Wicking bed advice

Hi all!

I'm converting my raised beds to wicking beds, and wanted to ask your advice about the sump layer.  I was originally thinking of using gravel, as it's readily available, but the more I read, the less suitable that seems.  I'm also trying not to bring things into my closed loop from outside.

A while ago I demolished some concrete block walls, so I have a large amount of rubble available, would this be suitable?

My concerns are...
1. Will there still be chemicals leaching from the blocks (even though they've been out in the hot sun for over a year)?
2.  Will the size of the rubble (large chunks) mean that the wicking system won't work? (I can break them down with a lump hammer if necessary, in which case, what size would be optimal?)
3.  Will the rubble take up too much space and not allow for an adequate reservoir?

I live in Spain and it regularly gets up to 45°C+ in summer, whilst receiving only 425mm rain annually, so water is crucial!

All advice gratefully received!

porous, light volcanic rock is ideal for the sump layer

Although crushed block kind of resembles this, it could throw off the ph because of the concrete ingredients leaching out.

I build a lot of sub irrigated planters/wicking containers.
The largest are about 150 gallons, made from 1/2 of an IBC tote.
I use inverted containers to create voids and pack a (peat based )wicking soil all around them.
They have worked great.
If you were to use broken up cement and the like, mixed with a wicking soil,  I imagine your beds will do fine.
If you are lining your beds with plastic, consider using inverted buckets instead of rubble.
I just turn them over, cut three slots around the bottom edge and pop them into the larger container.