Scrub with an abrasive soap, you can also use a machine. Never use a steel wire. If the stains remain seek the advice of a detergent manufacturer.

This is due to the crystallization of the soluble salts on the surface of the tile by the moisture underneath floorings and on the floor. Clean several times until darkening disappears completely.

The fastest way to degrease the tiles is by using a cleaning detergent containing organic solvents or high-alkaline detergents (pH>9).

Non-glazed tiles can be maintained clean by just sweeping while glazed tiles must be carefully cleaned with a dry cloth. Both types of tiles can be washed with warm water or with a natural, low-sulfite detergent added to the water. Once cleaned with the detergent the tiles must be rinsed a couple of times with pure water but should in no way be subjected to thermal shock. 

Darkening is caused by the crystallization of soluble salts by the moisture under the coverings and on the floor. Clean with water until it disappears completely. 

During the process of applying of the tiles, their surface and edges are periodically cleaned from glue and cement to avoid their solidification. Once the tiles are glued onto the working surface (preferably 1 m2), the surfaces and joints are wiped with a moist cloth. If there is still a residual cement layer you can remove it with acidic detergents. After cleaning wash the surfaces with pure water.