Theoretical and experimental study of structures and properties of cement paste: The nanostructural aspects of C–S–H Original Research Article

Ordinary Portland cement (OPC) paste with water to cement ratio 0.5 has been studied by FTIR spectroscopy and TEM/SAED techniques. The progressive shift in frequencies and change in intensity of characteristic peaks in the FTIR spectra of cement powder indicate polymerization of silicates due to hydration reaction. The appearance of new peaks around 975–985 and 3640–3650 cm−1 in FTIR spectra of cement paste has been attributed to the formation of two principal hydration products (C–S–H) and Ca(OH)2, respectively. There is a strong resemblance of peak shape at 980 cm−1 of cement paste with that of synthesized 1.1 nm Tobermorite. TEM/SAED techniques reveal the presence of three distinct types of C–S–H nanostructures viz. spherical particles, fibres and bundle of fibres, and nano tubes in cement paste. Theoretical study of two C–S–H clusters by MNDO and Density Functional Techniques shows excellent agreement between computed structural data and reported experimental results of Tobermorite and Jennite. The computed interplanar Ca–Ca distance (11.162 Å) of C–S–H cluster (Ca/Si=1.125) indicates greater structural similarity of this cluster to 11 Å Tobermorite. There is a strong similarity of IR profile of C–S–H cluster (Ca/Si=1.125) with 1.1 nm Tobermorite. Above study indicates that this cluster is probably the most likely basic unit of C–S–H phases formed during the early stage of OPC cement hydration.