Impact of Ceramic Tile Water Absorption Rate on the Color Stability of Cement-Based Grouts

Ceramic tiles are classified according to ISO 13006 based on their manufacturing method (dry pressing or extrusion) and their water absorption rate. This rate directly influences their behavior on the jobsite, particularly during grouting with Portland cement-based grouts or calcium aluminate-based grouts.

Among the defined categories, Group BIII type tiles — dust-pressed ceramic tiles with a water absorption rate exceeding 10% — present particular challenges during grouting. These tiles are thin, lightweight, glazed ceramics, generally made of red or white clay. They are highly porous and therefore intended for indoor wall applications only. They are commonly referred to as "wall tiles" or "subway tiles", often available in 4" x 4" (10 x 10 cm), 3" x 12" (7.6 x 30 cm), and 4" x 16" (10 x 40.6 cm) formats typically seen in kitchens, bathrooms, or backsplashes. Thanks to their high porosity and low thickness, they are easy to cut and install. They are not suitable for floors or exterior installations, as they lack mechanical strength and cannot withstand frost.

Their excessive porosity leads to high and rapid capillary absorption of the water contained in the grout, disrupting the complete hydration of binders and reducing the product’s workable time. This phenomenon can also alter the final grout color, compromise chromatic uniformity, and, in some cases, cause efflorescence.

Cement-based grouts, whether formulated with Portland cement or calcium aluminate, require a precise amount of water to ensure complete binder hydration, uniform polymerization, even pigment distribution, and proper setting to avoid dusting and to form a strong cementitious matrix.

In the case of highly absorbent tiles:

• The tile acts like a sponge, rapidly absorbing a significant portion of the water intended to hydrate the binder.
• This can lead to insufficient water remaining for full hydration of the hydraulic binder.
• This imbalance affects grout color, causing visible shade variations and a “blotchy” or marbled appearance depending on grout color.
• Excessive absorption also increases the risk of efflorescence by promoting the migration of dissolved salts to the surface when Portland cement–based grout is used.
• Accelerated grout setting makes cleaning more difficult.
• A decrease in surface strength of the joint may occur due to incomplete hydration.

These issues can be exacerbated by extreme climatic conditions, excessive ventilation, or when surfaces have not been properly conditioned prior to grouting.

The high absorption rate of BIII group tiles, now increasingly common in the market, significantly alters the hydration conditions of cementitious grouts. To date, there is no simple, effective, and reproduceable method to fully counter the negative impact of high absorption rates in these tiles.  For now, the only way to mitigate this issue is to use either a sand-free cement-based grout, such as Keracolor^® U, or even better – an epoxy grout, like Kerapoxy^® CQ, or a pre-mixed “Ready-to-Use” grout, such as MAPEI Flexcolor^® CQ.

Kerapoxy CQ is a two-component, 100%-solids, epoxy grout and mortar that is nonsagging/nonslumping in joints up to 3/8" (10 mm) in width, water-cleanable and easy to apply. It uses a proprietary aggregate to achieve its durable color, making it excellent for countertops, high-traffic areas, and areas needing stain and chemical resistance. It also does not contribute to efflorescence and has excellent resistance to chemicals, staining, alkalinity, chlorine, cracking and color deterioration.

MAPEI Flexcolor CQ is a professional-grade, ready-to-use specialty grout for precision commercial and residential installations with porcelain, ceramic and natural-stone tiles. Its dense composition helps to prevent water-based stains by providing water repellency to the grout surface.

Both of these specialty grouts are formulated with MAPEI’s BioBlock technology, which does not support mold/mildew/fungi growth after product installation as certified by external laboratories per industry standard ASTM G21: Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi.^[1]

To ensure uniform color, good surface density, and enhanced durability, it is essential to adapt jobsite practices to the absorption characteristics of the tile. A proactive approach based on preliminary testing, appropriate product selection, and effective cleaning techniques is key to achieving a flawless aesthetic installation.

If you have questions about MAPEI grouts and best practices for installing tile & stone, reach out to MAPEI Technical Services at 1-800-361-9309 or by email to [email protected].