A high-quality silicone sealant can be used against masonry walls. Depending on the manufacturer, the surface may need to be primed before the sealant is installed. Some silicone sealants tend to collect more dirt than polyurethane sealants, especially in urban environments, and this can be significant with light-colored units and sealants. Some new "clean seal" silicone products that address dirt accumulation problems are now available. Some silicone sealants also have shown a tendency to bleed silicone oil into absorptive masonry units, which can permanently stain the units. Although high-quality silicone sealants generally weather better than polyurethane sealants, most people choose polyurethanes for their aesthetic value. If appearance is critical, however, I recommend a trial application of any sealant. Long-term testing may prove that a high-quality silicone product will perform adequately.