**Conclusions**

From the present study the following could be concluded:

*1) Replacement (10% by weight of cement) of silica fume in concrete reduces the porosity, and increases the strength and impermeability of concrete. *

*a.* The porosity of silica fume concrete at 7 days was reduced by 10%-15% and by 20%-23% at 28 days. Thus it can be concluded that addition of silica fume in concrete reduces the porosity of concrete.

The average pore area of concrete with SF has increased and the average pore diameter has reduced. This is because of refinement of pore structure. Because of formation of further products of hydration the larger capillary pores have reduced and thus forming larger surface area.

It can be clearly visible fro the results that the intruded volume of mercury in concrete has reduced on account of addition of SF.

The intruded volume is decreasing in SF concrete. The intruded volume in concrete for:

- 0.35 w/c at 7 days without SF is 0.0489 ml/g and with SF is 0.0395 ml/g.
- 0.35 w/c at 28 days without SF is 0.0471 ml/g and with SF is 0.0338 ml/g.
- 0.5 w/c at 7 days without SF is 0.061 ml/g and with SF is 0.0499 ml/g.
- 0.5 w/c at 28 days without SF is 0.0496 ml/g and with SF is 0.0439 ml/g.

The number of finer pores increases with replacement of silica fume. It can be seen that the steepness of slope of SF concrete is comparatively more in finer pore range, showing the pore refinement and percentage variation can be seen.

*b.* Silica fume concrete has a considerably higher compressive strength than concrete without silica fume. Addition of silica fume in concrete with 0.35 w/c at 7 and 28 days, has 20% increment in its compressive strength. Similarly, the increment in its compressive strength for 0.5 w/c at 7 days is 16% and 19% at 28 days.

*c.* Addition of silica fume in concrete has considerably reduced the permeability. Silica fume concrete is 26.54 times less permeable when cured for 7 days and 64.41 time less permeable when cured for 28 days as compared to concrete without SF, with 0.35 w/c. Similarly the addition of SF results in reduction of permeability by 27.63 times for concrete cured for 7 days and 32.24 times for concrete cured for 28 days as compared to concrete without SF with 0.5 w/c.

*2) The porosity and permeability of concrete reduces with reduction in w/c ratio, while its compressive strength increases.*

*a.* Reduction in w/c from 0.5 to 0.35 reduced the porosity of concrete by 15-18%.

The intruded volume in concrete for:

- 0.35 w/c at 7 days is 0.0489 ml/g and 0.5w/c 7 days is 0.061 ml/g.
- 0.35 w/c at 28 days is 0.0471 ml/g and 0.5 w/c a 28 days is 0.0496 ml/g.
- 0.35 w/(c+SF) at 7 days with SF is 0.0395 ml/g and 0.5 w/(c+SF) at 7 days with SF is 0.0499 ml/g.
- 0.35 w/(c+SF) at 28 days with SF is 0.0338 ml/g and 0.5 w/(c+SF) with SF is 0.0439 ml/g.

The number of finer pores increases with the increase in the w/c ratio from 0.35 to 0.5. The steepness of slope of 0.35 w/c is more than 0.5 w/c and comparatively more in finer pore range, showing the pore refinement. Percentage variation can be seen.

*b.* There was an increment of 32%-35% in the compressive strength of concrete at 7 days, when the w/c was reduced from 0.5 to 0.35. Similarly, there was increment of 17% - 19% at 28 days.

*c.* The reduction in permeability of concrete without silica fume at 7 days was 1.16 times and at 28 days was 1.29 times, with reduction in w/c from 0.5 to 0.35. Similarly, for concrete with silica fume, the reduction in permeability was 1.1 times at 7 days and 2.58 times at 28 days.

*3) Extended curing after 7 days, till 28 days reduces the porosity and permeability, while it increases the strength of concrete as shown below:*

*a.* The porosity of concrete cured for 28 days as compared to concrete cured for 7 days was reduced by 2.5% for 0.35 w/c, 19% for 0.5 w/c, 14% for SF concrete with 0.35 w/c and 12% for SF concrete with 0.5 w/c.

The intruded volume in concrete for:

- 0.35 w/c at 7 days is 0.0489 ml/g and at 28 days is 0.0471 ml/g.
- 0.5 w/c at 7 days is 0.061 ml/g and at 28 days is 0.0496 ml/g.
- 0.35 w/c at 7 days with SF is 0.0395 ml/g and at 28 days with SF is 0.0338 ml/g.
- 0.5 w/c at 7 days with SF is 0.0499 ml/g and at 28 days with SF is 0.0439 ml/g.

The number of finer pores increases as the curing period increases from 7 to 28 days. The steepness of slope in 28 days cured concrete is comparatively more in finer pore range, showing the pore refinement.

*b*. The strength of concrete cured for 28 days as compared to concrete cured for 7 days was increased by 19% for 0.35 w/c, 44% for 0.5 w/c, 20% for SF concrete with w/c 0.35 and 49% for SF concrete with 0.5 w/c.

*c. *The permeability of concrete cured for 28 days as compared to concrete cured for 7 days was reduced to 1.17 times for 0.35 w/c, 1.05 times for 0.5 w/c, 2.85 times for SF concrete with 0.35 w/c and 1.23 times for SF concrete with w/c 0.5.

**Future Recommendation **

Mercury intrusion Porosimetry test can be used as an additional Non Destructive Testing with help of established models. But a further study is required with more numbers of specimens and variables to validate the models.