As in any masonry system, mortar is a crucial component of glass block construction. The basic ingredients of mortar- portland cement, lime, sand, and water-are the same regardless of the type of units being laid. But several aspects of glass block construction are unusual. First, glass block are not porous, so they don't draw in or absorb moisture from the mortar as do most other masonry units. Second, glass block walls generally are designed and built without cavities or weep holes to allow water to drain out of the wall. Therefore, both the head and bed joints should be solidly packed with mortar. Third, most glass block are transparent or translucent, so the appearance of the joints through the full thickness of the wall is very important. MOISTURE CONTENT Glass block mortar should be as workable as conventional mortar, but a bit drier and stiffer: It should be just wet enough not to fall off the block. BOND STRENGTH To develop strong bond between nonporous glass block and mortar, glass block manufacturers recommend using ASTM C 270 Type S portland cement-lime mortar, whether or not its high compressive strength is needed. This is especially important for exterior walls and panels. Glass block manufacturers recommend using white portland cement and white quartzite sand free of iron compounds to produce whiter, more aesthetically pleasing joints. WATER REPELLENT Glass block manufacturers recommend using a metallic-stearate admixture to increase water-repellency of the hardened mortar. According to admixture manufacturers, the activating agents are proprietary formulas of styrene butadiene rubber (SBR) latex or acrylic polymers.