Well certainly it’s the biggest variable that differs between the two walls. However the sand colors were very similar I must say. The other thought I had was maybe the finer-grain on the sand used for the whiter left wall’s coat allowed me to work more of the “fatty” component to the surface when I was applying it, which was inherently more lime-rich and therefore turned whiter on the surface.
- Thread starter aockenden
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Cubist
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Ah but.... which is Cause and which is Effect I wonder?
Stevers
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Probably all about the aggregate, but if it was caught by rain as it set, that may have caused lime or salts to come to the surface.
That happened here after I carefully filled the gaps round my new garage door frame with an NHL mortar mix a bit late in the season. After rubbing, bashing and brushing it looked good, but then a bit of rain caught it overnight and by the morning it was covered in tiny white dots that are only now starting to fade.
That happened here after I carefully filled the gaps round my new garage door frame with an NHL mortar mix a bit late in the season. After rubbing, bashing and brushing it looked good, but then a bit of rain caught it overnight and by the morning it was covered in tiny white dots that are only now starting to fade.
I covered it from rain with a tarp for its first 24 hours (as it was raining hard). But it’s also on a north wall with a gable-ladder overhang. So it hardly gets hit by any rain as typically wind blows from south / southwest.
Different aggregates are going to have a different amount of voids between the grains. Is the pale wall an example of when the amount of lime significantly exceeds the volume of voids in the aggregate?
The mix ratio section on this site describes it - Conserv
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Mix Ratios
The lime mortar mix ratio is not a predefined constant that is drawn from experience and industry wisdom. The popular recommendations of 2:1 or 3:1 ratios for bricklaying or repointing may not always work well, it all depends on the sand you're using and varies from quarry to quarry.
A mortar mix ratio is directly related to the percentage of voids in the sand, this is established by void testing. The void percentage determines the amount of lime binder that can be safely introduced, excessive binder (over-binding) can produce a heave effect and cause cracking, insufficient binder (under-binding) will impair the bond strength. Finally, with the optimum proportion of lime binder calculated, these percentages are converted to a mix ratio.
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Having said that I did a rough coat of plaster internally a couple of months ago from exactly the same mix and the bits on a thick internal wall carbonated rapidly compared to the bits on a cold single skin brick wall, which remains dark and is only now slowly becoming pale.
The mix ratio section on this site describes it - Conserv
"
Mix Ratios
The lime mortar mix ratio is not a predefined constant that is drawn from experience and industry wisdom. The popular recommendations of 2:1 or 3:1 ratios for bricklaying or repointing may not always work well, it all depends on the sand you're using and varies from quarry to quarry.
A mortar mix ratio is directly related to the percentage of voids in the sand, this is established by void testing. The void percentage determines the amount of lime binder that can be safely introduced, excessive binder (over-binding) can produce a heave effect and cause cracking, insufficient binder (under-binding) will impair the bond strength. Finally, with the optimum proportion of lime binder calculated, these percentages are converted to a mix ratio.
"
Having said that I did a rough coat of plaster internally a couple of months ago from exactly the same mix and the bits on a thick internal wall carbonated rapidly compared to the bits on a cold single skin brick wall, which remains dark and is only now slowly becoming pale.