This natural compound will make the black joints between the tiles white again

The natural compound you're looking for is baking soda (sodium bicarbonate) mixed with white vinegar.

How it works:
Baking soda is mildly abrasive and alkaline, which helps lift dirt and grime.

White vinegar is acidic and breaks down mold, mildew, and mineral deposits.

How to use it for grout cleaning:
Make a paste of baking soda and water.

Apply the paste to the blackened grout lines.

Spray or pour white vinegar over the paste. It will fizz and help loosen dirt.

Let it sit for 5–10 minutes.

Scrub with a small brush (like an old toothbrush).

Rinse with warm water.

This combo is very inexpensive (under 1 euro for both) and safe, restoring grout to a clean white look naturally without harsh chemicals. Would you like a detailed step-by-step guide?