Crystalline Cementitious Waterproofing: How It Works in Tanks and Basements

What is crystalline waterproofing and how does it work?

Crystalline cementitious waterproofing is a cement-based powder containing active chemicals. It is mixed with water into a slurry and brushed onto saturated concrete. Unlike coatings and membranes that form a layer on the surface, it works inside the concrete itself.

The active chemicals diffuse through the water in the concrete capillaries and react with moisture and with cement hydration products, including calcium hydroxide (free lime) and calcium silicates. ACI 212.3R describes the reaction as producing calcium silicate hydrates and pore-blocking precipitates that become integrally bound with the hardened cement paste. The result is insoluble, needle-like crystals filling pores, capillaries and hairline cracks, blocking liquid water while the concrete can still pass vapour.

Its distinctive feature is reactivation: unreacted chemicals remain dormant, and if new hairline cracks later form and water enters them, the reaction resumes and starts sealing them. ACI therefore classifies crystalline materials as permeability-reducing admixtures for hydrostatic conditions (PRAH).

• Surface slurry (coating): brushed or sprayed onto existing or new concrete.
• Admixture: added to concrete at batching on new projects.
• Repair mortars: for filling chased cracks, construction joints and leak points before coating.

Recommended uses

It is used in concrete structures that hold or face water: ground tanks, swimming pools, basements and lift pits, and construction joints. Application details are on the tank insulation and pool insulation pages.

Positive side and negative side

The positive side faces the water source: inside a tank or pool, or the outer face of a basement wall. The negative side is the opposite face, such as the inside of a basement leaking from the soil. Most membranes only work from the positive side, whereas Xypex and Penetron data sheets state the crystalline slurry can be applied from either side; this makes it the practical fix for existing basements that cannot be excavated around.

Advantages and technical properties

• Becomes part of the concrete, so it does not peel or delaminate like surface films.
• Works from both sides, positive and negative.
• Self-seals static hairline cracks in the presence of water.
• Withstands high water pressure according to published manufacturer tests.
• Vapour-permeable, so it does not trap moisture inside the concrete.

Disadvantages and cautions

• Static cracks only: it does not treat moving cracks or expansion joints and does not bridge wide new cracks.
• Needs sound concrete: it does not work on block, plaster or paint, and the surface pores must be opened by high-pressure water jetting or sandblasting.
• Curing is critical: rapid drying before the reaction completes weakens the result; the Xypex data sheet calls for protection from wind, rain and puddling.
• Not instant: crystals grow over time in the presence of water, so results cannot be judged on the day of application.
• Active leaks: are stopped first with a fast-setting plug before coating.
• Drinking water: check the product data sheet for potable-water certification before using it in tanks.

Standard application steps

1. 1

   Survey and documentation

   Locate leaks, cracks and construction joints, and measure crack widths to separate static hairline cracks from moving ones.

2. 2

   Surface preparation

   Remove paint, laitance and oil, and open the pores by high-pressure water jetting or sandblasting until sound concrete is exposed.

3. 3

   Cracks and joints

   Chase wider cracks and construction joints into a groove, stop active leaks, then fill with crystalline repair mortar as the data sheet specifies.

4. 4

   Pre-wetting

   Saturate the concrete with water to SSD condition and remove standing water from the surface before coating.

5. 5

   Coating

   Mix the slurry at the specified ratio and apply the first coat with a stiff brush at the required consumption, then the second once the first has set but is still damp.

6. 6

   Curing

   Mist with water several times a day for 2–3 days (or keep damp for 5 days depending on the product), shading the surface and protecting it from wind and direct sun.

7. 7

   Testing and handover

   Fill the tank or pool after the period the data sheet specifies, monitor the water level and former leak points, then hand over with a written warranty whose term depends on the insulation type and structure condition under the warranty policy.

Climate suitability by Saudi region

For local work: tank insulation in Riyadh, pool insulation in Jeddah, waterproofing in Dammam and tank insulation in Abha.

Estimated cost

Tank waterproofing typically costs SAR 60–120 per internal square metre depending on the system, tank condition and preparation, and general waterproofing of basements and walls SAR 18–40 per square metre. These ranges are indicative, exclude VAT and are confirmed after a site inspection. Cost drivers: the length of cracks and construction joints to be chased, active leaks, and access and ventilation. To assess your tank or basement, request an inspection.