PapersEffect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress
Introduction
The majority of concrete deterioration cases is connected to corrosion of reinforcement due to carbonation- or chloride-induced depassivation of steel bars [1]. In urban and industrial areas, where environmental pollution results in a significant concentration of carbon dioxide, carbonation-initiated reinforcement corrosion prevails. Numerous surveys have indicated that chloride ions, originating from deicing salts or seawater, are the primary cause of reinforcing steel corrosion in highways and marine or coastal structures [2]. The chlorides that are transported through the concrete pore network and microcracks depassivate the oxide film covering the reinforcing steel and accelerate the reaction of corrosion. Even high-performance concrete may not necessarily ensure long-term durability in a severe environment unless it is designed for dimensional stability and soundness [3].
On the other hand, it has been well established [4] that sustainable development of the cement and concrete industries can be achieved by complete utilization of cementitious and pozzolanic by-products, such as fly ash, slag, and silica fume, produced by thermal power plants and metallurgical industries. In addition to the effect on usual structural properties, such as strength and volume stability, the durability of concrete incorporating these supplementary cementing materials (SCM) should be taken into account. Despite the numerous contributions of a practical or experimental character regarding the effect of SCM on concrete durability 5, 6, 7, it is still not possible to identify the “ideal” concrete to provide optimum performance in a particular corrosive environment because of the numerous material, design, and environmental parameters involved in this problem [2]. In addition, efforts in the direction of a fundamental approach of SCM effect on concrete durability are very limited.
In previous publications 8, 9, a simplified scheme describing the activity of silica fume and fly ash in terms of chemical reactions is proposed, yielding quantitative expressions for the estimation of the final chemical and volumetric composition of an SCM concrete.
In the present work, an experimental investigation of the effect of SCM (silica fume, low- and high-calcium fly ash) on Portland cement systems resistance against carbonation and chloride penetration was carried out, defining precisely what material is replaced when an SCM is added to the volume unit. Moreover, a theoretical approach to this effect is presented.
Section snippets
Materials and mixture proportions
Three typical SCM (i.e., a silica fume, a low-calcium fly ash, and a high-calcium fly ash) were used, representing a wide range of chemical compositions, from highly pozzolanic to almost cementitious. The silica fume (SF) originated from Norway (Elkem Materials A/S, Kristiansand ) and is a highly pozzolanic material. The low-Ca fly ash (FL) was produced in Denmark (Danaske I/S, Aalborg) and is categorized as normal pozzolanic material. The high-Ca fly ash (FH) was produced in Greece (Public
Experimental results
The measured carbonation depths for all specimens are shown in Fig. 1 (mean variation about 10%). For all SCM tested, the carbonation depth decreases as aggregate replacement by SCM increases and increases as cement replacement by SCM increases. The frequently cited statement that as fly ash or SF reduce the amount of calcium hydroxide they may increase the carbonation rate is valid only for cement replacement. This is because not only the calcium hydroxide is carbonated, but also the calcium
Rapid chloride permeability results
The rapid chloride permeability results for all specimens are presented in Fig. 2. The electrical charge passed through the control specimen may be higher than 4,000 Coulomb. This quite high value is due to high cement paste volume and the absence of coarse aggregate. All specimens incorporating an SCM, whether it substitutes aggregate or cement, exhibited lower electrical charge. Silica fume exhibited the lowest charge (10% SF, about 550 Coulomb), then low-calcium fly ash (10% FL, about 1,250
Conclusions
- 1.
It was established that for all supplementary cementing materials tested (silica fume, low- and high-calcium fly ash), the carbonation depth decreases as aggregate replacement by SCM increases, and increases as cement replacement by SCM increases. The lowest carbonation depth is observed for high-Ca fly ash, then for low-Ca fly ash, and the highest for silica fume. New parameter values were estimated and used in an existing mathematical model to describe the carbonation of concrete
Acknowledgements
The European Commission (DG XII- Training & Mobility of Researchers Programme) and the Danish Technological Institute (DTI—Building Technology Division, Concrete Centre) provided financial support for this work. The assistance of Mr. E.J. Pedersen (DTI) and the DTI-Concrete Centre staff is gratefully acknowledged.
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