Back walls and light partition walls are today often built in light brick or pore concrete blocks with thin joint/glue mortar. It is a different technique than brick with KC mortar: first course in mortar, blocks glued with thin joint, tight tolerances and dust considerations when cutting dry.
Introduction
Much of the backing wall and the light partitions in modern construction are not built in brick with mortar, but in large lightweight concrete blocks that are glued together with a thin joint. It is faster — the blocks are large — but it is also a different craft technique with its own requirements for tolerances and dust. Two material families dominate: expanded clay concrete and autoclaved aerated concrete.
To bloktyper — DS/EN 771-3 og 771-4
Lightweight concrete blocks made from aggregate (light clinker, e.g. Leca) are declared according to DS/EN 771-3. They are porous and robust and are used for back wall, foundation blocks and load-bearing walls. Autoclaved aerated concrete (AAC, e.g. Ytong/H+H Celcon) is declared according to DS/EN 771-4. Aerated concrete is very light and insulating and can be sawed with a hand saw. Both are CE-marked with a DoP where density, compressive strength, stone group and tolerance category are shown.
| Block type | Standard | Grade |
|---|---|---|
| Lightweight clinker concrete (aggregate) | DS/EN 771-3 | Lightweight clinker aggregate (Leca); robust backing wall and foundation blocks |
| Autoclaved aerated concrete (AAC) | DS/EN 771-4 | Very light, insulating; saws easily, for light partition walls and backing |
Thin joints and adhesive mortar
The blocks are assembled with thin joint mortar (bond mortar) instead of an ordinary 10–12 mm joint as with brick. DS/EN 998-2 defines thin joint mortar as mortar designed for joints of about 3 mm at most (typically 1–3 mm). The adhesive is applied with a notched trowel, a comb board or an adhesive slide matched to the block width so the joint is uniformly thin and fully covering. The thin joint requires dimensionally stable blocks and a completely flat substrate.
First course in mortar — it carries the rest
The very first course is laid in ordinary, thicker mortar on the foundation/plinth so it stands perfectly level and plumb. That is the course that bears the accuracy for the entire wall: if the first course is skew, the thin joint cannot correct the error, and the skewness propagates up through all courses. Thereafter the following courses are bonded with thin joint.
Tolerances — the thin joint forgives nothing
Precisely because the mortar joint is only 1–3 mm, you cannot straighten a crooked block in the joint as you can with the brick's thick joint. The blocks are therefore manufactured to close tolerances (the tolerance category is in the DoP), and the coursing is continuously checked with a level and straightedge. Small height differences between blocks are sanded or scraped level with a sanding board before the next course is bonded on.
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