the concrete would almost certainly have been mine waste with two main problems: - Mundic - where when the conrete gets wet, the water reacts with sulphides in the waste, creating weak sulphuric acid which eats the alkali cement, leaving just sand and aggregate - if you ever go up to Williams shaft, look at the boundary wall and you will see there is practically no cement left. The other issue is what is called Alkali Silica Reaction (Also known as concrete cancer) - which is more a problem with the cement itself - a lot of the cement that came out of Plymouth Cement Works in the 1950's/60's suffers with this, which is what has led to the premature replacement of structures like the Dunheved bridge and Marsh Mills interchange. With this the water mixes with silica in the cement, creating a silica gel, this puts an intense pressure on the concrete from within causing the concrete to split.
Old mass concrete like this also suffers problems in poor workmanshp - the concrete being insufficently tamped (especially before the days of vibrating pokers), poor shuttering, insufficent reinforcement and insufficeint cover on the reinforcement - Cement naturally protects reinforcing steel and prevents corrosion, however on the surface of concrete you have a carbonation front wich extends about 30mm into the concrete - it is caused by a reaction of the cement with carbon dioxide in the atmosphere - if the carbonation front reaches the reinforcemnt, the steel will corrode very quickly, blowing the concrete and accelerating the frther corrosion of the steel.