how can I Stop white powdery substance coming through concrete?

You can also pour diesel over the cement floor and then paint over it. The cement will keep the paint it better shape longer.

Esmer
October 2010

Sorry big softy yours is a good idea but I would have removed the existing step and re-laid a new step with horizontal and vertical dpc's. Did one, still there, no more problem with damp, Even the local authority planner liked it, he was next door checking on an extension so the house owner asked him if he would check what I was doing.[sure he thought I was conning him]

bob
February 2007

First try using Google. That's what it's there for!

It's called efflorescence, and it's caused by salts leaching through the concrete.
As it's an ongong situation it almost certainly means that the bottom of the concrete is subject to damp.
Any efflorescence will re-occur as long as the step remains damp.

To remove efflorescence first to saturate the surface thoroughly with water. This will prevent deep penetration of any acid.
Wash with diluted muriatic acid consisting of five (5) parts hydrochloric to one hundred (100) parts water, or twenty (20) parts acetic acid or vinegar to one hundred (100) parts water. When the acidic reaction (bubbling) has ceased, brush
and rinse thoroughly with water. Immediately wash with an alkaline wash consisting of diluted household ammonia to neutralise the acidity.

Care must be taken when applying acid to portland cement based concrete surfaces. Acid will attack not only the calcium carbonate/sulfate efflorescence but will attack other calcium compounds in the concrete.
Therefore it is very important to neutralize the acid before it can attack other compounds in concrete.

Big Softy
January 2007