Cement and waterproofing: what cement can (and cannot) do

A classic site question: "which cement should I choose to make my structure waterproof?" The direct, honest answer: no cement, even waterproofed, seals a structure on its own. Waterproofing is a system matter: a dense, well-dosed concrete forms the first barrier, and dedicated complements take over depending on exposure. Cement is the indispensable base — not the whole answer.

Cement's real job: building the first barrier

A compact concrete holds water back almost completely; a porous one lets it through even under the best membrane, because bubbles and honeycombs create chimneys. What makes concrete dense is not the bag but the placing: a sufficient dosage (at least 350 kg/m³ for exposed work), a controlled water/cement ratio (≤ 0.5), careful vibration, and serious curing over several days. These four gestures are worth every additive on the market.

Waterproofing complements, by exposure level

The decisive criterion is pressure: the harder water pushes, the higher the system must step up. Mass water-repellent — the admixture added at mixing — covers capillary rise; it is not a barrier against a column of water.

The case of demanding structures

A pool or a tank works like a hull: sound structure first (well-dosed, vibrated, cured concrete with treated construction joints), waterproofing system second, installed per the chosen product's rules. That is the position we detail in our "cement and pools" article: cement builds the shell, waterproofing is entrusted to specialised systems. Skipping the first step condemns the second: a membrane over cracked concrete follows the cracks.

Geomembranes: when the membrane takes over from mortar

For pressure exposure — pools, reservoirs, roof terraces — there is a second route, complementary to waterproof mortars: the geomembrane, a continuous sheet of elastomer or polyolefin (EPDM, PVC, HDPE) rolled over a prepared substrate with seams welded or bonded together. Its principle differs from mortar: instead of a barrier integral to the substrate, it is an independent membrane that stays watertight even if the substrate cracks slightly — its strength on structures that move. In practice the two systems combine: a sound substrate built with well-dosed concrete and waterproof mortars, on which the membrane brings the final guarantee.

Three rules decide a geomembrane's outcome: substrate preparation (smooth, clean, no sharp edge to pierce the sheet), seam quality — water passes through joints when it passes at all — and mechanical protection above (sand bed, geotextile, protection slab), because a membrane punctured by a stone is no longer a membrane. It is a specialist trade, a petrochemical product laid with a welding machine: DAM neither manufactures nor installs geomembranes. Our contribution plays upstream and underneath: the CPJ 45 or CPJ 55 cement of a compact crack-free substrate, and the waterproof mortars for works that need no membrane. To arbitrate between mortar alone and a membrane system, describe your structure — we will tell you frankly what belongs to our bags and what belongs to a specialised installer.

What DAM produces — and does not

We produce grey CPJ 45 and CPJ 55, compliant with NM 10.1.004, for all your structures: foundations, tanking, pools, tanks. We produce no waterproofed cement, no admixtures, no waterproofing systems — our role stops at a quality binder, with a technical team to help you place it well. For specialised products, rely on the "waterproof mortar" and "mass water-repellent" guides on this blog, then on dedicated suppliers.

Will your structure expose cement to water? Describe the project: we will tell you frankly what our cement covers — free personalized quote, delivered in Dakhla City.