Originally Posted by ;
Curing has nothing to do with this
The with powder is salt coming out of the block. It also comes out of brick and once in a while flagstone. Its not mold. The block is effervescing. This is mainly caused because the block is remaining wet to long and the salts are weeping out of the block and drying as it hits the air.( powder) The contractor may not have Sealed the back of the block wall against the bank properly. This is usually the main reason but not always. If the block stays wet long enough it will do this also brick façades above grade will show the white powder.
As a mason I can't tell you how many retaining walls and foundations I built. Not one effervesced and I'm in a wet area.
Effervescent is the alkaline components that make up masonry materials. They are being leached out of the masonry and collecting at the surface. Often light deposits can be removed with water and a scrub brush but heavy deposits will need more work.
For interior areas use TSP, trisodium phosphate, and a lot of scrubbing. Plain white vinegar and water, mixed 50-50, can also neutralize these deposits. The vinegar can damage delicate materials and change the color of brick, be careful.
For exteriors a pressure washer is the best tool. This alone can remove most deposits.
Commercial masonry cleaners also do a good job on heavy deposits. These are available at your local paint store or home improvement center. A strong TSP solution can also be injected into the water stream if a commercial pressure washer is used. It is most likely that a little scrubbing will be needed.
Both interior and exterior masonry can be treated with the same chemicals. Make sure you rinse the affected areas well. Don’t worry if a little effervescent still remains, this will be neutralized and sealed with a good masonry primer like Zinsser 123 or a sealer like Seal Krete Heavy Duty Sealer.
The contractor is blowing smoke when he says its "curing" I Can show him, 30 year old block "curing" and has white powder He more than likely knows the wall not being sealed properly is the cause. If it was sealed right there would not be powder on 50% of the wall. I'm been doing masonry work over 30 years and never saw 50% of a new block wall doing that.
Part of what I do for a living is replacing failed retaining walls. Maybe I shouldn't say this but cement block can ROT like wood in the cores. I have seen a 30 year old walls do this that had water puddling in the cores. And yes it effervesced all the time. I guess what I'm saying if that wall was parged/ pitched and stone with perforated pipe out the end to drain this probably would never happen. At best it would be a couple of blocks if at all.