Mixing mortar on site

Read the requirement for the mortar's M class in the project, and decide whether to use KC prescribed mortar (mixed on site) or CE-marked designed mortar with declared strength (DS/EN 998-2). Use designed mortar for load-bearing structures and the plinth zone, where strength is critical.

The KC designation «KC x/y/z» gives the mix ratio by weight (dry mass): x kg lime : y kg cement : z kg sand, where lime + cement = 100. Compressive strength of the prescribed mortars according to Mur & Tag (Danish Technological Institute): KC 60/40/850 approx. 0.4 MPa, KC 50/50/700 approx. 0.9 MPa, KC 35/65/650 approx. 2.0 MPa and KC 20/80/550 approx. 4.5 MPa — none of them reaches M5.

If the project requires an M class, use CE-marked designed mortar. Mur & Tag gives e.g. MC1.0–MC2.0 instead of KC 50/50/700 and MC5 instead of KC 20/80/550.

Weigh and measure lime, cement and sand precisely according to Aalborg Portland's mixing table for the chosen mix. Imprecise dosing changes both strength and workability — the mix must be repeatable from tub to tub.

Mix in a mortar mixer in the order sand + lime → cement → water. Add the water gradually until the mortar has the right consistency. If cement and water are mixed too early without sand, it forms lumps.

The mortar must be neither too runny (so it runs off the brick) nor too dry (so it does not adhere). A correct mortar stays on the trowel and can be drawn out into an even strip.

Mark the tub with the mixing time. The normal working time is approx. 1–2 hours and depends on temperature (and on the CE instructions for factory-made mortar). Discard mortar that is starting to stiffen — it must not be ‘revived’ with extra water.

Record the mortar type/recipe in the receiving and process inspection form (x.300/x.400). Site-mixed KC mortar is not CE-marked like factory-made mortar; it must comply with DS/EN 1996-2 and be documented in the QA plan. Where documentation is required: use CE-declared designed mortar.