PROGRESS REPORT ON THE EFFLORESCENCE AND SCUMMING OF MORTAR MATERIALS1
Authors: Wilson Hewitt 
Published: June 2006

A general discussion is given on soluble salts in clays, clay products, and mortar materials and on the addition of barium compounds to make the salts insoluble. Twenty‐one, 4‐ × 4‐foot panels, 8 inches thick were constructed of the same glacial‐clay, red, face brick with variations in the following common mortar materials: sand, slaked lime, hydrated lime, and Portland cement. Saturated brick was tried versus dry brick; sea water was used to replace fresh water; machine‐mixing of the mortar batch was tried against hand‐mixing; and various percentages of barium carbonate, barium hydroxide, and barium chloride were introduced to make the calcium sulphate insoluble. Temporary scum or that obtained in the original drying of the wall was noted and later or permanent scums, produced by artificial and natural soaking and drying the walls were studied for over a year.

Results All the mortars studied can be made to scum if sufficient water and time of contact of water and mortar are provided. The troublesome salts are: (1) calcium sulphate largely introduced by the Portland cement, which makes a very bad, early scum; (2) calcium hydroxide from the free lime and hydrolyzed calcium aluminates and silicates in Portland cement and from uncarbonated lumps of lime in the lime mortars; in addition, some minor salts from special sources, such as sodium chloride from sea water Lime mortar streaks were only produced after a large quantity of water had been washed through the panel, and were apparently formed by finely divided or colloidal lime hydroxide or carbonate in suspension in the water running from the porous mortar down the side of the wall. This is a very severe soaking condition and is not common in the ordinary building wall. The lime sulphate dissolves readily enough to pass in solution through the pores of the brick and t o spread uniformly over the brick surface. Lumps of hardened lime should he removed from both slaked and hydrated lime to prevent streaks of lime hydroxide and carbonate on the surface under wet conditions for long periods of time. Lumps of lime also produce spalling. The interior of a wall should be well protected from leaky drains, gutters, etc., which permit water to get inside. A hard rain of short duration is not as bad as a long, soaking drizzle which permits the water t o remain in contact with the mortar for a longer period of time. Cement products, such as artificial cement stone, commonly release salts which drain onto other facing materials.

The tests indicated that barium carbonate is effective under normal conditions in preventing or minimizing calcium sulphate scum from Portland cement. This has been checked on large Seattle school buildings where the conditions are favorable for scum. Five per cent barium carbonate in terms of Portland cement has been used successfully. I t has been suggested (but not tried) that diatomaceous earth may combine with the free lime of the mortar materials to produce a more insoluble calcium silicate and reduce the temporary scum of lime hydroxide.

The study is being continued on the chemical analysis and composition of the scums taken from the surface of the walls and leached in the laboratory from separate batches of the mortar materials. Preliminary analyses have already been made. The strength of the mortar materials to note the effect of barium salts is also under study. No deleterious effects have been found from barium salts.