BATCH LEACHING TESTS VERSUS COLUMN LEACHING TESTS: WHICH METHOD IS BETTER FOR EVALUATING THE GEOCHEMICAL PROPERTIES OF CONSTRUCTION AGGREGATES?

Leaching tests for construction aggregates are essential for determining whether toxic and/or corrosive elements are solublized from the rock. However, selection of the appropriate leaching method is not straightforward. Batch leaching tests involve the placement of aggregate in a known volume of water for a given period of time. Differences in the sizes of the aggregates, water/aggregate ratio, and leaching time are common but typically vary within narrow ranges. Column leach tests allow water to flow through the pore spaces of a cylinder packed with aggregate. The contact time and number of pore water volumes exchanged are the commonly adjusted parameters. We performed batch and column leaching experiments with carbonate aggregates collected from six quarries in Texas. Batch leaching tests were conducted using 100g of rock to 1 L of DI water for 1 week. Column tests were performed by saturating the aggregate with one pore volume of water for 2 days, draining and collecting the water, letting the rock air dry for 5 days, and then re-saturating the rock with one pore volume of water for the next cycle. This was done until 50 pore water volumes were exchanged. The leachate from both test types was analyzed in-situ for pH and conductivity and filtered samples were measured for their ion concentrations. The pH and concentrations of major ions like Ca, Mg, SO₄ and Cl were typically higher in the short-term batch experiments than in the column tests. Moreover, the pH and major ion concentrations of the column tests decreased further as a function of the number of pore water volumes exchanged. For example, in one column experiment the SO₄ concentration decreased from 87 to 3 mg/kg over 50 cycles, while SO₄ in the corresponding batch experiment was 104 mg/kg. The results suggest that the column tests are most useful for situations involving water flow percolating through an unsaturated zone, while the batch tests more accurately reflect the chemical conditions of rocks saturated with water (and therefore probably the worst-case scenario). The decrease in ion concentrations in the column tests as a function of the pore water volumes exchanged is attributable to the fact that the smallest, most-readably leachable fractions are dissolved and flushed from the system over time.