Flex R20 Unmodified Mortar

As the name suggests, an unmodified mortar is a mortar that contains no polymers, which limits its use – it can only be installed on concrete, indoors. However, SCHLUTER® Systems® recommend its use with DITRA® and KERDI® membranes. But why an unmodified mortar? According to data provided by SCHLUTER ® Systems®, modified thin-set mortars are not recommended to set tile over DITRA® or KERDI® because these mortars must air dry to cure properly. When sandwiched between two impervious layers such as DITRA® or KERDI® and porcelain tile, drying takes place very slowly through the open grout lines, and extended cure times would be required before grouting. If extended cure times were not observed, the results could be unpredictable. On the other hand, unmodified mortars have a chemical cure which is not affected by moisture level. An unmodified mortar is therefore the best choice for this type of application.