1972]. In plain cements exposed to such a mixed sulfate environment, MgSO4 react
with Ca(OH)2 to form gypsum as shown in the following equations:
Ca(OH)2 + Na2SO4 + 2H2O → CaSO4. 2H2O + 2 NaOH (i)
Ca(OH)2 + MgSO4 + 2H2O → CaSO4.2H2O + Mg(OH)2 (ii)
Since in microsilica blended cements Ca(OH)2 is consumed and C-S-H gel is formed,
MgSO4 reacts more directly with C-S-H gel in the following manner:
3CaO.2SiO2.xH2O + 3 MgSO4 + (10-x) H2O → 3 (CaSO4.2H2O) + 3 Mg(OH)2
+ 2SiO2H2O (iii)
4 Mg (OH)2 + SiO2 . nH2O → 4 MgO.SiO2.8.5H2O + (n-4.5) H2O (iv)
[Al-Amoudi et al., 1995]
Thus, in the case of microsilica blended cements presence of more Ca(OH)2 in the pore
solution will lead to formation of more C-S-H which improves the strength and
corrosion protection behavior of concrete in chloride environment while on the contrary
will have adverse effect in the seawater environment due to presence of Mg2+
cations
which converts C-S-H to less protected M-S-H. It is known that the ratio of C3S/C2S
controls the quantum of Ca(OH)2 in hydrated cement, higher the C3S more the
liberation of Ca(OH)2 [Rasheeduzzaffar et al., 1990]. As the C3S/C2S ratio of OPC is
3.52 while that of SRC is 2.49, formation of more C-S-H in microsilica blended OPC
improves the corrosion protection in chloride environment (Figure 2B) while in
seawater it deteriorates (Figure 2A).
4.3 Immersion Test
During visual inspection of the concrete specimens, immersed in seawater for one year,
signs of chipping off was observed in the specimens of OPC blended with microsilica
(Figures 3-6). It is important to mention that chipping was more in air-exposed portion
of the specimens as compared to the portion which was immersed in the seawater. The
surface condition of OPC specimens blended with UDMS and DMS (Figures 3 and 4)
indicates that the chipping problem has aggravated on blending with microsilica, while
this type of adverse effect of microsilica has not been observed on blending SRC with