1 Introduction
Reinforced concrete structures in coastal and snowy regions are generally subjected to chloride attacks and steel corrosion often occurs due to the penetration of chloride ions (Vaysburd and Emmons
2004; Raupach
2015). To reduce future maintenance costs, concretes are required to have a high resistance to chloride ingress (Ministry of Land, Infrastructure and Transport
2015). Such concretes would have a denser structure and achieve excellent durability in various environments.
Mineral admixtures such as fine blast furnace slag powder and fly ash have been used to improve durability of concrete (Juenger and Siddique
2015; Hossain et al.
2016). Celik et al. (
2015) reported on physical properties of concrete mixed with fly ash (cement-replacement ratios of 30 and 60%) and limestone powder (cement-replacement ratio of 15 to 25%). In addition, Celik et al. (
2014) reported the relevant properties of concrete that incorporated 15% limestone powder and 30% natural volcanic pozzolan by mass of cement. Their investigations confirmed that concretes incorporating these powder materials significantly improved resistance against chloride-penetration. However, it was concerning that the compressive strength of concrete at an early age was significantly lower.
A number of investigations have dealt with the improvement of the early-age strength of concrete incorporating large amounts of admixtures. Meddah et al. (
2014) reported on relevant properties of concrete with either 30% blast furnace slag fine powder, 20% fly ash, or 5% silica fume. Another aim of the use of such by-products as an alternative cementitious material is to mitigate environmental impact.
However, when using large amounts of such by-products for the conservation of the environment, additional storage facilities, such as silos, are required.
Madani et al. (
2014) focused on concrete incorporating small amounts of admixture to improve early-age strength and chloride-penetration resistance. They examined the chloride-penetration resistance and compressive strength of concrete incorporating several percentages of nanosilica and silica fume. It is worth noting that the admixture used in this study effectively increased the compressive strength at an early age, even for small additions of the admixture. The manufacturing efficiency of the very fine powdered material is a concern.
The authors of this paper have developed an alternative cementitious admixture to improve resistance to chloride ingress of concrete (Ishida et al.
2015). The standard mixture volume of this admixture is in the range of 5–13 mass% (20–40 kg/m
3), therefore it can be added manually at a batching plant. The admixture can also be used as an alternative binder for Portland cement in addition to other general admixtures, such as fly ash. Use of the admixture may be suitable for precast concrete production factories with limited storage facilities. The admixture is a fine pozzolanic mineral powder with a Brunauer–Emmett–Teller (BET) specific surface area of 13,000 m
2/kg or higher and therefore it can also improve strength development of concrete at an early age.
Mechanical properties and durability of steam-cured concrete mixed with the admixture were examined in our previous investigation (Yamato et al.
2017). The admixture is also useful in general ready-mix concrete plants. To confirm applicability of the admixture for general purposes of ready-mix concrete, the present study examined mechanical properties and durability of standard-cured concrete incorporating this admixture. In addition, the study investigated the chloride resistance mechanism of concrete incorporating the admixture.
Resistance to chloride penetration may be due to (1) densification of hardened cement and (2) chloride-ion adsorption by hydrates (Ishida et al.
2007; Lia et al.
2015). Madani et al. (
2014) and Ahmed et al. (
2008) reported that hardened cement was densified by the microfiller effect and pozzolanic reaction. To confirm the densification effect (1), pore-size distribution and pore structures were examined using mercury intrusion porosimetry (MIP) measurement system (Elrahman and Hillemeier
2014; Fan et al.
2014). It is well known that chloride ions are chemically bound in Friedel’s salt (Lia et al.
2015; Neville
2012). Previous investigations showed that chloride ions were physically and/or electrically adsorbed on the C–S–H surface (Lia et al.
2015; Yoshida et al.
2002). To examine the chloride-ion adsorption effect (2), the study investigated the hydration products, such as Friedel’s salt, in cement paste immersed in 3% NaCl solution. The hydrated products were examined using X-ray diffraction (XRD) (Shi et al.
2017; Heisig et al.
2016). In addition, Rietveld analysis was performed to quantify the amount of Friedel’s salt and to examine chemical binding of chloride ions in hydrated cement (Shi et al.
2017; Schepper et al.
2014). The amount of bound chloride-ions concentration was estimated by subtracting the amount of free chloride from the total amount of chloride present in the cement paste (Lia et al.
2015).
This paper presents the properties of the developed admixture and describes the mechanism behind the improved resistance to chloride penetration when the admixture is mixed in concrete.
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