Cement is the binder that makes concrete, mortar and render work. Yet walk into any builders’ merchant and you will find several bags with different designations. Choosing the wrong type will not necessarily cause immediate failure, but it can lead to premature cracking, frost damage or chemical attack — all of which are expensive to put right. This guide explains the four types you are most likely to encounter on UK domestic renovation projects, with their specifications, typical uses and 2026 costs.
CEM I / CEM II — Ordinary Portland Cement
CEM I (formerly labelled OPC — Ordinary Portland Cement) is the workhorse of UK construction. It meets BS EN 197-1 and carries a strength class of 42.5 N or 52.5 N, meaning it achieves that compressive strength at 28 days in N/mm². CEM II blends Portland clinker with up to 35 % of a secondary material such as fly ash (CEM II/B-V) or ground-granulated blast-furnace slag (CEM II/B-S), reducing embodied carbon without meaningfully compromising performance for most domestic work.
CEM I/II is suitable for:
- In-situ concrete foundations, slabs and pads
- General masonry mortars (designations M4 and M6)
- Render and screed
A standard 25 kg bag weighs roughly 0.017 m³ in loose powder form. Mixed at a typical 1:2:4 cement:sand:aggregate ratio for C20 structural concrete, one bag yields approximately 0.011–0.013 m³ of finished concrete. For a 100 mm slab you would therefore need around eight to nine bags per square metre, though in practice ready-mix concrete is more economical above about 0.5 m³.
Rapid-Set and Fast-Setting Cement
Rapid-set cements (sometimes sold under trade designations such as “Postcrete” or “FastSet”) achieve workable strength — typically 10–20 N/mm² — within 15–30 minutes of mixing. They contain calcium aluminate or accelerated Portland clinker blends. BS 8500-1 still governs the concrete mix design; the cement itself must meet BS EN 197-1 or BS EN 14647 (calcium aluminate cement).
Rapid-set products are almost always sold pre-blended with aggregate to fill fence-post holes and small pad foundations — you simply add water in the hole and compact. They are not recommended for:
- Large pours (heat of hydration causes cracking)
- Sulfate-rich ground (unless specified as SR rapid-set)
- Structural elements requiring controlled, supervised placement
Sulfate-Resisting (SR) Cement
Ground conditions in large parts of the UK — particularly clay soils, brownfield land and sites near mining activity — contain soluble sulfates (SO₄²⁻) that attack ordinary Portland cement by forming ettringite, a crystalline compound that expands and fractures concrete over time. BRE Special Digest 1 (2005) classifies ground sulfate exposure as DS-1 to DS-5.
SR cement (CEM I-SR3 or CEM I-SR5) limits the tricalcium aluminate (C₃A) content to 3 % or 5 % respectively, which dramatically reduces ettringite risk. It complies with BS EN 197-1 and is specified under BS 8500-1 for exposure classes XA2/XA3.
For DS-3 and above ground conditions, Building Regulations Approved Document A (foundations) effectively requires either SR cement or the use of DC-3 / DC-4 designated concretes from a ready-mix supplier.
Masonry Cement
Masonry cement is a factory-made blend of Portland clinker, mineral filler (often limestone) and an integral plasticiser or air-entraining agent, conforming to BS EN 413-1. It simplifies site mortar batching: you mix masonry cement with sand only (no separate lime), typically at 1 part cement to 4–5 parts sand to produce a designation M2 or M4 mortar.
The built-in air entrainment gives it improved freeze-thaw resistance and workability compared with a plain cement:sand mix. It is not suitable for structural concrete. Typical uses are:
- Above-DPC brickwork and blockwork in sheltered to moderately exposed locations
- Cavity wall mortar bedding
- General pointing in M2 to M4 designations
Bag Sizes, Coverage and 2026 Costs
| Cement Type | Standard Bag | Approx. Cost/bag (2026) | Typical Mix | Coverage |
|---|---|---|---|---|
| CEM I 42.5 N | 25 kg | £6–£9 | 1:2:4 (concrete) | ~0.012 m³ concrete/bag |
| CEM II/B-V 32.5 R | 25 kg | £6–£8 | 1:2:4 (concrete) | ~0.012 m³ concrete/bag |
| Rapid-set (pre-blended) | 20 kg | £5–£7 | Add water only | 1 bag per 75–100 mm dia. post |
| Rapid-set cement only | 25 kg | £10–£14 | 1:2:3 | ~0.010 m³/bag |
| SR (CEM I-SR3) | 25 kg | £8–£11 | 1:2:4 | ~0.012 m³/bag |
| Masonry cement | 25 kg | £7–£10 | 1:4–1:5 w/sand | ~0.016 m³ mortar/bag |
Prices are merchants’ trade prices in London/South-East; add 15–25 % for retail or remote locations.
Where Each Cement Type Is Used
Concrete foundations (Strip, trench fill, pad): CEM I or CEM II/B, typically ordered as ready-mix designated concrete (GEN3 or RC25/30 depending on reinforcement). On sulfate-affected sites, specify DC-2 to DC-4 with SR cement or GGBS.
Blockwork and brickwork mortar: Masonry cement (1:4) or CEM I with hydrated lime (1:1:6 for designation M2, 1:0.5:4.5 for M4). The lime is critical for flexibility and preventing differential shrinkage cracking.
Render and scratch coats: CEM I or masonry cement at 1:4 to 1:6 (cement:sand). Never use a stronger mix than the substrate — render harder than the background will debond and crack.
Fence posts and small groundwork: Rapid-set pre-blended bag concrete for speed; avoid where frost is forecast within 24 hours of placement.
Chemically aggressive ground: SR cement-based concrete to BS 8500-1 Table A.5, matched to the DS/DC exposure class from a site ground investigation.
Reading a Cement Bag — What the Markings Mean
UK cement bags carry a CE mark and a designation such as CEM II/B-V 32.5 R:
- CEM II — Portland composite cement
- /B — secondary material content 21–35 %
- -V — secondary material is fly ash (pulverised fuel ash, PFA)
- 32.5 — minimum 28-day compressive strength class (N/mm²)
- R — rapid early strength (achieves 10 N/mm² at 2 days)
For most domestic projects CEM II 32.5 R is adequate. Specify 42.5 N or 52.5 N when early striking of formwork is needed or for high-strength structural mixes.
Key Standards
- BS EN 197-1:2011+A3:2024 — Classification and composition of common cements
- BS EN 413-1:2011 — Masonry cements
- BS EN 14647:2005 — Calcium aluminate cement
- BS 8500-1:2023 — Concrete: complementary British Standard — specifying and producing concrete
- BRE Special Digest 1 (2005) — Concrete in aggressive ground, sulfate exposure classes
- Building Regulations Approved Document A (2004, amended 2013) — Strip and trench-fill foundation concrete
[!note] Lime versus masonry cement Many practitioners use masonry cement for convenience, but traditional cement–lime–sand mortars (BS EN 998-2) give slightly better vapour permeability and self-healing of micro-cracks. For conservation or listed building work, lime-based mortars are usually specified to BS 8000-3 workmanship standards.
