As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
For most cast-in-place concrete water tanks, the best choice is a general-purpose structural cement such as ASTM Type I/II portland cement or Type IL portland-limestone cement, used in a properly designed concrete mix. Type I/II is especially useful where moderate sulfate resistance is wanted. Type IL is also widely used for general concrete construction, including tanks and reservoirs.
The cement name alone does not make a tank strong or watertight. The concrete mix, reinforcement, wall thickness, joints, placement, curing, foundation, and crack control usually matter more.
Cement vs. Concrete in a Water Tank
Cement and concrete are not the same thing.
Cement is the fine powder that reacts with water and binds the mix together.
Concrete normally contains:
- Cement
- Sand
- Gravel or crushed stone
- Water
- Sometimes supplementary cementitious materials and admixtures
A water tank should normally be built from reinforced structural concrete, not from cement mortar alone.
This matters because adding extra cement does not automatically produce better concrete. A poorly proportioned mix can shrink and crack even if it contains a lot of cement.
For a tank, the goal is dense, durable concrete with controlled cracking and properly sealed joints.
Best Cement Choices for a Concrete Water Tank
Type I/II Portland Cement
Type I/II is a practical choice for many concrete tanks.
It meets the requirements for both general-purpose Type I cement and Type II cement. ASTM describes Type II as a general-use cement with moderate sulfate resistance.
It can be a good choice for:
- Above-ground water tanks
- Underground cisterns
- Rainwater storage tanks
- Irrigation tanks
- Household non-potable storage
- Tanks where the surrounding soil has some sulfate exposure
Do not assume Type I/II is suitable for every soil condition. Severe sulfate exposure may require a different concrete specification.
Type IL Portland-Limestone Cement
Type IL is another good general-purpose option where it is locally available.
Type IL is a blended hydraulic cement made with portland cement and finely ground limestone. It is covered by ASTM C595 and is now widely used for general concrete construction. The American Cement Association lists tanks and reservoirs among normal applications for general-purpose cement.
For a normal water tank, there is usually no reason to reject Type IL simply because it is not traditional Type I portland cement. The important question is whether the complete concrete mix meets the structural and durability requirements of the tank.
Type II or Sulfate-Resistant Cement
Sulfates can occur naturally in some soils and groundwater. They can attack unsuitable concrete over time.
Where sulfate exposure is known to be a concern, the concrete specification may call for cement with moderate or high sulfate resistance.
ASTM Type II provides moderate sulfate resistance. Type V is intended for high sulfate resistance. Blended cements can also carry moderate-sulfate or high-sulfate designations.
Do not choose Type V automatically for an underground tank. Soil conditions, groundwater, the complete concrete mixture, and local structural requirements should determine what is needed.
For a large buried cistern or a site with known aggressive soil or groundwater, have the exposure conditions evaluated before choosing the mix.
What Matters More Than the Cement Type
A good cement can still produce a leaking tank if the rest of the concrete work is poor.
Liquid-containing concrete structures have stricter demands than an ordinary patio or walkway. ACI 350 specifically covers concrete structures used to store or convey water and other liquids, with special attention to durability and liquid containment.
Several factors deserve as much attention as the cement.
Concrete Strength
Water creates continuous pressure against the walls and floor of a tank.
ACI 350-20 specifies a minimum concrete compressive strength of 4,000 psi for structures within its scope. That does not mean every small residential tank everywhere is legally required to use exactly 4,000 psi concrete. Local plans and codes control your project. It does show that liquid-containing structures are normally treated as serious structural concrete work rather than ordinary low-strength flatwork.
For a structural tank, order or design the concrete for the tank rather than buying a generic concrete mix simply because it is convenient.
Water in the Concrete Mix
Too much mixing water can make concrete easier to place, but it can also leave the hardened concrete more permeable and can contribute to shrinkage and durability problems.
Do not add water at the job site just to make a stiff mix easier to pour unless the mix supplier or concrete specification allows it.
If better workability is needed, the concrete supplier can design the mix appropriately and may use a suitable water-reducing admixture.
Reinforcement
Concrete is strong in compression but much weaker when pulled apart.
Steel reinforcement helps control the stresses that can cause a tank wall or floor to crack.
Tank reinforcement must account for:
- Water pressure
- Soil pressure on buried tanks
- Temperature changes
- Concrete shrinkage
- Loads above the tank
- Connections between floors and walls
- Seismic loads where applicable
A large cistern is not a good place to guess at reinforcing-bar size and spacing.
Joints
Many tank leaks occur at joints rather than through otherwise sound concrete.
Important locations include:
- Floor-to-wall joints
- Wall construction joints
- Pipe penetrations
- Corners
- Changes in wall thickness
Liquid-containing concrete construction often uses carefully designed joints and waterstops to control leakage. ACI guidance for environmental concrete structures specifically addresses joint detailing and water containment.
A waterproof coating applied later should not be treated as a substitute for proper joint design.
Curing
Concrete needs time and moisture to cure properly.
Poor curing can increase surface cracking and reduce durability. This is especially important in a tank because even small cracks can become leakage paths.
Protect freshly placed concrete from drying too quickly, freezing, excessive heat, and physical damage. Follow the concrete specification and curing method selected for the project.
Is Waterproof Cement Needed?
Usually, no special product called "waterproof cement" is required to build the main structure of a concrete tank.
Regarding key facts about the minimum grade of concrete required for a water tank, evaluate tank venting and overflow control during extended storage.
A well-built tank gets most of its water resistance from:
- Dense concrete
- Controlled water content
- Correct reinforcement
- Proper consolidation
- Good curing
- Well-designed joints
- Properly sealed penetrations
- Crack control
Waterproofing admixtures or surface treatments may be included in some tank designs, but they do not fix poor structural concrete.
Be especially cautious with products sold as hydraulic cement.
Hydraulic cement is a broad term for cement that sets through a reaction with water. Some products sold under that name are fast-setting repair materials intended for plugging holes or active leaks. They are not necessarily suitable for casting an entire structural water tank.
Check the intended use rather than choosing a product because the package says "waterproof."
What About Drinking-Water Tanks?
A concrete tank that holds drinking water needs additional planning.
Potable means water intended to be safe for drinking. Non-potable water is water that is not intended for drinking, such as rainwater used for irrigation or some other approved household uses.
Cement choice also depends on whether repairing a concrete water tank from the outside can reach and seal the actual crack.
Do not assume that a concrete rainwater tank is suitable for drinking water simply because the walls do not leak.
For potable storage, consider the complete system:
- Roof and collection surface
- Gutters and debris removal
- First-flush or other runoff management
- Tank materials
- Pipe and fitting materials
- Sealants and coatings
- Water treatment
- Ongoing cleaning
- Current laboratory water testing
- Local drinking-water and plumbing requirements
NSF/ANSI/CAN 61 addresses health effects for materials and components that contact drinking water. NSF listings include some cement products for specified drinking-water-contact applications, but certification applies to the particular listed material and conditions rather than to every cement of the same general type.
If a concrete tank will supply drinking water, verify the water-contact materials and treatment requirements with the relevant local authority and, where needed, a qualified water-treatment professional.
Above-Ground vs. Underground Tanks
The best concrete specification can change with tank location.
Above-Ground Tank
An exposed tank may face:
- Daily temperature swings
- Direct sun
- Freezing weather
- Wind
- Wetting and drying
In a freezing climate, the concrete needs to be designed for the expected freeze-thaw exposure. The required air-entrainment and other durability details should be part of the concrete specification rather than improvised at the mixer.
Underground Tank
A buried tank must handle conditions from both directions.
Water pushes outward when the tank is full. Soil and groundwater can push inward or upward, especially when the tank is empty.
The design may need to account for:
- Soil loads
- Groundwater
- Buoyancy
- Sulfate exposure
- Vehicle loads above the tank
- Drainage
- Access openings
Do not assume a buried tank can safely be made by simply making the walls thicker.
Structural design becomes especially important with large tanks, deep burial, unstable soil, high groundwater, or vehicle traffic above the tank.
Should You Use Concrete Blocks Instead?
Concrete masonry blocks can be used in some small storage structures, but a block wall is not automatically a watertight water tank.
Mortar joints, reinforcement, wall filling, structural loads, and waterproofing all need to be considered.
For a substantial cistern, a properly engineered reinforced-concrete shell is usually easier to design as a continuous liquid-containing structure than an improvised block enclosure.
If you already have a block tank that leaks, do not assume coating the inside will solve structural movement or cracking.
A Practical Way to Choose the Concrete
For a typical residential concrete rainwater tank, start with these questions:
| Question | Why It Matters |
|---|---|
| Is the tank above or below ground? | Changes soil, groundwater, temperature, and structural loads |
| How large is the tank? | Larger walls and longer spans increase structural demands |
| Will the water be potable? | Changes material and water-quality requirements |
| Does the site have sulfate-rich soil or groundwater? | May change the cement and concrete exposure specification |
| Does the climate freeze? | Freeze-thaw durability may be required |
| Are pipes cast through the walls? | Penetrations need proper sealing and detailing |
| Will vehicles travel over it? | The tank roof may need substantial structural design |
| What do local rules require? | Structural and potable-water requirements vary |
For many ordinary tanks, Type I/II or Type IL cement in professionally proportioned structural concrete is a sensible starting point. Then adjust the concrete specification for the actual exposure and structural conditions.
For a ready-mix order, it is better to tell the supplier that the concrete is for a liquid-containing reinforced concrete tank and provide the engineer's specification than to order concrete based only on a cement type.
When a Water Tank Stops Being a Simple DIY Concrete Job
A small decorative water feature is very different from a buried cistern holding thousands of gallons.
Water is heavy. One U.S. gallon weighs about 8.34 pounds, so large tanks place substantial loads on the floor, foundation, and supporting soil.
Professional structural design is worth considering when the tank is:
- Large
- Buried
- Supporting a building or vehicle load
- Located on unstable soil
- Exposed to high groundwater
- In a seismic area
- Connected to household plumbing
- Intended for potable water
Excavation also creates separate safety risks. Deep unsupported excavations and confined tank interiors should not be treated as routine DIY work.
Frequently Asked Questions
Is Type I Portland cement good for a water tank?
Yes. Type I is a general-purpose portland cement and can be used in concrete tanks when no special cement properties are required. However, the complete concrete mix, reinforcement, joints, placement, and curing determine how well the tank performs.
Is Type I/II better than Type I for a water tank?
Type I/II can be a useful choice because it provides general-purpose performance while also meeting Type II requirements for moderate sulfate resistance. Whether that extra sulfate resistance is needed depends on the tank's exposure.
Can Type IL cement be used for a rainwater tank?
Yes, Type IL portland-limestone cement is used for general concrete construction and can be suitable for tanks when the complete concrete mixture meets the project's structural and durability requirements.
Does adding more cement make a tank waterproof?
No. More cement does not automatically make concrete watertight. Water content, aggregate proportions, consolidation, reinforcement, joints, curing, cracking, and workmanship all affect leakage.
Do I need waterproofing on the inside of a concrete tank?
Not every properly designed concrete tank requires the same internal coating. Some systems use coatings or liners, while others rely mainly on dense concrete and carefully detailed joints. If the stored water will be potable, any material in contact with the water must also be suitable for that use.
Can I use hydraulic cement to build a whole water tank?
Do not assume so. Many products sold as hydraulic cement are fast-setting repair or patching materials. Use structural concrete designed for liquid containment unless a particular material is specifically specified for constructing the tank.
What cement should I use in sulfate-rich soil?
The required concrete may use Type II, Type V, or a blended cement with an appropriate sulfate-resistance designation, depending on the severity of exposure. Have known aggressive soil or groundwater conditions evaluated rather than selecting the cement by guesswork.
What is the best cement for a potable concrete water tank?
There is no single cement type that makes a tank potable. A general-purpose structural cement may be appropriate, but drinking-water suitability depends on the entire collection, storage, treatment, plumbing, and maintenance system. Water-contact materials should meet applicable local requirements, and drinking-water safety should be verified with suitable treatment and current laboratory testing.
