CEM I or CEM II? Choosing the right cement type for your project

TS EN 197-1 divides cement into five families according to clinker content and main constituents. CEM I and CEM II are the two most frequently encountered on site. Both can be produced in the same strength class; the difference lies in how quickly strength develops, how much the concrete heats up and how it performs over the long term.

CEM I: Portland cement

CEM I contains at least 95% Portland clinker. The balance consists of minor constituents added during production. Its high clinker content means hydration starts quickly and early strength rises rapidly.

This is important where formwork turnaround controls the programme. Precast production, prestressed concrete and site operations requiring early stripping are natural applications. Rapid early strength is also useful in cold weather because it lowers the period of vulnerability to freezing.

The trade-off is heat. Rapid hydration releases more heat. In a thick foundation, heat accumulates in the core while the outer surface cools; the resulting temperature gradient may cause thermal cracking.

CEM II: Portland-composite cement

In CEM II, part of the clinker is replaced by ground granulated blast-furnace slag, fly ash, natural pozzolan, silica fume or limestone. The constituent is identified by a letter and its proportion by A or B: for example, CEM II/A-S contains 6–20% slag, while CEM II/B-S contains 21–35%.

Early strength develops more slowly, but gain continues beyond 28 days. Lower heat of hydration helps reduce thermal-cracking risk in mass concrete, large raft foundations and hot-weather pours.

The durability difference

Pozzolanic additions react with calcium hydroxide released during hydration and create additional binding phases. This refines the pore structure and reduces permeability, slowing the ingress of chlorides and sulfates. That is why cements with additions are frequently selected for marine structures, sulfate-bearing soils and wastewater facilities.

Four points to consider

• Formwork cycle: CEM I for early stripping; CEM II when the programme is more flexible.
• Section thickness: low heat of hydration becomes a priority in sections over about 80 cm.
• Exposure: consider cement with additions where sulfates, chlorides or seawater are present.
• Season: early strength matters in winter; lower heat is often preferable in summer.

In practice, cement selection must be made together with the concrete mix design. Water-to-binder ratio, admixtures and curing conditions are at least as important as cement type. Share your project conditions with our technical team for a combined type and strength-class assessment.