Grout is a material that is applied in the spaces between tiles to help them adhere to a surface. Even the most practically minded DIY enthusiast or professional craftsman can feel gutted when watching carefully applied grout crack and crumble to pieces. There are a variety of reasons for such deterioration. They range from age and structural damage, to the nature of the grout mixture and the application methods used.
Too much water or polymer
Grout is a type of mortar powder that mixes with water or a polymer additive to form a substance with the consistency of peanut butter. This substance is applied between tiles. It hardens, or cures, as the water or polymer evaporates. The evaporation creates hairline cracks that usually are no problem over the next few years. But if the mixture contains too much water or polymer, it becomes too liquid, structurally weak and creates large cracks as it dries. These holes become bigger with time and temperature changes causing the grout to crumble.
Grout, like other mortar applications and plaster, degrades with age. Over time, water and mildew can penetrate hairline cracks in solid grout and degrade the material from both within and from the surface. Small cracks can form in it due to the building wall or floor settling unevenly. The grout could also have been damaged by the impact of a hard object. In all these cases, water, temperature and mildew will further degrade the grout and cause crumbling.
Grout must be applied deep into the space between tiles. If any air holes remain between the grout and the surface to which the tiles adhere, they can expand during warm weather, creating a crack that eventually causes the grout to crumble.
Grout cures very quickly. This means it’s important to mix just enough for the job in hand so that it does not start curing in the mixing bucket. The addition of water to grout that has started to cure in a mixing bucket or on a surface – a common mistake someone may make when eager to finish off a job -- will weaken grout and make it crumble as the water evaporates.
Grout is very porous and needs a sealant to protect it from water infiltration and damage. Thinset is a type of glue that helps the grout adhere better to tiles. If either the sealant or thinset is applied too thinly, air pockets between these and the grout can expand, creating cracks and crumbling.
Grout, like other mortar and plaster, will crack if the structure behind it moves. This could happen when a building settles over time, or after a drought, flood or earthquake. Such cracking is visible at wall and floor junctions, and can progress along the surface through the grout around two sides of each tile. Grout between a bathtub and wall tiles will also crack if the bath flexes. This is a common problem with light steel or plastic baths. Eventually, the cracks cause grout crumbling.