Throughout your home you probably have either tile (ceramic, porcelain, etc) or stone (marble, travertine, etc.). Seemingly it is an easy surface to clean and keep clean, but beware, you will cause significant damage if in-correct cleaning agents are used!
If you’re like most homeowners, you may be wet mopping and that’s ok. The cleaning solution you should be using should be neutral in pH (7), not acidic (<7). Use warm water and a bit of detergent. The crucial step to take when mopping your floor is to not rinse the mop back into your fresh cleaning solution. Rinse the mop under a faucet using fresh water and then go back and dip into the bucket of cleaning solution. If this sounds like a pain, I agree, but usually rinsing is done in the bucket of cleaning solution. As a result you end up “cleaning” with dirty solution and just pushing the dirt around!
This pushed dirt ends up in your grout lines, which you have probably noticed as they get darker and dirtier. Now you’re at the point to where you can’t stand it anymore, and you try scrubbing the grout to get it clean. However, this is extremely difficult, so maybe you’ve heard of an at-home remedy to use an acid like muriatic acid. This should be used with extreme caution! As I mentioned before, acidic cleaning (pH <7) solutions are very damaging to your grout or stone.
Any natural stone (marble, travertine, granite, etc) would never be cleaned using an acid because it etches the surface (noticeably duller/scratched surface). Technically speaking, the acid reacts with the calcium carbonate (CaCO3) in the stone as well as in the grout lines. Etching within grout lines shows up by a loss of grout. Over a few washes with acid, most of your grout will be destroyed and removed and new grout will need to be installed.
That being said, proper tile cleaning will never use an acid, but many “professional cleaners” will get the job done that way and you will never know you paid someone to damage (“clean”) your grout!
In reality, there may have been nothing else that could have been done at the time. However, an attempt should be made by every professional cleaner to use an alkaline cleaning (pH >7) solution along with very hot water, powerful agitation, and vacuum extraction. The dirt is removed by vacuum extraction and the truckmount (used by many good carpet cleaning companies) gives the superior extraction necessary. Add a grout brush along with a little elbow grease and the dirt will be removed. Your grout can be cleaned without damage.
However, keep in mind that some grout just simply can’t be cleaned properly. Acid, along with the etching damage it creates, is the only alternative if you want to eliminate those stubborn stains in your grout. But instead of continuing this “cycle of damage” you must apply a proper sealant, called an impregnator, aka penetrating sealer, not a coating.
A sealer may not have been applied on installation, and wear away within 3-4 years. They should be applied (reapplied) to all CaCO3 (Grout or Stone) surfaces. It will absorb into the surface without any cosmetic loss to the honing or polishing.
You can easily test to see if your stone or grout is protected by simply pouring a drop of water onto the surface of question and wiping it up within a 10-15 seconds. The water soaks into the surface like any spill would. If the area you dropped water on is darker, the stone or grout is not protected.
Be sure to apply an impregnator/penetrating sealer to your grout or stone and eliminate the "cycle of damage".