Environ. Sci. Technol. 2001, 35, 788-793
Leaching of Concrete Admixtures Containing Thiocyanate and Resin Acids A° S E C . A N D E R S S O N * , † A N D ANN-MARGRET STRO ¨ MVALL‡ Department of Building Materials and Water Environment Transport, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden
There is an increasing concern about the emission of pollutants during the construction and lifetime of buildings. The leaching of concrete admixtures containing thiocyanate and resin acids was studied using standard leaching tests and chemical analysis. Ecotoxicological risk was assessed for each admixture. Thiocyanate leaching from concrete, with a chlorine-free accelerating admixture, was determined by ion chromatography. Of the total amount of thiocyanate added, 6-8% was emitted within 30 d. The thiocyanate diffusion curve indicates a fast dissolution process from the surface layer, followed by a slower continuous diffusion process. Thiocyanate exhibits both acute and chronic toxicity, which makes it of immediate environmental concern. Resin acid leaching from concrete test specimens containing an admixture of air-entraining agents with tall oil was determined by solid-phase extraction, methylation, and GC/MS. Of added resin acids, 10% was emitted over 143 d. The leaching curves for the resin acids indicate a continuous diffusion that is proportional to the square root of time and follows Fick’s first law of diffusion. The chemical composition of the resin acids in the leachate demonstrates degradation and rearrangement of the resin acids during diffusion. Resin acids emitted from concrete are of environmental concern because they are persistent and have the ability to bioaccumulate in aquatic organisms.
Introduction Studies on the environmental effects of building materials have focused on manufacturing and destruction processes with emphasis on energy use and depletion of resources. This study discusses potentially hazardous substances in concrete and the diffuse emission of these substances to surrounding environments during construction and use. Concrete is one of the most widely used construction materials. In Sweden alone, the annual use of concrete is approximately 7.5 million tons (1). The use of admixtures in concrete is steadily increasing since it promotes workability and durability (2, 3); as much as 700 000 tons of admixtures were used in Western Europe in 1999 (4). The growing interest in environmental issues in the building sector has drawn attention to these admixtures, particularly concerning the risk of environmentally hazardous substances released from concrete. * Corresponding author phone: +46 31 772 23 33; fax: +46 31 772 22 96; e-mail:
[email protected]. † Department of Building Materials. ‡ Water Environment Transport. 788
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 4, 2001
Several studies have been carried out on the topic of leaching from concrete, particularly the leaching of heavy metals originating from concrete additives such as fly ash and blast furnace slag (5, 6). The release of organic substances has focused on water-reducing superplasticizers (7, 8), which represent the largest volumes used. However, the active substances in superplasticizers are usually not toxic to humans and aquatic organisms (9). A chlorine-free accelerating admixture with thiocyanate (SCN-) was investigated in this study. When concrete is hardening at low temperatures, thiocyanate accelerates the hydration and increases the strength (10). Thiocyanate is of environmental concern because of its acute toxic effects (11) as well as its chronic toxicity to fish (12) and humans (13). The general toxicity and environmental fate of thiocyanate have been reviewed elsewhere (14). Air-entraining admixtures provide an even distribution of air bubbles in hardened concrete and thereby frost resistance. Natural polymers, such as tall oils, are the main products used (2). Crude tall oil, a byproduct from the kraft pulp industry, is mainly composed of fatty and resin acids. Resin acids are diterpenoid carboxylic acids present in most softwood species (15); dehydroabietic acid and abietic acid being the most abundant. The relative amount of fatty and resin acids in distilled tall oil products is of environmental concern because of the high toxicity and persistence of the resin acids (16, 17). Resin acids have acute toxic effects on fish and other aquatic organisms; their toxicity in wastewater from pulp mills has been reviewed elsewhere (18). Resin acids are also persistent and have the ability to bioaccumulate in aquatic organisms (19, 20). Direct estrogenic effects on fish (21) and indirect effects through biodegradation products released from sediments and/or effluents from pulp mills have also been reported (22). Results from a recent study indicate a mutagenic potential as measured by genotoxic response in fish (23). Synthetic emulsifiers such as alkylaryl sulfonates (24), alkyl sulfates (25), and nonylphenol ethoxylates (26) are also used as air-entraining agents and have serious environmental effects on aquatic organisms, although these were not studied in this project. In this study, the aim is to localize elements in concrete admixtures that are reported to have effects on aquatic organisms at low concentrations and to study their leaching potential during exposure to water. Concrete test specimens with additives containing thiocyanate and resin acids were subjected to a diffusion test to determine time-dependent release. This provides a basis for calculating the effective diffusion coefficient, which is required for modeling and predicting leaching behavior under real conditions.
Experimental Section Leaching Tests. The test method used is based on a Dutch standard test NEN7345 (27); this is a diffusion test that determines time-dependent leaching, assuming that diffusion is the dominant leaching mechanism for concrete (28). Since the method was developed for leaching inorganic substances, it was slightly changed here. As it is important that the leaching process stabilizes toward a diffusion-controlled process, the test durations in this study were 30 and 143 d. Materials and admixtures used for the concrete test specimens are presented in Table 1. The abbreviation used for each admixture is later used for the results of the concrete specimens. The water-cement ratio was 0.7 for all of the concrete specimens to get good concrete quality without adding superplasticizers and to make sure that detectable concentrations were achieved. Normally, outdoor concrete 10.1021/es000138h CCC: $20.00
2001 American Chemical Society Published on Web 01/12/2001
TABLE 1. Materials and Admixtures Used for Concrete Leaching Test Specimens cement aggregates additives admixtures
Portland cement (PC1) complying with ENV 197 (Sko¨ vde) crushed rock of 8-12 mm, gravel of 0-8 mm, sand of 0.068-0.354 mm and 0.125-0.707 mm, fine quartz sand (as filler) Danish fly ash (FA) complying with DS/EN 450 Swedish blast furnace slag (GGBS) from Merox (1) accelerator (A), dosage 1.25% of the cement weight, containing ∼7% sodium thiocyanate (4 specimens) (2) air-entraining agent (AEA1), dosage 0.4% of cement weight, containing 1-5% tall oil and 5-10% alkyl ethoxylate sulfate (1 specimen) (3) air-entraining agent (AEA2), dosage 0.05% of cement weight, containing 18% tall oil and
