huangyhg

cmu block construction
i have two questions: 1) i have read somewhere either aci or a spec book that a cmu wall should not be grouted on the same day as the block was placed. does anyone know if there is a spec for this? 2) what would turn cmu block or brick veneer the color black (new construction)?
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mildew?
mold - mildew ? you are probably correct. if they didn't construct their weep holes properly and moisture got trapped within the cavities. i just thought maybe there was some known impurity that can contaminate the water or cement during the block casting or mortar or grout mixing processes.
you can grout the same or next day if you follow proper grouting procedures. just make sure the wall is properly braced. that is sort of an old wives from back when people just dumped the large cavity of a pilaster as fast as they could. some people advocate grouting soon if it is possible, but many times the schedules and logistics do not permit it.
make sure your grout is approximately the same strength as the block. an other, more practical terms, avoid rich grouts. the ideal grout will have the same cured properties as the masonry units.
how soon after construction did the black color appear? did you spray a sealer(?) on the block wall before veneering? some could be water soluable. black is an unusual colore and suggests either organics or magnesium (very rare). unlikely it is from the units since they are essential cured (most chemical reactions completed) before delivery to the site. on rare cases, i have seen block "sun burn" (orange stain) due to uv and strange mortar mixing water reactions.
i actually had a chance to go see what's up with the block. it is not mildew/mold. however, the mortar joint itself, along the roof line, in conctact with wood blocking, is actually turning colors (gray/ black). the block wall has been up for a few months. i am still stumped though. i am not a masonry expert so i don't know what impurities can be doing this. any ideas?
qctech -
could it be coming from the pressure treated lumber? also it is a possiblity that the nails were not compatitble with the p.t. wood. the p.t. wood is very hard on certain metals (nails, aluminum flashing, etc.). both of these would require moisture to carry a stain, but p.t. lumber is usually very wet and can take a long time to dry out.
the mortar may have been relatively fresh and could be receptive to any staining moisture. - just a thought.
dick
pt wood? possibly. again i am not an expert at this, so, how about a petrographic exam.

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