Curbstones (edge restraints) hold your paved surface in place, but only if they are set in proper concrete. Trass cement (CEM II, pozzolanic cement) is the usual binder because it causes far fewer efflorescence stains than ordinary Portland cement and cures slightly more flexible. To save money, you can blend trass cement with cheaper standard cement. This page covers why, in what ratio, and how much you need.
Why trass cement instead of Portland cement?
- Less efflorescence: Trass binds free calcium hydroxide, which otherwise migrates to the surface as a white haze.
- Safe for natural stone: Granite, basalt and concrete pavers stay free of stains when set with trass mortar.
- Slightly more flexible: The cured mortar is less brittle, which helps at edges that take minor ground movement.
Mix ratio: trass cement + Portland cement + sand
Pure trass cement works well but costs more. A common compromise for curbstone foundations:
| Component | Proportion | Role |
|---|---|---|
| Trass cement (CEM II/B-P) | 1 part | Efflorescence protection, workability |
| Portland cement (CEM I 32.5) | 1 part | Strength, cheaper base binder |
| Sand 0/4 mm | 4 parts | Aggregate, bulk |
That gives a 1 : 1 : 4 mix. If efflorescence must be avoided entirely, for example with light-coloured natural stone, use pure trass cement: 1 : 0 : 4.
Note: The shopping list counts about 40 kg of mortar per 1 m curbstone. That covers a foundation bed of about 10 × 20 cm plus the haunching on both sides.
How much trass cement do I need?
The planner calculates the mortar from the perimeter of your surface:
- Draw your patio or path.
- The shopping list shows the curbstone count and the mortar weight under “Curbstones / edging”.
- In a 1 : 1 : 4 mix, trass cement is one part in six. Divide the mortar weight by 6 to get the trass cement, and the same again for Portland cement.
Worked example: 4 × 3 m patio
- Perimeter: 14 m, so 14 curbstones of 1 m each
- Mortar: 14 × 40 kg = 560 kg
- With the 1 : 1 : 4 mix: about 95 kg trass cement, 95 kg Portland cement and 375 kg sand
- In 25 kg bags: 4 bags of trass cement, 4 bags of Portland cement, sand loose
Use the Share button at the top right of the planner to send the plan as a link, for example to the builders’ merchant or the neighbour helping you.
Practical tips
- Consistency: Mix the mortar earth-damp. It should hold its shape but not flow.
- Foundation: At least 10 cm wide and 15 cm deep beneath the curbstone.
- Haunching: Back-fill both sides of the curbstone to two-thirds of its height.
- Curing: Do not load for 24 hours; cover it when frost is forecast.
Related planner features
- Load class: Pedestrian, Drivable or Tank. It changes the sub-base and therefore the excavation depth.
- Excavation depth: The planner shows how deep to dig (base layer + bedding + paver height).
- Export and share: Save the plan as PNG for the project folder or share a link.
- Optimizer: Finds the joint width and area size with the fewest cuts.
FAQ
Can I skip trass cement entirely?
Yes, pure Portland cement is structurally fine. Expect efflorescence, though, especially on light or dark stones. For concrete curbstones in a garden that is often acceptable; for natural stone it is not.
Where do I buy trass cement?
Any DIY store sells it in 25 kg bags. Sopro, Tubag, quick-mix and PCI sell CEM II/B-P 32.5 as trass cement. For larger quantities, order a pallet through a builders’ merchant.
How long does trass cement keep?
About 6 months, stored dry and frost-free. Use opened bags quickly.