Efflorescence on grout is a white powdery feathery deposit, caused by salts dissolving in moisture retained by the grout and finding their way to the surface. The deposit can appear as splotches or a white crust all over the grout surface. The minerals that form the salts can be present in the grout itself or the water that was used to mix it; the salts are invariably sulfates or carbonates. Although unsightly efflorescence can initially be easily removed by brushing it off with a fairly stiff brush.
Returning Grout Efflorescence
If the efflorescence returns it might be tempting to wash it off with water. However, you mustn’t do this as using plain water will only encourage more salts to leach out and make the matter worse. Rather than using plain water, look to see what mildly acidic cleaners you have and try using a mild solution of those first. If that doesn’t work you’ll need to get one of the preparatory grout efflorescence cleaners form your DIY store. If that still doesn’t work you’ll probably be best off replacing the grout.
Constantly returning efflorescence has to be down to one of two problems; either an excess of minerals in the water or in the grout. You can determine if it’s the water you’re using when cleaning the grout that’s the problem, by either boiling it or filtering it to remove the minerals. Dependant on your local water supply boiling the water might help. However, you really are recommended to filter it, using a domestic water filter from any DIY store or a store selling kitchen utensils.
If changing the quality of the water you use doesn’t help - then it has to be the grout itself. This could be due to the conditions at the time that the grout was applied or it could simply be due to the quality of the grout; either way it probably needs replacing.
Replacing Grout to Eliminate Efflorescence
The consistency of the grout mixture when it’s applied and the temperature and humidity conditions affecting the evaporation rate of the grout; can all play their part in the development of efflorescence on grout - not to mention the actual quality of the grout itself. So, your first step is to buy a top grade grout that is clearly labeled as being anti-efflorescence. Having a good quality grout and using water that is known not to have a high mineral content are the starting points.
The next step is to mix them properly, the worst thing you can do is have a grout mix that is too wet. Having a grout mix that is too wet will exasperate any efflorescence problems especially if the substrate onto which it is being applied is unable to soak up any excess moisture. So mix the grout to the consistency of a wet paste that will slip off your applicator, but not immediately run off it as soon as the applicator is lifted.
Also, don’t just set about mixing the grout and water without actually measuring the amounts properly. If the packaging says a certain weight of grout to a certain volume of water - measure it properly. Also, trying to clean up the grout before it has reached a firm consistency can encourage efflorescence, by introducing fresh water too quickly.
Finally do be aware of the ambient temperature when mixing and applying grout. Applying grout in cool temperatures, between 50F and 70F, will prolong the drying period for the grout and will encourage efflorescence.