Amelioration of physical strength in waste foundry green sands for reuse as a soil amendment

To avoid increasing costs of landfill disposal, it has become increasingly important for U.S. foundries to identify beneficial reuses for the 8 to 12 million tons of waste foundry sand (WFS) generated annually. A major drawback to the reuse of some WFSs as a soil amendment is their high soil strength, under dry conditions, where root growth may be limited. Fifteen WFSs were analyzed for strength to rupture using lab-formed clods, exchangeable cations (Na, Mg, Ca), metal oxide concentration (Fe, Mn, Al, Si), cation exchange capacity (CEC), and % clay. Several WFS samples from gray iron foundries demonstrated high strength to rupture values (> 1.5 MPa), and could potentially restrict root growth in amended soils. The percentage of Na-bentonite exhibited a positive correlation (R² = 0.84) with strength to rupture values. When WFSs containing more Na-bentonite were saturated with 1 mol L^-1 Ca ions, strength values decreased by ∼70%. Waste foundry sands containing less Na-bentonite were saturated with 1 mol L^-1 Na ions and exhibited a threefold increase in strength. Additions of gypsum (up to 9.6 g kg^-1 sand) to high strength waste foundry sands also caused decreases in strength. These results indicate that high strength WFSs have properties similar to hardsetting soils which are caused by high Na⁺ clay content and can be ameliorated by the addition of Ca²⁺.