My basement has a white chalky residue on it. It is cement. How do i

Interior?? Walls or floor?

Efflorescence is a chalky white salt residue that can occur with any product containing cement. As moisture migrates up to the surface of the concrete, it carries along with it calcium salts from within the concrete. When the salts reach the surface, they react with CO2 in the air and form insoluble calcium carbonate. This white, dusty, scaly salt can be minimal or dramatic, depending on the amount of free calcium salt present in the concrete. Exposure to rain, standing water, and sprinklers only make the situation worse, as water triggers the reaction and creates more efflorescence.

To fix the problem at this point, clean the surface with a mild acid or efflorescence remover (some manufacturers make special efflorescence cleaners) followed by sealing. To avoid the problem altogether on future projects, consider using a colored curing compound or cure and seal to match the color of the concrete.

it is alkaline powder from moisture underneath

May be efflorescence. Check Google search for remedies, such as spraying with vinegar, scrubbing with brush, rinsing and dry. It's caused by moisture leaching through the concrete. Also try running a dehumidifier with a hose to drain into a basement drain.

Efflorescence. It's caused by moisture from within the concrete. The "salts" are coming out of the concrete to the surface. You may have a water problem form outside.