Portland Cement: What It Is, How to Use It, and How Much You Need
The definitive guide to Portland cement for UK homeowners: CEM I vs CEM II, mix ratios for mortar and concrete, how many bags per m², storage, safety, and current prices.
Builders talk about cement as if everyone knows what it means. Most first-time homeowners arrive on site believing "cement" is the grey stuff that ends up as the hard floor of their extension. It isn't. Cement is a powder. It's the binding agent that, mixed with sand, aggregate, and water in different proportions, becomes mortar, concrete, or screed. Buy the wrong type, mix it wrong, or use bags that have been sitting in your garage for eight months and you'll discover the distinction the expensive way.
What it is and what it's for
Portland cement is a grey powder manufactured by grinding limestone and clay together, heating the mix to around 1450°C in a rotary kiln (the resulting material is called clinker), and then grinding the clinker with small amounts of gypsum to control how fast it sets. When you add water, a chemical reaction begins (hydration) and the paste hardens into a rigid, rock-like solid. This is not drying. It's a chemical reaction. You can lay mortar on a cold, damp day and it will still cure, provided the temperature stays above 2°C. But if the mix freezes before that reaction completes, the ice crystals physically disrupt the microstructure and the mortar or concrete will crumble.
Portland cement on its own is never used in construction. You always mix it with aggregates. The ratio of cement to aggregate determines the strength and workability of the final material:
- Mortar (cement + building sand) is used to bed bricks and blocks, point joints, apply render, and lay floor screed.
- Concrete (cement + sharp sand + coarse aggregate) is used for foundations, bases, lintels, and poured floors.
- Screed (cement + sharp sand, no coarse aggregate) is used to level and finish floors ahead of tiling or flooring installation.
The standard for all Portland cement sold in the UK is BS EN 197-1. Any compliant bag will reference this standard and carry a UKCA mark.
CEM I vs CEM II: what you actually buy at the merchants
This is the piece most guides skip, and it catches people out. Walk into Wickes, B&Q, or any builders' merchant and pick up a bag labelled "General Purpose Cement." The product inside is almost certainly CEM II/A-LL (Portland-Limestone Cement), not pure Portland cement (CEM I). That distinction matters if you're trying to specify correctly, even if it makes very little difference in practice for domestic work.
BS EN 197-1 defines five families of cement:
| Type | What it contains | Strength class | Common UK product | When to use |
|---|---|---|---|---|
| CEM I | 95-100% clinker, max 5% other minerals | 42.5N or 52.5N | OPC (Ordinary Portland Cement) - usually a premium product | Where specifically specified by an SE. Sulphate-resistant applications (requires SR cement, see below). High early-strength needs. |
| CEM II/A-LL | 80-94% clinker + 6-20% limestone filler | 32.5R | Blue Circle GP, Hanson GP - the standard bag at every retailer | All domestic mortar and concrete work. Blockwork, bricklaying, screeds, footings. Performs identically to CEM I for these uses. |
| SR (Sulphate Resistant) | CEM I or CEM III/A base with low C3A | 42.5N | Blue Circle Sulfacrete, Tarmac SR | FND3 and FND4 ground conditions where sulphate attack is a risk. Your SE will specify if required. |
For a kitchen extension, loft conversion, or garden room, the bag of GP cement from any retailer is what you need. It's CEM II/A-LL and it will meet BS EN 998-2 (the mortar standard) and BS 8500-2 (the concrete specification standard) for all domestic applications. The only time you specifically need to request CEM I (OPC) is when your structural engineer has called it out by name, which for domestic work almost always means sulphate-resistant conditions.
Mix ratios for every application
Water quantity is the thing nobody can give you a precise number for, because it depends on how damp your sand is, how warm the weather is, and how your particular mixer works. The right consistency for mortar is "peanut butter that drops off a trowel in one clean motion." The right consistency for concrete is "porridge that slides to fill a void when tamped, without being sloppy." Add water gradually. Stop when it looks right. Too much water is the single most common mixing error, and it doesn't just make the mix harder to work with - it physically weakens the cured result.
A rough starting point for water is around 0.5 litres per kilogram of cement, but treat that as a starting point, not a rule.
Mortar
Mortar strength must match the bricks or blocks being laid. Too strong a mortar on soft Victorian bricks causes the brick face to spall (flake off) over time as thermal movement can't be accommodated. Too weak a mortar in an exposed or below-DPC application means crumbling joints and water ingress.
| Application | Mix ratio (cement:sand) | Notes |
|---|---|---|
| General blockwork above DPC | 1:5 or 1:6 with plasticiser | Adding hydrated lime (1:1:5 or 1:1:6) improves workability |
| Below DPC / exposed masonry | 1:3 or 1:4 | Stronger mix, no lime substitution |
| Engineering brick courses (DPC level) | 1:3 | NHBC requires minimum M4 designation below DPC |
| Pointing - hard modern bricks | 4:1 sand:cement with plasticiser | Slightly richer than bedding mortar |
| Pointing - soft period bricks | Use hydraulic lime, not Portland cement | Cement mortar will cause spalling |
| Render scratch coat | 1:4 (1 part cement, 3 parts rendering sand + 1 part building sand) | |
| Render float coat | 1:5 with plasticiser |
The NHBC requirement for below-DPC mortar is M4 designation (1:3 cement:sand) minimum. Anything softer and your NHBC inspector will flag it as a defect. Do not add lime to below-DPC mixes - lime softens mortar and reduces its resistance to water.
Concrete
Concrete ratios follow the cement:sand:coarse aggregate pattern. UK practice uses either a three-part ratio (cement:sharp sand:20mm aggregate) or a two-part ratio using pre-mixed all-in ballast (cement:ballast).
| Application | Mix ratio | Approx strength | Notes |
|---|---|---|---|
| Non-structural paths | 1:3:6 (M10) | ~10 MPa | Light use only |
| Domestic floor slabs, footings | 1:2:4 (M15) | ~15 MPa | Standard domestic specification |
| Driveways, foundations | 1:1.5:3 (M20) | ~20 MPa | Required for most extension strip foundations |
| Structural elements, heavily loaded | 1:1:2 (M25) | ~25 MPa | SE-specified; rare in domestic work |
| Using ballast (all-in aggregate) | 1:5 cement:ballast | ~M15 | Equivalent to 1:2:4, just pre-blended aggregate |
For an extension strip foundation, M20 (1:1.5:3) is the standard specification. Your structural engineer's drawings will state the required mix or concrete grade. Don't assume M15 is sufficient if the drawings say C20 or C25 - those are the same as M20 and M25 but referenced under the BS 8500 designation system.
Ready-mix concrete is almost always the right choice for foundation pours over about 0.5m3. Hand-mixing 1m3 of concrete requires roughly 14 bags of 25kg cement plus the sand and aggregate. That's a very large quantity of physical mixing. For anything over half a cubic metre, call a ready-mix supplier.
Screed
Sand-and-cement screed for a floor uses a drier, stiffer mix than mortar. The "droppings off a trowel" test doesn't apply; screed should be a firm, crumbly-damp mix that holds its shape when squeezed in a fist. Too wet a screed shrinks as it cures and cracks.
Standard mix: 3:1 or 4:1 sharp sand:cement (not building sand - building sand is too fine). A 4x6m floor at 65mm depth requires approximately 1.56m3 of screed material (4 x 6 x 0.065), which works out to around 16-20 bags of 25kg cement plus sharp sand at a 4:1 mix.
How many bags do you need
Each 25kg bag of general purpose cement yields approximately 0.012m3 of concrete when mixed at 1:2:4, or covers approximately 1.5-2m2 of mortar joints at 10mm depth.
For extension tasks, the reference quantities from the brief are:
- Blockwork for a full extension (walls and blockwork): approximately 50-90 bags for bedding mortar. The wide range reflects extension size and wall height. Budget 1 bag per 35-45 blocks laid (standard 440x215mm blocks at 10mm joints).
- Screeding a 4x6m floor (65mm screed thickness): 16-20 bags.
- Below-DPC course (engineering brick DPC layer): 2-4 bags.
- Steels and lintels (mortar packing around beam bearings): 1-2 bags.
- External works (paving, steps, pointing): 5-10 bags depending on scope.
For concrete quantities, the rule of thumb: 1m3 of finished concrete at M20 (1:1.5:3) requires approximately 16 bags of 25kg cement, 0.45m3 of sharp sand, and 0.9m3 of 20mm aggregate. At M15 (1:2:4), it's about 14 bags of 25kg cement per m3.
If in doubt, overestimate slightly. A bag of cement left over costs less than a special re-delivery to top up a pour that's run short.
Cost and where to buy
General purpose cement (CEM II/A-LL) retails at £6.50–8.50 per 25kg bag inc. VAT at national retailers. Trade pricing through builders' merchants runs lower. Buying a full pallet of 60 bags brings the unit cost down further.
Blue Circle (Tarmac) and Hanson (Heidelberg Materials) are the two brands you'll find at most UK retailers. Wickes stocks Blue Circle GP. Jewson carries Heidelberg Materials (previously labelled Hanson). Travis Perkins stocks both. For a full extension's worth of mortar (50-90 bags), buying by the pallet makes sense if you have storage space. Call your local builders' merchant and ask about pallet pricing.
Jewson list prices are typically the highest among national merchants but are not representative of what trade account holders pay. If you're building an extension, open a trade account with your local merchant before placing any orders. The savings across all your materials add up quickly.
Sulphate-resistant cement (sold as "SR cement" or "Sulfacrete") costs more than standard GP cement. Don't buy it unless your structural engineer has specified it. Ground conditions requiring SR cement are identified by soil testing against BRE Special Digest 1. In many parts of the UK, standard cement is perfectly adequate. In areas with sulphate-bearing ground (certain clay and brownfield sites), using standard cement in below-ground concrete will cause it to expand and crack over time.
Storage and shelf life
Paper bags: 3 months. Plastic-wrapped bags stored completely dry: up to 6 months. That's the practical rule.
Beyond that point, two things happen. First, the chromium VI reducing agent added to comply with REACH regulations (European chemical regulation carried over into UK law, restricting hexavalent chromium to a maximum of 2mg/kg in hydrated cement) degrades. The expiry date stamped on the bag is largely about this - cement past its expiry date may have elevated chromium VI levels, which is a health risk during skin contact, not just a quality issue.
Second, cement absorbs moisture from the air even through an unopened paper bag. The particles begin to hydrate and micro-lump. The bag feels fine from the outside but the powder inside has partially reacted and will give reduced strength.
Before using any cement that has been stored for more than two months, squeeze the bag. If it feels firm in patches, open it. Check for lumps. Free-flowing powder with no lumps: fine to use. Lumps that break apart easily under pressure: use for non-structural applications only (pointing, patio bedding) but not for structural concrete or below-DPC mortar. Hard lumps that won't break: the bag is ruined. Don't use it for anything. Cement with hard lumps will not re-hydrate correctly regardless of how much you mix it.
Store bags on a pallet or timber boards, never directly on a concrete floor. Ground moisture wicks up through paper bags faster than most people expect. Cover with a tarpaulin and keep in the most sheltered, dry location on site. If storing indoors, ensure ventilation so condensation doesn't form. Bags stacked more than five high become difficult to rotate (last in, last out), so the bottom bags age faster. Keep stock shallow and use oldest bags first.
Health and safety
Wet cement is highly alkaline, with a pH of 12-13. Skin contact causes both irritant contact dermatitis (chemical burn from the alkali) and, with prolonged exposure, allergic contact dermatitis from chromium VI. The chromium VI effect is cumulative: people who work with cement for years can develop a severe, lifelong allergy. You don't get a warning before the allergy develops.
The HSE's guidance is unambiguous. Wear waterproof gloves for any prolonged wet cement contact. The gloves must be rated for high-pH (alkaline) substances and must fit at the wrist so cement can't be trapped inside. Loose gloves are as bad as no gloves. For larger pours (foundations, floor slabs), wear waterproof trousers and wellington boots. Cement trapped against skin by clothing causes burns that may not appear for several hours.
Cement burns are delayed. You won't feel them immediately. A worker who spends two hours with wet concrete on the floor kneeling without knee boards may not notice the burn until that evening or the next morning. By then, the damage is done. If cement has contacted skin, irrigate with water for at least 20 minutes and remove contaminated clothing immediately. Deep cement burns require medical attention.
For mixing, cement dust is a serious respiratory hazard. When opening bags or mixing dry, wear a P3 dust mask (not a basic disposable paper mask). Silica in the sand and crystalline phases in the cement particles cause progressive lung damage that doesn't show up clinically for years.
Common mistakes
Adding too much water. Every forum thread about weak mortar or crumbling concrete points here. More water makes the mix easier to work. It also reduces strength, increases shrinkage cracking, and compromises durability. Add water gradually, stop when the mix is workable, and don't be tempted to add more just because the mix stiffens five minutes later (it's probably fine - stiffening on the trowel is not the same as going off).
Using old bags without checking. Hard lumps in a bag of cement are not a minor problem. That cement will not perform at full strength regardless of how thoroughly you mix it. Using lumpy cement in structural concrete or below-DPC mortar means the finished work is weaker than specified. If building control later requires a core sample or the NHBC inspector queries mix quality, there is no good answer.
Getting the mortar mix wrong for the application. A 1:6 mix that's fine for above-DPC blockwork is inadequate below the DPC. A 1:3 mix that's correct for below-DPC work is too strong for pointing soft period bricks and will cause spalling. The ratio isn't just about strength, it's about compatibility with the surrounding materials.
Hand-mixing volumes that need a mixer. One person with a spade can realistically mix three to four bags of cement by hand per hour. At that rate, mixing the mortar for a full day's bricklaying (6-8 bags) is a full morning's work before any bricks get laid. Hire a cement mixer. The day rate is around £25-40. The labour cost of hand-mixing makes it the most expensive option once you account for the time.
Re-tempering mortar that has started to set. Once mortar begins to stiffen on the board (typically after 1.5-2 hours in normal conditions), adding more water to bring it back to workability is called re-tempering. It looks like it works. The mix becomes fluid again. But the cement particles have already begun to hydrate and the structural bonds that form during curing will be weaker. Re-tempered mortar is non-compliant under BS EN 998-2. Mix only what you can use in 1.5-2 hours, and bin what's left.
Alternatives
Premixed dry mortar (sold in bags as "ready-mixed mortar") contains cement, sand, and often a plasticiser pre-blended in the correct ratios. You add water and mix. It costs more per square metre of coverage than mixing from scratch, but it eliminates the ratio guesswork and produces a consistent result, which matters on smaller jobs where inconsistent batching would show up as colour variation.
For laying period brickwork, repointing Victorian and Edwardian masonry, or any work on a listed building, Portland cement mortar is the wrong material. The correct choice is hydraulic lime or natural hydraulic lime (NHL). Cement mortar is too strong and too rigid for soft historic bricks, causing spalling as the brick face cracks under thermal movement that the hard mortar can't accommodate. This is a specific, separate topic covered in the lime mortar guide.
Where you'll need this
Portland cement appears at multiple phases of any extension or renovation involving masonry, concrete, or floor finishes:
- Walls and blockwork - bedding mortar for all brick and block courses; approximately 1 bag per 35-40 units laid, 50-90 bags for a complete extension
- Damp proof course - stronger 1:3 mix for engineering brick DPC courses; 2-4 bags
- Steels and lintels - mortar packing around beam bearing points; 1-2 bags
- Screeding - sand-and-cement screed at 3:1 or 4:1 sharp sand:cement; 16-20 bags for a 4x6m floor
- Garden and external works - mortar and concrete for paving, steps, and external walls; 5-10 bags
Cement appears across groundwork, structure, and second-fix phases of any project involving masonry or poured concrete. It's rarely a large proportion of project cost on its own, but it's a material where poor purchasing decisions (wrong type, expired stock, under-ordering) have consequences that are time-consuming and expensive to fix.