Evolution of the formation of calcium hydroxide during the hydration of cement studied by Near Infrared Spectroscopy

Cement is a complex mixture of inorganic compounds composed of calcium silicates and calcium aluminates (C3S, C2S, C3A etc.). When cement is mixed with water to form concrete, these silicates are hydrated, and calcium hydroxide is formed. Even though calcium hydroxide is formed as a byproduct in cement hydration, it is an important component in cement. It helps to form a layer of Ƴ-iron oxide on the steel surface and protects the steel reinforcements in concrete structures. The alkaline environment of calcium hydroxide (pH >12) contributes to the stability of the Ƴ-iron oxide on the steel surface and prevents corrosion of the steel reinforcements. Therefore, the detection and quantification of calcium hydroxide is important to understand the nature and state of the iron reinforcements in concrete structures. Testing for calcium hydroxide is routine in the cement industry. The water used in mixing the cement is found as pore waters and water molecules hydrated to cement components, In this work, a cement sample was mixed with water in four different w/c ratios, including 0.35, 0.40, 0.45 and 0.55, respectively in glass vials. The near-infrared spectra of the sample were measured continuously over 24 hours. The near-infrared spectroscopic analysis confirms the formation of calcium hydroxide during hydration and the presence of three different types of water molecules in the cement samples. These three types of water molecules are indicated by near-infrared absorptions at 5165, 5150, and 5130 cm-1, showing that the water molecules are associated with three different environments. Furthermore, the calcium hydroxide formed during hydration reaches a maximum after 80 hours of reaction.