Coloring and Curing

Coloring concrete can be done in two basic ways. Natural coloring consists of iron oxides, while synthetic metal oxides can be made from iron to make red, yellow or black pigments, from cobalt chrome aluminates to generate blue tones, or chrome oxides for green.

The concrete mix is fed into a tile-making machine, where it is extruded under high pressure into molds (to make various shapes and sizes of tile) that continuously pass through the machine, and immediately proceed down a conveyor belt to receive nail holes and surface treatments. The tiles are transported and collected into curing racks that are then moved into curing chambers for the initial cure prior to packaging.

The curing process accelerates the rate of cement hydration so that the tiles made are strong enough to be de-palleted (separated from the mold). The curing process also impacts other product quality factors affected by cement hydration, such as color variation, efflorescence, and moisture resistance.

There are two types of cures: ambient and heated. After six days of curing, the results of these two processes are the same. The heated cure reaches a stable point much faster, sometimes in one day. For this reason, a controlled heating process is most prudent while tiles are in the curing chambers. This eliminates weather as a factor in curing in order to better assure consistency.

Curing also represents the difference (aside from basic raw materials) between concrete, clay, and natural slate roof tile. Chambers producing cured concrete roof tile reach controlled temperatures of 150 degrees Fahrenheit. Clay tile kilns produce peak heat levels between 2000 and 2200 degrees Fahrenheit. The lesser curing time indicates concrete's greater inherent strength. Natural slate is not manufactured and therefore subject to natural structural inconsistencies which may result in reduced durability. Concrete tile, on the other hand, is manufactured to help ensure a consistent product.

In some areas, particularly warmer regions not subject to severe cold temperatures, a slurry coating, consisting of cement, pigment, sand, and water, may be applied to the tile in the factory. The slurry coating must be mixed to specifications, and is then applied evenly over the tile to a thickness of 400 microns while the tile is traveling on its palette mold at a speed of 1.6 tiles per second.

Efflorescence

For a few months after their manufacturing, concrete tile may exhibit efflorescence, a natural process of water penetrating the capillary structure on the surface and extracting soluble salts from the tile body. Efflorescence is a temporary condition and does not impact the functional qualities of the tile. Deposits from the efflorescence process on the tile surface will wash away in rain or by cleaning once the supply of salts accessible to water is exhausted. The duration depends on the amount and cycle of rain the tile is exposed to.

Efflorescence can be reduced or eliminated by applying an acrylic sealer to form a continuous film over the tile surface. The sealer blocks the migration of calcium hydroxide to the concrete's surface, while allowing carbon dioxide to pass through to form a plug of calcium carbonate in the capillaries.

In addition to efflorescence, the appearance of the tiles may at first be affected by slight mismatches in color from different palettes during curing. When the roofing contractor assembles a roof load, tiles should be gathered from two or more palettes in order to blend shades and reduce grouping of shades.