B-G keeps boiler water on the alkaline side. This reacts with or neutralizes the acids so they form into harmless non-corrosive salts. It also inhibits oxygen corrosion. As steam is formed and carried away from the boiler, the dissolved salts stay in the boiler water. As more steam is used more raw water with dissolved salts is introduced into the boiler.
More steam is driven off leaving more dissolved salts behind in the boiler water.
In this way the concentration of salts and other contaminants is built up in the water left behind in the boiler. Part of these salts are dropped out of the water by boiler treatment in the form of sludge. Other harmless water soluble salts stay dissolved in the water. Among these salts is chlorides. Common table salt is a chloride - sodium chloride.