The white, chalky substance is called efflorescence. This forms when salts naturally present in concrete products are brought to the surface by moisture.
Efflorescence typically disappears on its own as the salts are released, which may take anywhere from a few months to a few years. For this reason, it’s recommended to avoid sealing concrete products for at least one year post-installation.
If the efflorescence is still a concern, products are available to help clean it away.