Mortar consumption is first and foremost the volume of the joints, but mixing, suction and method shift the real consumption.
Introduction
The geometric mortar quantity is simple: the proportion of the wall face the bricks don't cover (the joint fraction), multiplied by the wall thickness. That gives the joint volume per m².
Eksempel: med 228×54 mm sten og 12 mm fuge er fugeandelen ca. 22 %. En halvstensmur på ~108 mm giver da ca. 0,22·0,108 ≈ 0,024 m³/m² ≈ 24 L/m² rent fugevolumen, før tillæg.
Why the real consumption is higher
- 01Absorbent bricks draw water and require a little more mortar.
- 02Spillage, hoses, and abandoned parts on the floor are classic causes of accidents. Wipe up immediately and keep walkways clear.
- 03The mix ratio and mortar type change the density and therefore the bag/kg consumption.
Mortar types — lime, cement and KC
Mortar is the mason's second main material alongside stone. The binder determines how strong, flexible and diffusion-open the mortar becomes, and there are three basic types. The choice is not taste — it must fit the stone and the loading.
- 01Pure lime mortar (K) — only slaked lime and sand. Soft, very diffusion-open and slow-curing (absorbs CO₂ from air). Used primarily for restoration of older and listed masonry, where the stone is soft and must be able to breathe.
- 02Pure cement mortar (C) — only cement and sand. Strong, dense, fast-curing and moisture-resistant. Used where there is heavy loading or water: plinths, floors, wells and similar — but is too strong and dense for ordinary façade masonry.
- 03KC mortar (lime-cement) — the compromise, and by far the most used for new masonry. The lime gives workability and diffusion openness, the cement gives strength and set speed.
KC marking — what the numbers mean
A KC notation such as KC 35/65/650 is a recipe: the numbers specify the mixing ratio directly. First number is kg of slaked lime, second is kg cement, and third is kg sand (aggregate). Lime plus cement together give 100 kg of binder — so KC 35/65 means 35% slaked lime and 65% cement in the binder, combined with 650 kg sand.
| Designation | Lime / cement / sand (kg) | Typical application |
|---|---|---|
| KC 60/40/850 | 60 / 40 / 850 | Non-load-bearing masonry, interior plaster |
| KC 50/50/700 | 50 / 50 / 700 | Load-bearing masonry, external rendering |
| KC 35/65/650 | 35 / 65 / 650 | Strong/bearing masonry, external and basement plaster, jointing |
| KC 20/80/550 | 20 / 80 / 550 | Heavily loaded brickwork, foundation zone |
Strength classes M1–M20 (DS/EN 998-2)
Ready-mixed mortar is CE-marked and declared according to DS/EN 998-2 in a compressive strength class: M1, M2.5, M5, M10, M15 or M20 — the number is the guaranteed compressive strength in N/mm² (MPa). An M5 mortar thus has at least 5 MPa compressive strength. The class is determined by testing according to DS/EN 1015-11 (compressive strength of hardened mortar); consistency is measured according to DS/EN 1015-3 and air content according to DS/EN 1015-7.
The standard distinguishes between two ways to specify mortar. Recipe mortar (prescribed mortar) is declared on the recipe itself — e.g. the KC ratio — and the strength follows from the mix. Design mortar (designed mortar), on the other hand, is declared on the property: the manufacturer guarantees an M-class regardless of the exact recipe. For load-bearing masonry according to Eurocode 6, it is the M-class (f_m) that is entered in the compressive strength calculation.
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