What Is Grout Made Of? Cement, Sand, Polymers, Resins and Pigments

Most tile grout is a mix of portland cement, fine aggregate (sand or mineral filler), pigments and small amounts of additives, which hardens when water reacts with the cement. Epoxy grout replaces cement with a resin and hardener, and premixed grouts use acrylic or urethane binders that cure as water evaporates.

Inside a bag of cement grout

A typical bag of powdered cement grout is a carefully proportioned blend. Exact recipes are proprietary and vary by manufacturer and product line, but the ingredient families are consistent across the industry.

Basic grouts may contain only cement, sand and pigment, plus a little water retention agent. High-performance cement grouts carry the fuller list, which is what lets them meet the tighter limits of ANSI A118.7.

How cement hydration works

Portland cement is made by firing limestone and clay or shale in a kiln, then grinding the resulting clinker with a little gypsum. The clinker consists mainly of four compounds that cement chemists abbreviate as C3S (tricalcium silicate), C2S (dicalcium silicate), C3A (tricalcium aluminate) and C4AF (tetracalcium aluminoferrite). White portland cement, which most light and bright grouts use, is made from raw materials low in iron and manganese, so it contains very little C4AF, the compound responsible for gray cement's color.

When water is added, these compounds dissolve and react. The silicates form calcium silicate hydrate (often written C-S-H), a gel-like solid that is the main source of strength, plus calcium hydroxide as a byproduct. The aluminates react with the gypsum to form other hydrate crystals, and the gypsum controls how fast that happens so the mix does not stiffen instantly.

• Early hours: the grout stiffens (initial and final set). This is the window for cleanup.
• First few days: strength builds fastest. Most grouts reach usable strength for normal service in about 72 hours at room temperature.
• Weeks: hydration continues, more slowly, for 28 days and beyond. Strength in standards is typically measured at 28 days for this reason.

Hydration is temperature sensitive. Below about 50 deg F (10 deg C) it slows sharply, and if the grout dries out before hydrating it never reaches its potential strength. That is why manufacturers specify minimum installation temperatures and why hot and cold weather change the rules. The grout cure time guide covers the practical timeline.

Why cement grout is alkaline

The calcium hydroxide formed during hydration keeps the pore water strongly alkaline, roughly pH 12.5-13. This alkalinity is why fresh grout irritates skin, why acids etch cured grout, and why cleaners with low pH should be used with care. See grout cleaners by pH for what that means for maintenance.

Water: the ingredient that causes most problems

Cement needs only a limited amount of water to hydrate fully. Classic cement research puts the water-to-cement ratio needed for complete hydration at roughly 0.4 by weight, with about 0.25 chemically bound. Grout is mixed wetter than that so it can be worked into joints, and manufacturers design their formulas around a specific mixing water amount.

Every bit of water beyond what the reaction and workability require eventually evaporates and leaves a network of capillary pores behind. More pores mean:

• Lower strength and abrasion resistance. Overwatered grout is a main cause of soft or crumbling grout.
• Higher absorption, so the grout stains and harbors mildew more easily.
• More drying shrinkage, which can pull grout away from tile edges or cause fine cracks.
• Shading and blotchiness, because excess water carries fine particles, pigment and soluble salts to the surface unevenly.

Efflorescence and shading: chemistry you can see

Calcium hydroxide is somewhat soluble in water. When moisture moves through grout and evaporates at the surface, it carries dissolved calcium hydroxide with it, which then reacts with carbon dioxide in the air to form calcium carbonate: a white, chalky deposit known as efflorescence. Dark grouts show it most. Other soluble salts from the substrate or setting materials can add to the deposit.

Formulators fight this in several ways: specialty cement blends that tie up more calcium in less soluble hydrates, polymers and water repellents that slow moisture movement, and pigments that tolerate the alkaline environment. Installers fight it by controlling water. If you are already dealing with white haze or bloom, see efflorescence on grout and grout discoloration and shading.

Aggregate: why sand matters

Cement paste on its own shrinks substantially as it dries and would crack badly in a wide joint. Sand and mineral fillers are dimensionally stable, so replacing part of the paste with them reduces shrinkage, adds abrasion resistance and lowers cost. Grading (the mix of particle sizes) matters: a well-graded blend packs tightly, so less paste is needed to fill the gaps between particles.

The particle size also sets the minimum joint the grout can enter. Sanded grout uses sand that is too coarse to pack fully into very narrow joints, which is the basis for the common rule of thumb: unsanded grout for joints under 1/8 inch, sanded for 1/8 inch and wider. Unsanded grout relies on very fine fillers and more polymer to control shrinkage, which is why it cracks and slumps if forced into wide joints. Many modern high-performance grouts use ultra-fine graded fillers that span roughly 1/16 to 1/2 inch in one product. See sanded vs unsanded grout.

Polymers and additives

Older practice was to mix grout with a liquid latex additive instead of water. Today most polymer-modified grouts contain redispersible polymer powder in the bag: spray-dried polymer particles that disperse when mixing water is added and coalesce into a fine film within the cement matrix as the grout cures. That film bridges micro-cracks, improves adhesion to tile edges and reduces water absorption. Because the polymer is already included, adding a liquid admixture is generally unnecessary and can upset the formula unless the manufacturer approves it.

Cellulose ethers, used in small amounts, act like a sponge that releases water slowly. They help the grout stay workable, prevent absorbent tile edges from stealing water, and give cement time to hydrate. Water repellents such as silanes or stearates give the cured surface its beading behavior, though they do not make grout waterproof. Grout is never a waterproofing layer; that job belongs to a membrane behind the tile.

Pigments and color

Grout pigments must survive a highly alkaline environment and, outdoors, UV light. Inorganic pigments do this well, which is why most grout colors are built from synthetic iron oxides (reds, yellows, browns, blacks), titanium dioxide (whites and tint strength), chromium oxide (greens), and carbon black. Some blues and brights use other inorganic or carefully selected organic pigments.

The base cement affects the final shade. Light and bright colors require white cement, while some deep colors are made on gray cement. Because pigment is a small fraction of the mix, the color you see is strongly influenced by surface texture, water content and cure conditions. Two joints from the same bag can look different if one was washed harder. The grout color guide and our grout color visualizer help with choosing a shade that tolerates these variations.

Epoxy, urethane and premixed grout chemistry

Epoxy grout is usually sold as two or three parts: a resin (Part A), a hardener (Part B) and, in many systems, a filler powder with pigment. Mixing triggers a reaction that cross-links the resin into a dense solid with very low absorption and strong chemical resistance. The reaction generates heat and is temperature sensitive. See epoxy grout.

Premixed grouts come ready to use in a tub. Their binder is a water-based acrylic or urethane dispersion loaded with fillers and pigments. They harden as water leaves, so cure depends on air, temperature and joint depth. That is why they can stay soft in cold, humid rooms or under sealed conditions; see premixed grout, urethane grout and grout not curing.

Grout vs thinset vs mortar

Thinset mortar and grout share cement, sand and polymers, but they are formulated for different jobs. Thinset is built for adhesion and is hidden under tile; it lacks the color control, packing behavior and surface hardness that grout needs. Grout is built to fill and finish an exposed joint, not to bond tile to a substrate. Masonry mortar is different again. The grout vs thinset vs mortar comparison covers when each applies.