Preventing Unacceptable Grout Joints
It has been said that, because they encompass so many different situations, grout joint problems generate the largest number of complaints. In this installment of Ceramic & Stone Close-Up, I'll cover three of the most common deficiencies associated with grout joints.
Cracking. Cracks in grout joints are most obvious even to a casual observer. Such cracks may result from any of a variety of causes. Among the most common causes is shrinkage of the grout due to excess water in the mix - in other words, the product was improperly blended. The cracks occur due to the loss of the excess water in the grout through evaporation which, in turn, reduces the grout's mass.
Some cracks in grout can be attributed to the use of dry, absorbent tile that absorbs the water in the mix, stops the hydration needed for strength and reduces the mass of the grout. If you know you are dealing with dry and very absorbent tile, you can avoid this situation by dampening the joint before grouting. Use of a latex-modified grout is also helpful in resisting this type of cracking.
Deflection in the structure, either by racking or substrate deflection, will also cause grout to crack. Again, the use of a latex-modified grout might help in such instances.
Anytime the curing process is thwarted by environmental conditions, meaning hydration has been curtailed, the likely result will be weak grout that is susceptible to cracking. ANSI specifies damp curing to prevent the hydration process (curing) from being interrupted. Curing is critical to obtaining strong, crack-resistant grout. Therefore, it's vital that the proper blending of the mix be obtained. Do not over water, do not speed mix (to avoid air bubbles), and do not add water to your mix - even if it is becoming stiff.
It is important to use the proper tools for finishing your joints. Avoid using too much water in the joints, as it will definitely cause crazing.
Efflorescence. A salt deposit visible on the grout or on the tile edge also becomes an unsatisfactory situation. What is efflorescence? Efflorescence results from a soluble salt present in your mortar setting bed, in your cementitous grout and in concrete slabs. It remains fairly inert unless moisture is present, in which case the salt moves to the surface where it dries into a white, crumbly powder.
Elimination of efflorescence is achieved by some of the following methods. When installing over an on-grade slab, a vapor retarder should be in place beneath the slab. Use a meter to check moisture emissions. Other preventive measures include allowing your setting bed to dry (and cure), using proper mixing techniques, installing dense grout joints, not using excessive water in dressing joints, and using kraft paper for curing.
Soft Grout Joints. These problems may occur when old grout mix is used. Cement-based products do have a limited active life span. Again, improper mixing may play a role in this problem. To prevent soft grout joints, I'd advise that the product be mixed at low speeds and, as usual, that the mix be allowed to slake. This is particularly important because the slake time allows the constituents of the grout mix to completely wet out and achieve satisfactory hydration.
The joint must be clean prior to grout placement to avoid powdery and soft joints. Work dust and gypsum dust also can inhibit the effectiveness of the hydration process to cause an unsatisfactory grout joint.
Soft grout joints may occur when the grout is placed under excessively hot and dry environmental conditions, which can abruptly end the hydration (strengthening) process. Of course, exceptionally porous and warm tile will also stop hydration by promoting rapid water loss which, in turn, creates a soft joint.
Grouting is like a painter's finishing touches. So use care, stand back and enjoy.