I don't know the precise answer to your question but I am pretty certain it will not be silicone and certainly not acrylic or anything you can get in the likes of B&Q. There are many different construction joint sealants available made by such firms as Fosroc, Sika, Thioflex, etc. I would suggest you consider polysulphide ones as I have good experience of them. Another worthwhile product is CT-1 which has numerous interesting attributes, including being overpaintable without any shade change at the sealant area, unlike most/all others.