Hydraulic Lime and Pulversied Fuel Ash

Q We are carrying out works to Barry Port Hydraulic Pumphouse. Analysis of the very hard brick pointing suggests lime based with a coal derived fuel ash of some description. The use (harbour works), date (1890), area (all coal powered), location (next to the S Glamorgan Blue Lias quarries) suggests a Hydraulic lime and ‘fly ash’ or PFA. No other aggregate was found. Has anyone experience of such mixes?

A It was common practice in Victorian times to use ‘black ash’ mortars in urban and industrial areas. These incorporated ash from factory boilers saving the cost of dumping and replacing at least some of the sand required. The ash, often in the form of large clinker that had to be crushed, had a certain pozzolanic activity and the hydraulicity of the lime would depend on the location. In this query it is probable that it was hydraulic whereas in my main experience from the Manchester / Liverpool area the lime was purer and had only a little hydraulicity. Current equivalent would be a NHL 3.5 / Fly ash / sand mix although the use of Metakaolin instead of fly ash would speed up set and strength gain thereby reducing protection periods in tidal exposure situations. Castle research shows the pozzolanic strength of fly ash as being above that of brick dust and natural pozzolana but below those of granulated slag, Metakaolin and silica fume. Paul Livesey, Technical Manager, Castle Cement Ltd.