Mix 7 cups water, 1/2 cup baking soda, 1/3 cup lemon juice and 1/4 cup vinegar. Spray the concoction onto the dirty grout, let sit, and scrub with a brush. Sparkling grout await.
Grout Cleaner: 7 cups water, 1/2 cup baking soda, 1/3 cup lemon juice and 1/4 cup vinegar - throw in a spray bottle and spray your floor, let it sit for a minute or two... then scrub :)
20 Cleaning Tips for Neat Freaks - One Crazy House
walk-in shower with small wall as divider