Civil Engineering Reference
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observed that cement containing 55% zeolitic tuff and 45% Portland exhibited
high strength, with 28-day values similar to the strength found for 100%
Portland cement.
compressive strength has also been reported to rise at all test ages with
respect to the control for concrete made with cement containing natural
zeolite (ahmadi and Shekarchi, 2010). The best results were obtained with
a replacement ratio of 15%. Opaline also yielded good results, even better
than those obtained with volcanic tuff. When the ratio was 80/20, the 28-
day values were similar to the findings for cement (99.6%). When the ratio
was 65/35, strength in the samples was 92% of the value for the unblended
cement. The excessive amount of mixing water required for diatomaceous
earth constitutes a substantial drawback to its use as a pozzolan, which
adversely affects its use as a pozzolanic material.
The Keciborlu amorphous silica rocks generally show strength enhancement
with the addition of aS at all curing ages and higher strengths are obtained
in comparison with that of control mix (Davraz and Gündüz, 2005) and the
comparative compressive strength analysis of bentonite mixes showed higher
strength than the reference cement (Memon et al. 2012).
The effect of MK has been studied in pastes, mortars and concretes.
Poon et al. (2001) and Badogiannis et al. (2005) explored the effect of the
presence of MK as a partial cement replacement (5-20 wt%) on cement
paste compressive strength. Mechanical behaviour was observed to improve
in both these studies, according to which the optimal replacement ratio is
10%. Other authors have focused on the effect of using metakaolin as an
addition (10-30 wt%) on the mechanical properties of mortars (Curcio et al.,
1998; Li and Ding, 2003; Potgieter-Vermaak and Potgieter, 2006), likewise
observing a rise in compressive strength.Wild et al. (1996) and Brooks and
johari (2001) reported that the greatest rise in compressive strength was
obtained with replacement ratios of 20 and 15%, respectively.
The effect of adding varying percentages (5-30 wt%) of MK to cement on
the properties of new concretes has also been researched (Wild et al., 1996;
Brooks and Johari, 2001; Qian and Li, 2001; Roy et al., 2001; Sabir et al.,
2001; Badogiannis et al., 2004; Güneyisi et al., 2008; Abbas et al., 2010).
All these studies revealed the beneficial effect of this material on concrete
compressive and tensile strength (Qian and Li, 2001).
4.5 Conclusion and future trends
As noted earlier, the future of natural pozzolans will depend on the availability
of quarries, i.e. the maintenance and enlargementof existing sites or the
discovery of new ones. In this regard, a review of the literature shows that
papers have been published recently on volcanic tuffs from Turkey (Uzal
and Turanli, 2003; Turanli et al., 2004), natural zeolites (Uzal and Turanli,
