Solid solutions amongst cement AFm phases containing nitrate and nitrite ions

Abstract

The AFm phase found in hydrated Portland cements refers to a family of calcium aluminate phases. Their layer structure is derived from that of portlandite, Ca(OH)2, but with one third of the Ca2+ ions replaced by a trivalent ion, nominally Al2+ or Fe2+. The resulting charge imbalance gives the layers a positive charge which is compensated by intercalated anions the remaining interlayer space is filled with H2O. Hydrated cements will contain mixtures of AFm phases and their solid solutions. Portland cement is often modified by addition of soluble nitrate or nitrite salts to protect embedded steel or to shorten the set time. These nitrate and nitrite ions are capable of entering and occupying AFm anion sites hence impacting overall phase balances between cement hydrates. In this study AFm phases and solid solution formations were investigated between AFm phases containing SO42+, CO32+, OH2+, Cl2+ and NO22+NO32+ ions. Samples were synthesized and subsequently characterized using X-ray Diffraction (XRD) and through the measurement of aqueous solution compositions after 180 days of equilibration in water.