Research into each of the materials used in masonry walls reveals that the main source of alkalies for the salts is the Portland cements used in the mortar and grout. Using a low alkali Portland cement will often eliminate the efflorescent problem. It is recommended that low alkali Portland cement be used to reduce the chances of efflorescence occurring. Low alkali Portland cement has 0.6% alkali or less, by weight in the cement (2, 3).
Another culprit is, of course, the clay brick itself. The natural clays used in the manufacture of brick often contain soluble alkali sulfates. Most modern fired clay brick have balanced chemical additives, such as Barium Carbonate (BaCO3), to immobilize the sulfates and render them insoluble (4). This prevents the salts from being dissolved into a solution that could migrate through the wall to the surface. Most fired clay brick do not greatly contribute towards the efflorescence problem.