Efflorescence on concrete paving when using recycled concrete aggregate

So my fears seem to be unfounded! Very nice!

On the terrace, we will also have concrete paving stones, but AS A PRECAUTION, design the last 10cm of the base layer with mineral gravel. Who knows if terrace slabs or natural stones might eventually be used there. That way, the subsoil is at least perfectly prepared.

I have heard especially about anthracite-colored paving that it effloresces, but why should that have anything to do with the substructure? I rather assume it comes from the paving itself.

how should something get up against gravity, against the flow of water, through the gravel?

Due to the capillary effect of the paving or clinker stones, which with the salts dissolved in water from concrete residues from the substructure brings them to the surface of the paving, the pure water evaporates there and can lead to efflorescence of the previously dissolved crystalline salts there. Under carports etc., the draining of the surfaces is additionally missing.

So much for the theory. Thin gravel layers with a fine grain content greater than 4 mm as a capillary-breaking layer separate and also drain the layers very well in practice.

In-depth search e.g.: possibilities to avoid the "carport effect" (by an expert, as a PDF on the Metten website)

In masonry, a horizontal barrier is therefore also used in practice to prevent the rising of moisture through the capillary effect.

Due to the capillary effect of the paving or clinker stones, which transports salts dissolved in water from concrete residues in the substructure to the surface of the paving, the pure water evaporates there and can lead to efflorescence of the formerly dissolved crystalline salts.

This effect occurs occasionally. However, the minerals often come from the stone itself. A capillary-breaking layer is naturally still required. These efflorescences can, if necessary, be very easily removed with a high-pressure cleaner. And after 1 - 2 times, it is usually over forever.