Types of lime binders in mortars used for the construction of the Ming Great Wall of China and their importance for the development of a conservation strategy

The most obvious characteristics of the Ming Great Wall are external masonry walls made of natural stones,bricks and lime mortars.According to the chemical and mineralogical compositions of the original bedding and pointing mortars,dolomitic lime binder was used dominantly in the construction of the Ming Great Wall in provinces such as those around Beijing and Hebei.Calcium-rich lime and air lime with low natural hydraulically reactive phases,which are hydrated and react slowly with the carbon dioxide in air to form calcium carbonate,were used in some western provinces.Chemical and microscopic investigations show that both dolomitic and calcium-rich lime mortars are almost aggregate-free.Historic dolomitic lime mortars are characterised by high strengths,low porosities and dense micro-scale textures.The most recent conservation principle is to preserve the Ming Great Wall as a ruin.Therefore,the conservation strategy should be redefined in terms of mortars.Since some of the damage to the Great Wall is related to reactions between dolomitic lime mortars and air pollutants,calcium-rich lime binders should be used for conservation and even for restoration of those parts of the Great Wall that were originally built with dolomitic lime.Binders based on natural hydraulic lime and calcium-rich lime gauged with natural pozzolana might be more compatible than other binders"for the structural consolidation of the ruins of the GreatWall.