skilled
mason will reuse a certain amount of sluff, but if you watch
carefully, sometmes he will discard a bunch and yell at his hod
carrier for 'more mud'. He knows from experience that it is no longer
A similar situation occurs with plaster of paris for well known
reasons. The initially formed hydrated crystals catalyze the formation
of more to complete setting reaction.. Setting type drywall compounds
take advantage of this to tailor the setting times by varying the
amount of initial crystal content.
Good advice for your boss: You fight chemistry, you lose. HTH
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