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TECHNICAL PAPER COLLECTOR’S EDITION

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resembles silica fume (65 percent content of SiO , fume . Compressive strength tests were made
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2
15m /g surface area). Three Portland pozzolana using 100-mm cube specimens at ages 3, 7, 28
cements were made with the pozzolan replacing and 90 days. In addition, a pore-size distribution
10, 20 and 30 percent by weight of cement, analysis was made with a mercury penetration
respectively. technique.

The water/cement ratio of the pastes was 0.50. The compressive strength tests showed that
Compressive strength tests were made according the early strengths of the concretes with fly ash
to ASTM C-109. Also, a pore-size distribution were lower than the corresponding strength of
analysis was carried out by a mercury penetration the controlled concrete. For example, at 7 and 28
technique. The compressive strength tests showed days the strengths of the fly ash- Portland cement
that the paste containing 10 percent pozzolan concretes were, respectively, 11 percent and 12
exhibited the higher strength (6 percent over the percent less than the strength of the controlled
control) al 28 days. In one year, the paste with 20 concrete. However, at 90 days the strength of
percent pozzolan content had the higher strength, the fly-ash concrete was similar to the controlled
a 10 percent increase over the controlled Portland concrete. On the other hand, the concretes with
cement paste. silica fume showed much higher strengths than
the controlled concrete, particularly, the strengths
These strength characteristics could be explained in at 7, 28 and 90 days. It was observed that at 7
terms of the pore-size distribution of the hydrated and 28 days both the total volume of the mercury
pastes. The higher strength achieved by the penetrable pores and the volume of large pores
paste containing 10 percent pozzolan at 28 days, (> 0.1 µm) was higher for the fly ash-Portland
was attributed to the low volume of large pores cement pastes than for the controlled Portland
(> 0.1µm) present in the paste at 28 days. Also, cement pastes.
the higher strength achieved by the paste with 20
percent pozzolan after 1 year was attributed to the As a result, at these ages, the concretes made from
absence of large pores and the large volume of the Portland-fly ash cement (at the same water/
small pores (< 0.05 µm). cement ratio), exhibited lower strengths than the
controlled Portland cement concrete. However, at

Based on these results, it was concluded that 90 days, even though the total volume of the pores
the volume of large pores, not the total porosity, of the fly ash-Portland cement pastes was much
adversely affects the strength of the hydrated higher that the pore volume of the Portland cement
cement pastes. Moreover, the transformation pastes, the volume of large pores was similar and,
of large pores into finer pores-pore refinement, consequently, the concrete strengths were similar.
as a result of the pozzolanic reaction, played
an important role in enhancing the strength of For the silica-fume pastes, it was found that at 7 days
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Portland pozzolana cements. the volume of the large pores was only 0.12cm /g
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compared with 0.30cm /g for the controlled
Portland cement paste and, therefore, the concrete
In a similar experimental work, Mehta and Gjory
have investigated the strength development of strength was 22 percent higher than the controlled
concretes in which 30 percent of Portland cement Portland cement concrete. In addition, at 28 and
was replaced by an equal volume of fly ash or silica 90 days, even though the total pore volumes were

The Indian Concrete Journal | November 2018 53