Hydration Chemistry of Cement Studied by Near Infrared Spectroscopy

Abstract

Cement is a complex mixture of inorganic compounds which mainly composed of calcium silicates and calcium aluminates. Cement is mixed with water to form concrete. During the mixing calcium silicate hydrate (CSH) and calcium hydroxide are formed. The ratio of water/cement (w/c ratio) is important to obtain a mixture that gives optimum strength to the concrete. In this work, three different cement samples were mixed with water in four different ratios, including 0.35, 0.40, 0.45 and 0.55, respectively. The hydration process of cement was investigated by using near infrared (NIR) spectroscopy over a period of 28 days. The combination frequency of OH stretching and bending of water molecules gives rise to an absorption around 5200 cm-1. This peak contains contributions of overtones from several types of water molecules in the cement. Fourth derivatives spectra of all cement samples showed three peaks in the combination band region of 5300-5100 cm-1. These peaks indicated the presence of three distinct types of water molecules in the system. First, the characteristic peak at 5260-5240 cm-1 represented the hydrogen bond between water molecules and silinol group of calcium silicates. This peak indicated the formation of CSH from hydration of cement. Furthermore, this peak experienced a slight red shift after a period of seven days indicating stronger hydrogen bonding of water molecules with silinol groups. The peak at 5130 cm-1 corresponded to hydrogen bonding between water molecules and the peak at 5165 cm-1 corresponded to hydrogen bonding between free water and bound water. The suitable w/c ratio for cement-1 is at 0.35-0.45, cement-2, and cement-3 are 0.45. In addition, real concrete sample showed two characteristic peaks at 5250 cm-1 and 5165 cm-1, demonstrating the presence of CSH and free water within concrete, respectively. Near infrared spectroscopy in combination with fourth derivative technique can be used to investigate the hydration chemistry of cement and concrete.