Remove Laitance to Enable Waterproofing Integrity!
Perfect and reliable waterproofing requires skilled applicators and quality systems. It is also essential that the substrate to be waterproofed is sound and prepared correctly too. Concrete slabs should be clean and firm, with no dusting.
Concrete slabs sometimes show laitance, a weak powdery layer of cement dust, calcium hydroxide (lime) and sand. It is caused by too much water in the concrete mix, overworking the wet concrete or by slabs that have been affected by rain during the curing stage. If you can scratch the surface of the slab with a nail chances are you have a laitance issue.
The powdery layer must be removed. Otherwise, there may be movement above the slab leading to cracked tiles or failed adhesion. Laitance can only be treated by grinding until solid concrete and the larger aggregate in the concrete is reached.
Some builders try to “fix” laitance by pouring on chemicals called “densifiers”. Densifiers use the same chemistry principle that was used to make ancient roman concrete which was to react volcanic ash with lime! Densifiers can improve good concrete by reacting with any leftover lime and turning it into a hard hydrated form which can then be polished, however they cannot turn water into wine, nor turn laitance into 20MPa concrete. When densifiers are poured on laitance, they will only create a harder form of dust at best. Yep, more dust.
Prevention is mightier than the cure. The best way to manage laitance is to make sure the concrete mix is protected from rainfall during the cure. Get the concrete mix right and keep water to a minimum.
Wet-seal provides both the quality membrane systems and the skilled applicators. Wet-seal Franchisees understand the substrate requirements necessary to ensure a water tight outcome for your builds.
Elisabeth Brazier, MSc. BSc.(Chem.)
National Research & Development Manager, Wet-seal.