Efflorescence on Grout: What the White Deposit Means and How to Stop It

Efflorescence is a white, powdery or crusty deposit left when water moves through grout and the cement beneath it, dissolves calcium compounds and salts, and evaporates at the surface. The fresh powder brushes off; hardened calcium carbonate needs a mild acid on acid-safe tile. It returns until you stop the water feeding it.

What efflorescence is, chemically

When portland cement hydrates, it forms calcium silicate hydrates (the binder that gives grout strength) and calcium hydroxide, a soluble byproduct. Cement, sand, water and some admixtures can also contain small amounts of soluble sodium, potassium and sulfate salts. When water moves through the grout or the mortar bed below it, it dissolves these compounds. At the surface the water evaporates and leaves them behind.

Calcium hydroxide left at the surface reacts with carbon dioxide in air to form calcium carbonate, the hard, white, insoluble deposit people usually mean by efflorescence. Sodium and potassium sulfates and carbonates form a fluffier, crystalline bloom that dissolves back into water. Knowing which you have tells you how to remove it. The underlying chemistry of grout is covered in what grout is made of.

Primary and secondary efflorescence

• Primary efflorescence appears in the first days to weeks after grouting, from the mixing water itself working its way out. It is usually light, uniform and self-limiting.
• Secondary efflorescence appears later or keeps coming back, because outside water (from a slab, rain, irrigation, a shower or a leak) keeps carrying new salts to the surface. This is the one that signals a problem.

Is it efflorescence? Telling white deposits apart

Several white deposits look alike on grout. Treatment differs, so test before you clean.

Other white-grout causes are compared in grout turning white. For a simple field test, put a drop of white vinegar on an inconspicuous deposit on cement grout (never on acid-sensitive stone). Fizzing indicates a carbonate. Then place a drop of plain water on another spot and let it sit for a minute; if the deposit disappears, it is a soluble salt.

Where the water comes from

Efflorescence is a moisture symptom. The deposit shows you where water is leaving the assembly. Finding its source is the actual repair.

Slabs on grade

Concrete in contact with the ground can wick moisture and vapor upward, especially without a vapor retarder beneath. Tile on these slabs, particularly in basements and older houses, can show efflorescence in joints for years. Tile itself is relatively impermeable, so moisture exits through the grout, concentrating deposits there.

Showers

In a traditional mortar-bed shower floor, water passes through the grout and the mortar bed to a waterproof pan below, then drains through weep holes at the drain. If the weep holes are clogged with mortar or debris, or the pre-slope beneath the pan is missing, water collects in the bed. The bed stays saturated, and minerals keep wicking up into the grout. Recurring efflorescence on a shower floor, especially near the drain or at the low side, is a classic indicator (see shower floor grout). Leaking valves, failed waterproofing and cracked grout at the curb produce similar signs on walls and curbs. Our guide to shower grout discusses assemblies that limit this.

Exteriors, patios, pools and balconies

Rain, irrigation, poor slope, and missing drainage layers drive large volumes of water through exterior tile assemblies. Efflorescence is common on patios and pool decks, and especially on balconies where water collects in the mortar bed above a membrane with no way to drain. See outdoor and pool grout.

Installation water

Too much mixing water, heavy sponge water, and grouting over a freshly placed mortar bed or slab all put extra water into the assembly, and that water carries salts out as it evaporates. This is mostly primary efflorescence and tends to stop once the installation dries.

Removing efflorescence

Start gentle and escalate only as needed. Wait until the grout is fully cured before using any acid; many grout makers specify 7 to 14 days or more. Check the grout TDS.

1. Dry brush. Use a stiff nylon brush on dry grout and vacuum the powder. This alone removes soluble salt efflorescence. Do not wash it away with lots of water; that just dissolves the salts and carries some back into the grout to reappear.
2. Light damp wipe. If residue remains, wipe with a barely damp cloth and let dry completely. Repeat brushing.
3. Mild acid for carbonate deposits. On cured cement grout with porcelain, glazed ceramic or other acid-tolerant tile, use a commercial efflorescence remover or a sulfamic acid cleaner at the label dilution. Pre-wet the area with clean water so the acid works at the surface rather than soaking in. Apply, scrub with a nylon brush, keep dwell time short (often a few minutes), then rinse thoroughly with clean water. Neutralize if the label calls for it.
4. Repeat if needed. A second, lighter application is better than a stronger mixture. Strong acid attacks the cement paste and exposes sand, leaving the grout rough, lighter and more porous.
5. Let it dry and watch. Leave the area to dry for several days, then check whether deposits return. Return is your evidence that water is still moving through.

For more on choosing cleaner strength safely, see grout cleaners by pH, and for other residues the grout stain removal guide.

On natural stone

On marble, limestone, travertine and other calcareous stone, acid is off the table. Use repeated dry brushing, a neutral cleaner, and for stubborn deposits on the grout lines, careful mechanical removal with a nylon pad or a fine abrasive pad tested in a hidden area. Some stone care product lines offer efflorescence treatments described as safe for calcareous stone; check the product's own claims and test first. The Natural Stone Institute and the stone's supplier are the best references.

Preventing efflorescence

Grout choice helps, but does not solve the water problem

Epoxy grout does not produce efflorescence of its own because it contains no portland cement, and its low permeability limits water passing through the joint. Polymer-modified high-performance cement grouts are less absorbent than standard cement grouts and tend to show less efflorescence. But if water saturates the mortar bed or slab beneath, salts can still appear at tile edges, at movement joints, or wherever moisture finds a path out. Switching grout treats the surface, not the source.

Sealers: breathable, not film forming

A penetrating sealer reduces how much water enters grout from the surface, which helps against efflorescence driven by surface water. Where moisture is coming up from below, though, a film-forming or membrane sealer can trap water and salts under the surface. Salts then crystallize inside the grout pores, a process called subflorescence, which can cause flaking and spalling. Use breathable penetrating sealers on slabs and exteriors, and never seal over active efflorescence; clean it first and let the grout dry. See penetrating vs membrane sealer.

When efflorescence signals bigger trouble

Efflorescence itself is cosmetic, but persistent efflorescence means water is moving through the tile assembly, which can lead to other problems:

• Soft or crumbling grout as water leaches lime from the cement paste over time. See soft or crumbling grout.
• Dark, persistently damp joints (see grout stays wet or dark) and blotchy color; see grout discoloration and shading.
• Mold growth in showers where the bed stays wet.
• Freeze-thaw damage outdoors, where saturated grout and mortar crack and spall in winter.
• Rot and corrosion in framing or metal components behind a leaking shower or balcony.