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LEACHING OF ALKALINE SUBSTANCES AND HEAVY METALS FROM RECYCLED CONCRETE AGGREGATE

USED AS UNBOUND BASE COURSE

Presenter: Jiannan (Nick) Chen Advisors: James M Tinjum and Tuncer B. Edil

9/11/12 1 Mid-‐Con-nent Transporta-on Research Forum

Recycled Concrete Aggregate (RCA) Demolition of existing concrete pavement, bridge structures, roadway

structures and airport runways

Uses Infrastructure backfill, e.g. Pavement base course

Advantages   Excellent mechanical properties

e.g. Higher resilient modulus, and freeze-thaw durability

  Significant life-cycle benefits e.g. Reducing greenhouse gas emissions, energy and virgin aggregate consumption

  Widely available and used An average of 140 million tons of RCA is produced annually (ACPA 2008)

Background

Mid-Continent Transportation Research Forum

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Environmental Issues

High alkaline leachate

  Cement-based material

  Wide pH range (7.5≤pH≤13) in field studies

Leaching of heavy metals

  pH-dependent leaching behavior

  Grain-size effect due to exposed surface area

Objectives   Leaching of alkaline substances and heavy metals

  Different leaching patterns of RCAs from new demolitions and stockpiles


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Problem Statement

A wide geographical range of RCA sources were used in this study.


Materials

California, CA RCA

Minnesota, MN RCA

Colorado, CO RCA

Texas, TX RCA

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Wisconsin, WR- F and WR-SP RCA

Note: CA, CO, MN and TX RCA were provided by pooled fund project.


On-Going Research Work

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Batch Tests (BLT)

Column Tests (CLT)

Field Tests (FLT)

CA, CO, MN and TX

MN, WR-F and WR-SP

All Testing Materials

Lysimeters


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A

C

B C

Field Leaching Tests MNRoad Facilities University of Wisconsin – Lot 60

A

B


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Results

Note: (C) Calcite, (D) Dolomite, (F) Feldspar, (Q) quartz.

Mineralogy


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Leachate pH – FLT& CLT

 WR-F RCA started with a high leachate pH (12.6) and remained constant during testing period (5 PVF).   WR-SP RCA started at a lower pH (7.3) but with a gradually increasing trend, up to pH = 12. 1 during testing period (2 PVF).

MN RCA Field vs. Column


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Elements Exceed MCL of USEPA Drinking Water Standard


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AW

 As and Cr were found to be mainly sourced from cement mortar when compared with AW.  Pb and Se had similar concentrations in two RCA samples of AW.  Pb and Se could come from both mortar and natural sediment.

Results of Total Elemental Analysis


Batch Leaching Test - EPA method 1313

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Gravels

Sands

Fines

TASK I (on Bulk Samples)

Gravels (75-4.75 mm )

Sands (4.75-0.075 mm )

Fines (<0.075 mm )

TASK II (on Fractionated Samples)

Test parameters   pH target: adjust pH to 2~13 with acid or base   L/S ratio: 10:1 (wt/wt)  Contact time: 72 h in a 30 rpm tumbler

Tumbler Batches

Data Comparison


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Results Material pH and weight percentage of each grain

Material pH : Batches without acid/base addition. Alkaline Leaching: Gravel > Sand > fines


Cu and Zn: maximum release at pH ≅ 2.0, and minimum release at pH > 6.5 for Cu and pH > 7.5 for Zn.

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pH-dependent leaching patterns – Cu and Zn

Cr: minimum between pH 5.0 and 6.5, and increasing release pH < 5 and pH > 6.5.


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pH-dependent leaching patterns – Cr


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Grain-size effect on Cu and Zn leaching

Fines faction released more Cu and Zn than coarser-grain fractions. Mid-Continent Transportation

Research Forum 9/11/12 16

No obvious differences in Cr leaching among the three fractions. Mid-Continent Transportation

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Grain-size effect on Cr leaching


Conclusion  Fresh RCA started with a high leachate pH (12.6) that remained constant throughout the testing period.Stockpiled RCA started at a lower pH (7.3), but gradually increased to pH = 12.1 during the same testing period.

 Laboratory column tests showed high alkaline leachate (pH = 10.8 to 12.5) from all five RCA materials, and no pH decline during the long testing period (100 PVF).

 As, Cr, Pb, and Se exceeded the maximum contaminant levels (MCLs) in the USEPA drinking water standard in the field leaching tests.

 Different pH-dependent leaching patterns of trace elements (Cu and Zn) and the oxyanion (Cr) were observed.

 In the most observed field pH scenario, Cr(VI) will have a higher leaching potential from RCA.

 Finer particles leached more Cu and Zn than coarser grains. However, the difference in leaching based on grain size alone is relatively small for Cr.

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•  This material is based on work supported by : TPF-5 (129) Recycled Unbound Materials Pool Fund Recycled Materials Resource Center (RMRC) Solid Waste Research Program of UW The National Center for Freight and Infrastructure Research and Education (CFIRE)

•  Professor Craig H. Benson, James M. Tinjum, Tuncer B Edil and Sabrina L. Bradshaw

•  Undergraduate Researcher: Brigitte L. Brown, Kyle Groves •  Lab Manager: Jackie B. Cooper and Xiaodong Wang •  Geo-friends, University of Wisconsin at Madison


Acknowledgement

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