[Strawbale] Hydraulic vs Hydrated Lime plaster

[Derek Roff]

> I would like to make some comments on Derek's statements.
> Hydraulic lime does not set in a short time. It is a product that can
> be  reworked up to 24 hours.

I am glad that Michel has come forward to offer his experience and 
expertise.  I will repeat that I am not an expert in lime.  I would 
like to learn more about the reworking that Michel refers to.  St. 
Astier, one of the biggest suppliers of natural hydraulic limes, 
mentions reworking times for various products ranging from 8 to 24 
hours.  8 hours means that mixed plaster could not be applied, if 
stored overnight.

In a workshop situation, I try not to mix any plaster after about 4 PM. 
Any plaster not used will sit until the workshop gets going again, 
usually between 9 and 10 the next day.  So for me to use yesterday's 
plaster in such a situation, I need at least 18 hours of rework time. 
This puts me into the last third of St. Astiers stated reworkability 
range.  Obviously, professionals might work later and start earlier. 
Unlike workshop participants and owner-builders, pros would be fully 
cognizant of their tools, materials, and skills.  Please, Michel, tell 
us more about what you have found with reworking different plasters 
after different periods of time.  How do temperature and humidity 
affect the reworking time, in your experience?

St. Astier mentions the tendency of reworked plasters to be weaker, due 
to higher water content introduced in the remixing.  This can be 
counteracted, they say, by greater physical compression of the plaster, 
increasing its density, during the trowelling.   Michel, can you 
comment on your experience with these techniques?

I agree with Michel that cold joints (where new plaster does not adhere 
well to plaster applied previously) can be a problem with any plaster, 
including earthen plasters.  I did not mean to imply otherwise.  I 
remain convince that being able to store mixed plaster for days, weeks, 
or months is an advantage for most owner-builders.

I think good quality natural hydraulic limes are wonderful products.  I 
think good quality hydrated limes are wonderful products.  I look 
forward to hearing more from Michel.

Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu