Impact of aging on leaching characteristics of recycled concrete aggregate

Abstract

The focus of this study was to evaluate the effects of stockpiling (aging) on leaching of elements in recycled concrete aggregate (RCA) that may contribute to tufaceous constituent formation. Speciation and leaching controlling mechanisms of these elements were identified via geochemical modeling. The effects of stockpiling were simulated by comparing freshly produced RCA with RCA aged as part of this study for 1 year both in the laboratory and in the field. Leachate samples were generated following batch water leach test (WLT) and US Geological Survey leach test (USGSLT) methods. USGSLTs were conducted both on the laboratory and field samples while WLT was only conducted on laboratory samples. During the laboratory aging, it is observed that the carbonate content of RCA, measured as calcite equivalent, increased 20 % (i.e., from ∼100 to 120 mg/g) within a year time frame. The leachate extracted from RCA showed minor changes in pH and more significant decreases in electrical conductivity (i.e., ∼300 to 100 μS/cm). A comparison between laboratory and field samples revealed that the RCA aged much slower in the field than in the laboratory within a year. Comparisons between two leach extraction methods on the laboratory conditions showed that the total leached concentrations (TLCs) of most of the constituents from USGSLT were appreciably lower than the ones measured via WLT method. The results of geochemical modeling analyses showed that Al, Si, Fe, Ca, Mg, and Cu exist in their oxidized forms as Al, Fe, Si, Ca, Mg, and Cu and results revealed that these elements are primarily controlled by the solubility of gibbsite, hematite, silica gel, calcite, magnesite, and tenorite solid phases, respectively. One of the significant findings of the study was to identify the changes in leaching behavior of Ca, Si, Mg, Al, Fe, and Cu due to carbonation.

Chemical and Leaching Behavior of Construction and Demolition Wastes and Recycled Aggregates

“…In fact, pH is strictly related to the carbonation degree, which is linked to the age of the building, the exposure conditions during its lifetime, and the type of concrete [28]. In particular, carbonation occurs when the cement hydrate phases react with CO 2 , precipitate as calcium carbonate (CaCO 3 ) [40], and develop carbonated material [41]. This process generally proceeds slowly during the service life of concrete structures, but when these concrete materials are demolished and transformed into recycled aggregates, their specific surface increases, resulting in greater CO 2 absorption.…”

Toxicity of Recycled Concrete Aggregates: Review on Leaching Tests

“…The Coudray's study [42] used concrete 50 years old in RCA production and, on the other hand, Lopez Meza [46] used new concrete formulated in the laboratory. In all analysed studies, RCA produced in the laboratory or collected in recycling centres are constituted by at least 90% of ground concrete and/or mortar elements [48] [51]. An exception is the Del Rey study [23], which classifies RCA as a blend of natural aggregate, concrete and mortar, representing more than 90% of their constitution, even if the concrete content is as low as 41%.…”

“…Abbaspour [39] states that concrete aging does not significantly affect pH values, but affects the released concentrations of Al, Fe, Ca, Mg, Si, SO 4 -2 and Cu. However, Galvín argued that, for pH values between 4 and 7, in the acidic region, there is an increased concentration of Zn, Cu, Ni, Pb and Cd [45].…”

Neutralization of high pH and alkalinity effluent from recycled concrete aggregate by common subgrade soil

“…Alkaline wastes, such as RCA, fly ash, and red mud, have been extensively reused as engineering fills or additives in concrete in recent years (Abbaspour, Tanyu, & Cetin, 2016;Bestgen, Cetin, & Tanyu, 2016;Cetin, Aydilek, & Li, 2014;Gomes, Mayes, Rogerson, Stewart, Burke, 2016;Sun et al, 2019). Leachate generated from these wastes are often associated with the concerns of high pH (usually pH = 9-14) and elevated concentrations of hazardous elements, especially oxyanions (e.g., arsenic [As] and chromium [Cr]) (Chen, Bradshaw, Benson, Tinjum, & Edil, 2012;Sun et al, 2019).…”

“…Leachate generated from these wastes are often associated with the concerns of high pH (usually pH = 9-14) and elevated concentrations of hazardous elements, especially oxyanions (e.g., arsenic [As] and chromium [Cr]) (Chen, Bradshaw, Benson, Tinjum, & Edil, 2012;Sun et al, 2019). Attenuation options for the hyperalkaline leachate usually include active aeration (high partial pressure of CO 2 ) for carbonation, recirculation of drainage waters, and acid treatments (Abbaspour et al, 2016;Mayes et al, 2009). For example, Roadcap, Kelly, and Bethke (2005) stated that strong acid, including hydrochloric acid (HCl) and sulfuric acid (H 2 SO 4 ), can effectively neutralize the pH of discharges from the active processing plants but may not successfully remove other contaminants (e.g., heavy metals).…”