Water and concrete--Part 1

Nowadays, of course, chemical admixtures can provide workability at low w/c, and mineral admixtures, such as fly ash and ground granulated blast-furnace slag, allow higher strength at the same water content. Cracks are often "healed" by water leaching the calcium hydroxide component of portland cement hydration from the paste and precipitating it in cracks, where it subsequently carbonates and provides a barrier to future passage of water.