Cement Types Explained: OPC, Rapid-Set, SR & Masonry

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 occupies roughly 0.017–0.018 m³ as loose powder. Mixed at a typical 1:2:4 cement:sand:aggregate ratio by volume, one bag makes roughly 0.08 m³ of finished concrete, or about 12 to 13 bags per cubic metre, because the mixed concrete occupies only about two-thirds of the loose dry volume. For a 100 mm slab you would therefore need a little over one bag 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 products (sold under trade names such as Postcrete) stiffen far faster than ordinary cement: Blue Circle Postcrete, for example, sets a fence post within 5–10 minutes.¹ They use accelerated Portland cement blends or calcium aluminate cement, and the strength reached at a given age varies by product, so check the bag. BS 8500-1 still governs the concrete mix design; the cement itself is usually made to BS EN 197-1 or, for calcium aluminate cement, BS EN 14647.

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. 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; in UK practice the concrete for sulfate-bearing ground is specified by design chemical class (DC class) under BS 8500-1 and BRE Special Digest 1. Bagged sulfate-resisting cement at UK merchants is not always a CEM I-SR: Breedon’s, for example, is classified CEM III/A 52,5 L (a blast-furnace slag cement).²

For foundations in chemically aggressive ground, Building Regulations Approved Document A points to BS 8500-1 and BRE Special Digest 1, which set the concrete’s DC class and may call for sulfate-resisting cement or a DC-3 / DC-4 designated concrete from a ready-mix supplier.³

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 (iii) mortar, broadly equivalent to strength class M4.

The built-in air entrainment gives it improved freeze-thaw resistance and workability compared with a plain cement:sand mix.⁴ 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

Prices are typical shelf prices including VAT at builders’ merchants and DIY stores; trade prices and pallet quantities are often cheaper. At Wickes in September 2026, a 25 kg bag of Blue Circle General Purpose Cement (a CEM II cement) cost £7.50.⁶ A 20 kg bag of Blue Circle Ready To Use Postcrete cost £7.50 there too.¹ Rapid-hardening cement on its own costs more: one merchant listed a 25 kg bag of Blue Circle Extra Rapid Cement at £18.82 including VAT.⁷ At Builder Depot, a 25 kg bag of Breedon Sulfate Resisting Cement cost £9.78 including VAT.²

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: Approved Document A gives GEN1 or ST2 for unreinforced strip and trench-fill foundations, with an RC grade where the concrete is reinforced.³ On sulfate-affected sites, specify concrete to the site’s DC class, for example with GGBS or a sulfate-resisting cement.

Blockwork and brickwork mortar: Masonry cement with sand (about 1:4 to 1:5) or CEM I with hydrated lime and sand at 1:1:5 to 1:1:6, which Approved Document A gives as the standard mortar for house walls (designation (iii), strength class M4).³ 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, 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.⁸ 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, Cement: composition, specifications and conformity criteria for common cements⁹
• BS EN 413-1:2011, Masonry cements⁴
• 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 classes³
• Building Regulations Approved Document A (2004, amended 2013), Strip and trench-fill foundation concrete³

Lime versus masonry cement

¹⁰ Many practitioners use masonry cement for convenience; the traditional alternative is a cement–lime–sand mortar, and factory-made masonry mortars are specified under BS EN 998-2.¹⁰ For conservation or listed building work, lime mortars are usually specified, and conservation guidance advises keeping the mortar weaker than the masonry around it.¹¹

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.