Effect of carbonation, chloride and external sulphates on the leaching behaviour of major and trace elements from concrete

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Abstract

The effect of the exposure of concrete structural components to CO2, chloride and external sulphates on the leaching of major and trace elements (Al, Ca, K, Na, S, Si, Ba, Sr, Cr and V) was investigated for concrete made with OPC and fly ash using a tank leach test and the Dutch availability test NEN 7341. Concrete specimens were carbonated under natural and accelerated conditions (2 vol.% CO2) before testing. Exposure to chloride or external sulphates was simulated by adding NaCl (30 g/L) or Na2SO4 (3or30gSO42-/L) to the tank leach test eluent. The results show that concrete leaching depends significantly on the abovementioned environmental factors. In particular, the cumulative release of Cr during the tank leach test increased proportionally to carbonation depth. More V was released from carbonated concrete. Replacement of the cement by a fly ash with more Cr and V had no discernible effect on Cr release, but resulted in additional V release in proportion to the level of cement replacement. Exposure of concrete to NaCl solution enhanced the release of Cr, but not V. Exposure to Na2SO4 solution increased the release of Cr and V considerably. It is suggested that the dissolution of CrO42- and VO43- substituted AFt/AFm governs the leaching of Cr and V.

Introduction

At present, increasing attention is being paid to the potential leaching of heavy metals from concrete structural components, in particular, in view of the increasing use of secondary fuels in cement production and additions like fly ash or slag in concrete which generally lead to larger amounts of heavy metals in concrete. Heavy metals are leached into water in contact with concrete surfaces by direct dissolution of surface phases and/or diffusion through the pore solution to the concrete surface. The solubility of heavy metals depends on their bonding in the hydration products as well as their ability to form aqueous complexes and precipitate solubility-controlling phases. Thus van der Sloot [1] observed no distinct correlation between the total amounts of Cr, Mo, Pb, V and Zn in mortar and their availability as defined by the Dutch availability test NEN 7341 [2], i.e. their soluble content with respect to dry specimen weight. Consequently, the evaluation of environmental risks cannot be based on the total amounts of trace metals contained in concrete alone, but requires better understanding of the mechanisms involved in leaching. In addition to this, leaching is affected by the actual environmental conditions at the surface of concrete structural components. These include exposure to CO2 and NaCl. The latter is commonly used as a deicing agent for concrete highways in moderate and cold climates. Sometimes concrete is exposed to sulphates in groundwater or soil. Such effects are not considered by contemporary leach tests with water which are used to assess the release of environmentally relevant substances from concrete. Incorrect assessment may lead, on the one hand, to underestimation of the actual release of toxic substances or, on the other hand, to the restriction of otherwise useful construction materials.

Since carbonation results in a reduction in pH of the pore solution of concrete, dissolution of the hydration products as well as changes in porosity and pore size distribution, it is expected to affect the solubility and transport of major and trace elements in the pore solution and therefore their leaching behaviour. Despite this, relatively little is known about the effect of carbonation on the release of heavy metals from concrete. Andac and Glasser [3] and Van Gerven et al. [4] investigated the effect of CO2 on the leaching of metals from cement-stabilized municipal solid waste incineration fly ash by bubbling pure CO2 through the leachant. The latter authors noted a significant increase in the leaching of Cr, Zn, Ni and Co. However, CO2 bubbling tends to produce very low pH values and amounts of dissolved CO2 much in excess of those occurring naturally. Another approach is to compare the leaching of carbonated and non-carbonated specimens. Usually the carbonation process is accelerated in climatic chambers with high CO2 concentrations or by the use of super critical CO2 [5], [6], [7]; the method chosen affecting the degree of carbonation and leaching behaviour. Van Gerven et al. [5] observed a reduction of the leaching of Ca, Ba, Cu and Pb from mortar specimens by carbonation.

Penetrating chloride is also expected to affect the solubility of major and trace elements in the pore solution. Thus, for example, according to You et al. [8] Cr may be released from ettringite which is able to incorporate Cr(VI). Opposed to this, it has been reported by Dai et al. [9] that Cr(VI) can be fixed by adsorption on Friedel’s salt, an AFm phase formed in the presence of chlorides.

Exposure of concrete structural components to external sulphates leads to deterioration owing to the formation of the expansive phases, ettringite and gypsum, whereas portlandite, AFm phases and C–S–H tend to dissolve, see for example Müllauer et al. [10], [11]. It is likely that external sulphates affect the solubility of trace metals and thus their leaching behaviour.

This paper focuses on the effect of carbonation, exposure to NaCl and Na2SO4 solutions on the leaching behaviour of Al, Ca, K, Na, S, Si, Ba, Sr, Cr and V from concrete made with OPC and fly ash. The Dutch availability test NEN 7341 and a German tank leach test [12] were used.

Section snippets

Materials and methods

The experiments were carried out with concretes made from OPC and a fly ash with the chemical compositions listed in Table 1. The materials were analysed by ICP-OES after digesting in nitric acid. This particular fly ash resulted in a marked increase in the amounts of V, Cr, Ba and Sr in the concrete.

Five different concrete compositions differing in the amount of fly ash were used, Table 2. The concretes were prepared at a total binder content of 450 kg/m3 and an equivalent water binder ratio w/b

Carbonation

The carbonation depths measured after 154 d are presented in Table 3. A decrease in carbonation depth with increasing fly ash content is apparent which is, as is well-known, due to the denser concrete pore structure obtained with fly ash. The phenolphthalein test yielded no measurable carbonation depth for the specimens stored in argon.

The effect of carbonation on Ca availability is shown in Fig. 2a where the marked decrease in Ca availability is obviously due to the formation of CaCO3 which has

Conclusions

Tank leach tests (56 d: duration, ratio of eluent volume to concrete surface: 90 L/m2) were performed to determine the effect of carbonation depth and NaCl or Na2SO4 in the eluent on the release of Al, Ca, K, Na, S, Si, Ba, Sr, Cr and V from concretes made with OPC and fly ash. The availability of the elements was determined between pH 4 and 13 following the Dutch standard test NEN 7341. The results show that the leaching behaviour of major and trace elements from concrete depends significantly

Acknowledgement

The authors express their thanks to the German Research Foundation (DFG) for supporting this project financially.

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