In Figure 4, the microstructure of cement pastes supports the result of compressive strength and chemical composition mentioned above that the cement structure is developed with higher packing density according to the curing time. SEM image of 3 days, in Figure4(a) reveals plenty of pores and needlelike ettringite structures. At the late age of 28 days, Figure 4(b) shows more dense structure of resultant cement which contains less pores and ettringite needles due to the supplementary cementing phase of C-S-H. C-S-H gel of the pozzolanic reaction is a principal coordination phase to increase compressive strength of resultant cement.