It's called Effllorescence.
It's caused by waterborne salts passing through the masonry.
It happens in new builds when the cement was allowed to get wet, but it usually dries out after a few weeks or months dependent on climate.
If it's continuous it means there is water seepage through the floor from groundwater.
Was a waterproof damp course installed before laying the concrete?
Big Softy
June 2010