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Cob: stucco question?Mike Wye mike at mikewye.co.uk
Fri Aug 10 12:50:49 PDT 2001
Charmaine wrote, > Mike, as a matter of course would you recommend hydraulic hydrated lime > for all base stone work (foundations, etc?) or is is priudent for owner > builders of stone bases mix their own clay-ash pozzolans into reg lime? Yes I would recommend an hydraulic lime mortar mix, by either route that I outlined, if the stone foundations/plinth are going to be permanently damp. How much pozzolan you add depends on how reactive the burnt clay portion is and how much compressive strength you need in the bedding lime putty mortar. The questions as to how a structure will resist earthquakes are academic if there isn't sufficient compressive strength in the foundations/footings/plinth to actually take the weight of cob above. Lime putty mortars have relatively low compressive and tensile strengths to start with. This is an advantage in many uses but not where the wall is loadbearing and the pure lime putty mortar's compressive strength would be insufficient. When adding a pozzolan its a question of how reactive is the pozzolan and how much you choose to add. At one end of the scale you can add a little pozzolan and barely change the characteristics of the lime putty mortar or you can add so much pozzolan that it reacts with every bit of available lime and makes a mortar as hard as any concrete. Where you want to be on that scale depends also on the location of your structure, the weight and thickness of the walls, climate its exposed to, position of water table etc etc. Historically there were wide differences in the lime/earth bedding mortar mixes used and their characteristics. We're aware of cases in England where people have used pure lime putty mortars for building and rendering externally without being aware of the history and existence of pozzolans and have "rediscovered" belatedly why they form an important part of lime building practice when they find that their pure lime mortar is still soft and uncarbonated months after application and then fails in the first frosts of winter. regards, Mike Mike Wye & Associates, Traditional & Ecological Building Products www.mikewye.co.uk 01409-281644 > Mike said:When working with a lime mortar in foundations and stone > plinths > they traditionally used an hydraulic lime mortar either by > 1. adding a pozzolan to a lime putty/sand mix to set off some part > of the mix prior to carbonation. > ( a pozzolan is a burnt clay such as volcanic ash, brick dust, > crushed > clay tiles etc which reacts with the lime to form cementitious > compounds) > 2. using a natural hydraulic lime mixed with sand. > Permanently damp stone structures don't allow sufficient air and > hence > carbon dioxide into the mortar matrix to carbonate the lime to the > extent > needed. >
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