Raman spectroscopy of cement - White Portland cement
The grey colour of ordinary Portland cement is due to the presence of ferrite, the presence of which is unwelcome in White Portland cement (WPC). Whilst efforts are made during manufacture to reduce the levels of iron and manganese in the raw materials, their complete absence is impossible. So as to minimise discolouration, the clinkers are produced under slightly reducing conditions and quenched rapidly to avoid oxidation of iron and manganese during cooling. Therefore, the major components of WPC are alite, belite, aluminate and gypsum, all of which give distinctive Raman spectra.
Figure 8 Raman spectra of fresh, unhydrated WPC and a WPC paste hydrated for 1 day.
Figure 8 shows the Raman spectra of fresh WPC and of a WPC paste hydrated for 24 hours. The fresh cement shows bands characteristic of bassanite, CaSO4.½H2O (1163, 1132, 1018, 678, 629 and 609 cm-1), C2S (978, 894 and 860 cm-1), and mixtures of C2S and C3S (846, 829, 552, 538 and 518 cm-1). Upon hydration bassanite was converted to ettringite (990 cm-1). The band at 860 cm-1, attributed to C2S became more prominent as C3S, which hydrates more rapidly, was consumed. Hydration of the C3S led to C-S-H (674 cm-1) and portlandite (358 cm-1) formation.
Figure 9 ESEM image of WPC hydrated for 1 day, showing the presence of C-S-H, Ettringite (AFt), Portlandite (CH) and unhydrated clinker (primarily C2S).