At one time, floor tiles were installed and grouted with a single mortar. This mortar was mixed with Portland cement or its equivalent, was on the wet side, and served as setting bed, adhesive, and grout. The mortar was floated and tiles were beat into the surface until some of the mortar oozed up into the joints. After the initial setup, the joints were struck, the tiles cleaned off, and the mortar was damp cured. Sometimes, to increase the bond strength between the setting bed and tiles, a thin layer of dry Portland cement was spread over the wet setting bed before the tiles were installed. This type of installation required a very highly skilled tile mechanic who was able to work both quickly and accurately. For best results, this type of installation required a rather lengthy damp curing period that involved covering the floor with loose-laid plastic film or building paper and, depending on ambient temperature and humidity, misting the surface one, two, or three times every day for about a week or longer.
Regular thinset mortars and unmodified grouts are modern advances and are relatively easy to use, but both still require damp curing to achieve maximum strength, and neither have any of the benefits available with modified materials. Unmodified thinset mortars have good bond strength, but like regular, unmodified grouts, are rigid and inflexible. These qualities are fine for buildings that are also rigid with limited flexibility, but these types of structures were far more common in the past and do not represent the type of construction now in use. Today’s residential and light commercial structures are designed to be flexible, and the materials used in construction must be able to withstand a certain amount of flexing.
Currently, tile installations need to be done quickly, with minimal time and expense spent on damp curing, and with installation labor that may not always be highly trained. Technological advances in mortars and grouts have resulted in materials that do not need long periods of damp curing nor seasoned professionals. These may cost a bit more than regular grout and adhesive mortars, but increased material costs are more than offset by reductions in the cost of labor. From an installer’s point of view, less time spent on fussing with damp curing means more time can be spent installing tiles.
Dry-set thinset mortar: The starting point for adhesive mortars is dry-set thinset mortar: a combination of sand, Portland cement and other additives that help this mix retain water. Dry-set thinset mortar has a high compressive strength, and good bond strength for dry- and wet-areas and for tiles that will be submerged. Some manufacturers offer a non-sanded version of dry-set thinset that is useful for installing small tiles (1-inch or less).
Modified thinset mortar: Modified thinset mortar refers to thinset mortar that has latex as an ingredient. The latex can be in liquid form and added on-site (normally mixed with dry-set thinset powder). Modified thinset can also have the latex added, in dry form, at the factory: this is referred to as polymer-modified thinset and it is mixed with water on-site. There are also some very high strength polymer-modified thinset mortars that require a companion liquid latex to be added, in place of water, on-site.
Epoxy thinset mortar: There are two types of epoxy thinset. One is a mixture of sand, Portland cement, epoxy resin, and epoxy hardener and it is called epoxy-emulsion thinset. Another type, made from sand, epoxy resin, and epoxy hardener, is called 100% solids-epoxy and it can be used as both tile adhesive and grout.
Applying thinset mortars: Thinset mortar is spread with a notched trowel. As tile sizes get larger and larger, the amount of thinset required achieving the industry-recommended 80% dry-area and 95% wet or exterior area adhesive contact must also increase. Charts are available on packaging to aid in selecting the right size notch, but in my experience, most charts underestimate the amount of thinset actually required. As well, tile industry practice says that in order to achieve good adhesion, the flat side of the trowel should be used first, to key the material onto the setting bed surface, and the notched side is used next to comb out a uniform layer of thinset: good advice for any setting bed, but what about adhesion to the tiles? In my opinion, in order to achieve maximum adhesion of the tile to the setting bed, all tiles should be back-buttered before they are set into the thinset spread on the setting bed. In fact, when I am installing large-format tiles (10-inches or larger), rather than apply a skim coat to the back of each tile, I may use another notched trowel to ensure 95% uniform coverage (on my installations – dry or wet area – I strive for greater than 95% coverage). Photos 1 and 2 show the method I use to install large tiles. In photo 1, I use a portable table to make back-buttering easier. Back-buttering is not a slap-dash process but rather applying a uniform layer of adhesive. Photo 2 shows a method I use to ensure that the most vulnerable sections of a tile – the corners – are completely supported. After back-buttering, I apply a small amount of thinset to each corner before installing the tile.
Achieving proper adhesive coverage when installing small tiles such as sheet-mounted mosaic tiles can be difficult, especially when slightly irregular tiles are selected. Normally, back-buttering sheet-mounted tiles is not recommended because this method usually results in joints clogged with thinset that is very difficult to remove. Photos 3, 4, 5, and 6 illustrate a method I use to back-butter small tiles without the usual thinset bleed-through. First, I spread a layer of thinset on the setting bed. Next, using a notched trowel, I spread a layer of thinset on a scrap piece of plywood (Photo #3). After that, I use the smooth side of the trowel to gently flatten the adhesive ridges (Photo #4). Then, I set a sheet of tiles over the adhesive and press each tile into the thinset (Photo #5). When this is done, I peel off the tile sheet (Photo #6) and install it on the setting bed. With the right trowel selection on both the setting bed and the tile sheet, maximum adhesion is attained with little or no bleed-through. Note: the ridges on the setting bed must be knocked down smooth before each back-buttered mosaic sheet is installed.
Regular grout: Regular grout comes in two flavors: sanded and unsanded. Generally, unsanded grout is used for joints less than 1/8-inch wide, while sanded is used to fill wider joints. Regular grout is composed primarily of sand and cement with a colorant and other trace ingredients that help retard setup. Regular grout is mixed with water and it should be damp cured for three or four days after installation. Regular grout is inexpensive, but if used properly – with damp curing – labor costs can be significantly higher than if a modified grout is used.
Modified grout: Regular grout can be modified by mixing with latex instead of water. Grout modified by the manufacturer with dry polymer latex is site mixed with water. Dry polymer modified grout has several advantages over a regular grout/liquid additive. The most obvious are less weight to carry and more room in the truck. A not so obvious advantage for a general contractor working with a sub installer is that with dry polymer mixes, it is hard to cheat by diluting the liquid latex with too much water. The benefits of modified grout include more flexural strength, better color retention, and of importance to the installer, limited damp curing requirements. Some modified grouts require no damp curing while others may require one or two light mistings. In hot weather, it is a good idea to mist all types of modified grout to improve hydration.