From bitter personal experience Crittall = without doubt the worst window manufacturer I have ever had the misfortune in dealing with ! (several times)
Various versions of fly-ash such as GGBS (ground granulated blastfurnace slag) or PFA (pulverised fuel ash) are used in specific concrete mixes, they can give resistance to sulphates in dodgy ground and act as a cement 'replacement' making it a (bit) greener
