During the 1960s and into the 1970s heritage bodies and organisations were becoming increasingly concerned at the accelerated decay of the overall fabric and detail of many of our churches, cathedrals and historic buildings. The three main culprits were environmental pollution, the extensive use of cement mortars since the 1860s and the presence of ferrous fixings throughout these complex structures.
By the 19th century the increase in pollution was rapid, due to an explosion of industry and population in cities, towns and countryside, and the consequent acceleration in the burning of fossil fuels. Limestone, as well as being a porous building material, is rich in calcium carbonate. This makes it highly susceptible to atmospheric sulphurous acids from the burning of fossil fuels. The calcium carbonate binder in the stone undergoes a chemical change to become calcium sulphate, which is one of the soluble salts most closely associated with the decay of the limestone through repeated cycles of salt crystallisation. By the middle of the19th century some of the carved surfaces of our historic buildings had accumulated a thick carbon deposit behind which the soluble salts were wreaking havoc. The result was the exfoliation and blistering of surfaces behind which the stone had been reduced to powder.
Hot on the heels of environmental pollution came the invention of Portland cement in the first half of the 19th century. This new and exciting material was highly regarded for its hydraulic properties, fast setting times and great strength. It is easy to see why the custodians of our churches and cathedrals must have thought that Portland cement was the ideal repair material which would cope with the increase in stone decay due to environmental pollution, particularly as it had the added benefit of colour matching the blackened surfaces. However, Portland cement is now known to be a potential source of sulphates, thereby contributing to the decay process. Of even greater significance is the fact that Portland cement has a dense and impervious nature which encourages concentrations of moisture and soluble salts at the interface of the cement repair and the limestone. In most cases where a cement repair is removed, the limestone beneath will be found to have been reduced to powder by the crystallisation of salts within the interface.