Using Mortar


Hi,
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.
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Greg writes:

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.
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Unless its setting which it isnt leave him alone he knows more than you.
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"The only reason"? Hardly. Depends on what it spills onto.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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Doug Miller writes:

One would think that goes without saying.
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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 'workable'. 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
Joe
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Thanks all for your replies.
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