2012 Theses Master's

Dolomitic Lime Mortars: Carbonation Complications and Susceptibility to Acidic Sulfates

Hartshorn, Heather

Dolomitic lime mortars, widely used in historic construction and as repair mortars today, are distinguished by their high magnesium content as well as their high plasticity, water retention, and workability. However, the dolomitic lime creates a more complicated chemistry in the resulting mortar in which the magnesium and calcium compounds carbonate at different rates. Although portlandite readily carbonates to calcite, the carbonation of brucite is delayed and could result in a variety compounds in the cured mortar. The reaction of these magnesium compounds with acidic sulfates in the environment could lead to the formation of magnesium sulfate salts, which have the potential to deteriorate the mortar itself and surrounding materials. Mortar deterioration could also occur due to the dissolution of the magnesium compounds in the mortar and subsequent material losses. A series of experiments have been conducted that examine the carbonation of dolomitic lime mortars as well as the extent to which these mortars interact with acidic sulfates. First the production of dolomitic lime was simulated in a laboratory setting, using XRD and SEM to determine the chemical and physical changes that occur during the calcining process. The same analytical techniques were used to examine the level of carbonation in dolomitic lime model mortars. The fundamental reactivity of acidic sulfate solutions with certain compounds in dolomitic mortars was explored using AA. Acidic sulfate attack via acid rain was also simulated on model mortars, and the effects of that exposure analyzed by AA and XRD. Laboratory work for this thesis was conducted at Columbia University GSAPP's Architecture Conservation Laboratory, the Department of Scientific Research at the Metropolitan Museum of Art, and Highbridge Materials Consulting, Inc.