Release of major elements from recycled concrete aggregates and geochemical modelling

• Published in: Cement and concrete research 2009-05, Vol.39 (5), p.446-459
• Main Authors: Engelsen, Christian J., van der Sloot, Hans A., Wibetoe, Grethe, Petkovic, Gordana, Stoltenberg-Hansson, Erik, Lund, Walter
• Format: Article
• Language: English
• Subjects: Aggregates and other concrete constituents; ALUMINATES; ALUMINIUM HYDROXIDES; Applied sciences; ATOMIC FORCE MICROSCOPY; Buildings. Public works; CALCIUM SILICATES; CARBONATES; Cement concrete constituents; CEMENTS; CONCRETES; Equilibrium; Exact sciences and technology; Geochemical modelling; GEOCHEMISTRY; Hydrate phases; HYDRATES; IRON HYDROXIDES; LEACHING; MAGNESIUM COMPOUNDS; Materials; MATERIALS SCIENCE; Miscellaneous (including polymer concrete, repair and maintenance products, etc.); PARTICLE SIZE; PRECIPITATION; SILICA; SIMULATION; SOLUBILITY; SULFATES; WAIRAKITE
• ISSN: 0008-8846; 1873-3948
• DOI: 10.1016/j.cemconres.2009.02.001

The pH dependent leaching characteristics were assessed for different types of recycled concrete aggregates, including real construction debris and crushed fresh concrete samples prepared in laboratory. Carbonation effects were identified from the characteristic pH dependent leaching patterns for the major constituents Al, Ca, Fe, Mg, Si and SO 4 2–. The original particle size ranges were different for the samples investigated and this factor influenced the cement paste content in the samples which in turn controlled the leachable contents. Cement paste contents for concrete samples with fine particle size fractions (0–4 mm) were found to be higher than the originally present amount in the hardened concrete. Geochemical speciation modelling was applied over the entire pH range using the speciation and transport modelling framework ORCHESTRA, for which mineral saturation, solution speciation and sorption processes can be calculated based on equilibrium models and thermodynamic data. The simulated equilibrium concentrations by this model agreed well with the respective measured concentrations. The main differences between the fresh and aged materials were quantified, described and predicted by the ORCHESTRA. Solubility controlling mineral phase assemblages were calculated by the model as function of pH. Cement hydrate phases such as calcium silicate hydrate, calcium aluminate hydrate (AFm and AFt) and hydrogarnet were predominating at the material pH. The concentration of carboaluminates was found to be strongly dependent on the available carbonates in the samples. As the pH was decreased these phases decomposed to more soluble species or precipitates were formed including iron- and aluminium hydroxides, wairakite and amorphous silica. In the most acid region most phases dissolved, and the major elements were approaching maximum leachability, which was determined by the amount of cement paste.