Influence of fly ash fineness on the chloride penetration of concrete

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Abstract

In this paper, the resistance to chloride penetration of concrete containing fly ash of various finenesses was examined. Three different fly ash finenesses viz., original fly ash, 45% fine portion and 10% fine portion fly ashes were used for the study. Chloride resistance of concretes was evaluated using the measurement of the Coulomb charge as per ASTM C1202 and by determination of chloride ingress after different periods of exposure to 3% NaCl solution in full immersion and partial immersion modes. From the tests, it is found that the resistance to chloride penetration of concrete depends on the fineness of fly ash. The rapid chloride permeability test (ASTM C1202) clearly indicates that the Coulomb charge of concrete at the age of 28 days is significantly reduced with the incorporation of fly ash. The decrease is promoted with an increase in fly ash fineness. The immersion of cut cylinders in the 3% NaCl solution for 3 and 6 months confirm that the resistance of the chloride penetration of concrete increases with an incorporation with fly ash and with an increase in the fly ash fineness.

Introduction

Corrosion of reinforced steel resulting from the ingress of chloride ion is one of the most important issues concerning the durability of concrete structures. When the chloride concentration of the concrete exceeds a certain threshold value, which is dependent on several material and environmental factors, the reinforced steel would start to corrode [1], [2], [3]. It is generally recognized that the introduction of pozzolan in blended cements improves concrete protection against chloride-induced corrosion of steel reinforcement by reducing its permeability/diffusivity, particularly to chloride ion transportation and increasing the resistivity of the concrete [4], [5], [6], [7].

Fly ash is the most common pozzolan and is being used worldwide in blended cements. The incorporation of fly ash increases the porosity of the hardened cement paste at early ages, but the average pore size is reduced, and this often results in a less permeable paste [8], [9]. The dense interfacial zone between aggregate and the matrix is also a result of the use of fly ash [10], [11]. The concrete containing fly ash is, therefore, less susceptible to the ingress of the harmful chloride ions.

It has been shown that the use of the finer fly ash results in a better mechanical properties of mortar and concrete as compared to the coarser ones [12], [13]. The fine fly ash with spherical particle and smooth surface reduces the water requirement of mortar, and increases the strength and the resistance to sulfate solution. The use of finer fly ash also reduces the average pore size of the paste as compared to the coarser one [9], and this should further improve the resistance of concrete to the ingress of harmful solutions.

The knowledge of the use of fly ash of different finenesses to increase the resistance of concrete to chloride penetration would therefore be beneficial to understanding of the mechanism as well as for future applications of these materials for increasing the durability of concrete.

Section snippets

Materials and concrete mixes

In this experiment, ASTM Type I Portland cement (PC) and ASTM class F lignite fly ash (FA) from Mae Moh power station in the north of Thailand were employed as cementitious materials. Three fly ash finenesses: coarse, medium and fine were used. The coarse fly ash was 100% original fly ash (100FA). The medium fly ash was the 45% fine portion of the original fly ash (45FA). The fine fly ash was the 10% fine portion of the original fly ash (10FA). The medium and the fine fly ash were obtained by

Water requirement and strength

The water contents of the mixes are given in Table 2. The incorporation of fly ash results in a large reduction in the water-to-binder ratio of the fly ash concrete mixes for constant workability, i.e., similar slump of 75 ± 25 mm. Results also suggests that the reduction is enhanced with finer fly ash because of the more spherical and smoother surface of finer fly ash particles. The water-to-binder ratios (W/B) of PC1, PC2 and PC3 mixes are 0.540, 0.480 and 0.250 and those of the corresponding

Conclusions

From the results, it is concluded that the resistance to chloride penetration of concrete is significantly increased with the incorporation of fly ash and the increase is enhanced with an increase in fly ash fineness. The increase results from the reduced water-to-binder ratio, the reduced average pore size of the paste and the improved interfacial zone. The incorporation of fly ash is especially enhanced for the fine fly ash, due to the spherical and smooth surface of the fine fly ash

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