(i) In the initial 100 days chloride ingress in OPC is very fast, however, after this
duration, it slows down and there is very little increase in the Cl-
ingress rate
(Table 6).
(ii) In general Cl
-
ingress in SRC is very fast, however, slight decrease in the ingress
rate is observed after 400 days of immersion (Figure 10).
(iii) Blending of microsilica with both the types of cement suppresses the ingress of
Cl-
ions in the concrete, however, blending of densified microsilica in OPC
appears to be more effective than blending in SRC or blending of undensified
microsilica in OPC (Figure 11).
Low chloride ingress in OPC as compared to the SRC seems to be due to high C3A
content in OPC as compared to SRC. The C3A phase of portland cement has the ability
to complex with the dissolvable chloride, resulting in the formation of insoluble
Friedleís salt (3CaOAl2O3. CaCl2 + 10H2O). This combining of C3A phase with free
chlorides in hydrated cement, results in the reduction of the corrosion-inducing soluble
chlorides in the pore solution and also retards further ingress of chloride ion in the
concrete. These results are in consistent with the finding of Rasheeduzzafer et al.,
[1990, 1991]. Blending of microsilica to these portland cement further enhances the
chloride binding capacity of C3A by reducing the alkalinity of the pore solution which is
due to conversion of Ca(OH)2 in C-S-H. Also, it seems that the tiny particles (0.1-0.15
µm) of microsilica improves the packing of the system, which results in low
permeability of the chloride solution in the concrete. Low chloride permeability on
addition of microsilica has also been observed in RCP test carried out after a curing age
of 28 days (Figure 12). These results also show that at the age of 28 days, chloride
permeability rate in OPC and SRC are very high and are nearly equal, however,
blending of these cements with microsilica slows down the permeation of Cl-
ions
drastically.