Cement is one of the most widely used construction materials in the world. It is an integral component of concrete, mortar, stucco and other building materials. Cement is a binding agent that, when mixed with water, sand and aggregates, binds them together to form concrete. While concrete is undoubtedly waterproof, cement in its pure form is not entirely waterproof. Let’s take a closer look at whether pure cement is waterproof.
What is Cement?
Cement is a fine, grey powder that is made by grinding a mixture of limestone, clay, silicon dioxide, iron oxide and aluminum oxide and heating it to very high temperatures. The key ingredients that make up cement are:
- Calcium oxide (CaO) – Also known as lime. This is obtained by heating limestone to high temperatures in a kiln.
- Silicon dioxide (SiO2) – Also known as silica. This is found in clay and sand.
- Aluminum oxide (Al2O3) – Also known as alumina. This is found in bauxite ores.
- Ferric oxide (Fe2O3) – Also known as iron oxide. This gives the cement its usual grey color.
These core ingredients are proportioned, grounded and blended to form a fine cement powder. Some amount of gypsum is also added during grinding to control the setting time of cement. The four main types of cement are:
- Portland Cement – The most common type used in general concrete construction.
- White Cement – Used when a light color is required.
- High Alumina Cement – Formulated for use in specialized applications.
- Quick Setting Cement – Sets faster but has lower strength than traditional cement.
Chemical Composition of Cement
The typical chemical composition of cement is:
| Compound | Formula | Percent by Weight |
|---|---|---|
| Tricalcium silicate | Ca3SiO5 or 3CaO.SiO2 | 50% |
| Dicalcium silicate | Ca2SiO4 or 2CaO.SiO2 | 25% |
| Tricalcium aluminate | Ca3Al2O6 or 3CaO.Al2O3 | 10% |
| Tetracalcium aluminoferrite | Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3 | 10% |
| Gypsum | CaSO4.2H2O | 5% |
When cement is mixed with water, a series of complex chemical reactions called hydration takes place between the compounds in cement and water. The products of this hydration process are what give strength and binding properties to concrete and mortar.
Is Pure Cement Waterproof?
Now coming to the key question – is pure cement waterproof? The short answer is no, cement in its pure form is not waterproof. Here’s why:
- Cement particles have voids and pores that allow water to pass through when cement is in its dry powder form.
- During the hydration process, the calcium silicate compounds in cement react with water to form calcium silicate hydrate gel and calcium hydroxide. These products fill up the pores, making concrete water-resistant.
- But cement on its own does not contain aggregates like sand and stone that are necessary to make concrete. So it cannot undergo adequate hydration and has pores that allow water to seep through.
- Pure cement is also brittle and can develop cracks when dried, allowing water ingress.
So while concrete can be made waterproof by proper mixture design, selection of low porosity aggregates and correct curing, pure cement lacks the physical and chemical composition to be completely waterproof. It readily absorbs moisture when exposed to water.
Factors that Influence Water Permeability of Cement
While pure cement is not waterproof, some factors can influence the degree of permeability and water absorption in cement to a certain extent:
Fineness of Cement
Finer cement particles have lower permeability compared to coarse particles. Higher fineness leads to higher strength development in concrete, reducing pores.
Ratio of Compounds
Higher ratio of tricalcium silicate results in higher strength. Tricalcium aluminate reacts quickly with water, refining pores.
Hydration Level
Higher hydration of compounds fills up capillary pores, reducing permeability.
Water-Cement Ratio
Lower water-cement ratio leads to low porosity and permeability.
Compaction
Proper compaction of cement reduces pores and leakage.
Curing
Adequate moist curing hydrates cement compounds and reduces capillary pores.
While the above factors reduce permeability of cement to some degree, it does not make the cement completely impervious to water on its own without aggregates.
How to Make Cement Waterproof?
Although plain cement is not waterproof, there are certain ways in which cement can be made water-resistant:
Addition of Waterproofing Admixtures
Waterproofing admixtures such as crystalline waterproofing chemicals can be added to cement. These chemicals react with moisture and the byproducts block capillary pores in cement.
Hydrophobic Cement
Hydrophobic agents can be introduced during cement grinding to make it water repellent. But this does not completely prevent water ingress under pressure.
Polymer Modified Cement
Polymers like latex can be added to cement to make it more impermeable. However, only up to 5% polymer can be practically added.
Surface Treatments
Cement surface can be coated with waterproofing paints, slurries and membranes to prevent water contact with the pores.
However, the best way to utilize cement’s waterproofing potential is to use it to create concrete. The aggregates bind the cement particles tightly, reducing pores. The hydration process also fills up pores in concrete, making it impermeable. With proper concrete mix design and construction practices, a waterproof concrete structure can be built using cement as a key ingredient.
Properties of Waterproof Concrete
Unlike pure cement, concrete can be made waterproof by following certain guidelines:
- Low water-cement ratio of 0.45 or less
- High cement content of 350 kg/m3 or more
- Low porosity aggregates like granite
- Pozzolanic materials like fly ash to reduce pores
- Careful compaction to avoid honeycombing
- Adequate curing for minimum of 14 days
- Low permeability less than 10 x 10-8 cm/sec
- High strength above 35 MPa
Such waterproof concrete has very low capillary pores making it impermeable even under hydrostatic pressure. It prevents seepage of water and is suitable for underwater structures, water tanks, basements, etc.
Waterproofing Admixtures for Concrete
Special waterproofing admixtures can further enhance the waterproofing properties of concrete by chemically reacting with the concrete pore structure. Some common admixtures are:
- Crystalline waterproofing admixtures – React with moisture and chemicals in concrete to form insoluble crystals that block pores.
- Hydrophobic admixtures – Coat pore walls with water-repellent film while allowing vapour transmission.
- Flexible sealant admixtures – Seal micro-cracks and capillary pores.
Such specialty admixtures make the concrete impermeable to water ingress even under high hydrostatic pressure.
Applications of Waterproof Cement and Concrete
Although cement on its own has limited waterproofing properties, waterproof cement products and concrete are extremely useful in a variety of applications:
Water Tanks and Reservoirs
Waterproof concrete is used to build storage tanks and reservoirs for water. This prevents loss of precious water through leakage or seepage.
Bridges and Dams
Underwater parts of bridges and dams use waterproof concrete to prevent flow of water through the concrete. Dams in particular need to withstand immense hydrostatic pressure.
Basements
Basements are prone to water ingress due to underground water table levels. Waterproof concrete construction prevents leakage into basements.
Underground Structures
Tunnels, underground railways, pipelines and other underground structures need to use waterproof concrete to stop water ingress.
Swimming Pools
Swimming pools contain huge volumes of water. Waterproof pool concrete prevents loss of water and damage to structures.
Water and Wastewater Treatment Plants
Concrete tanks and channels in such facilities handle water and corrosive effluents. Waterproofing is vital to contain liquids and prevent deterioration of concrete.
Coastal Structures
Marine structures like ports, jetties and sea walls are exposed to seawater. Waterproof concrete prevents corrosion and deterioration due to saltwater.
Conclusion
To summarize, pure ordinary cement is not waterproof as it has pores and capillaries that allow water ingress. Cement on its own lacks the physical and chemical composition to be impervious unlike concrete. However, measures like waterproofing admixtures, hydrophobic agents and surface treatments can enhance water-resistance of cement to some degree. The most effective way to utilize cement’s waterproofing capabilities is by using it to create high performance waterproof concrete through proper mix design, ingredients and construction practices. With the right techniques, concrete made from cement can be made completely waterproof for various applications even under high hydrostatic pressures. So while plain cement may not be waterproof, the versatility of cement-based concrete is unmatched when it comes to building watertight structures.