From waste to function: Ground walnut shells as a reactive additive for heavy metal removal in pervious concrete
This study examines the effects of grounded walnut shell (GWS) incorporation on the structural, transport, and contaminant-sequestration properties of pervious concrete, including its water purification efficiency from heavy metal-zinc. Two complementary experimental protocols were developed to quantify heavy-metal immobilization under both static and dynamic flow regimes, thereby simulating quasi-static ponding and percolative stormwater infiltration, respectively. Additionally, the influence of GWS as an internal curing agent on key mechanical and physical characteristics of the pervious composite was evaluated. It was found that the considered modification had a statistically significant influence on the investigated properties. GWS acted as an internal curing agent, increasing the porosity of the cement matrix. A slight deterioration in the mechanical performance of pervious concrete was observed at lower GWS contents (2 % and 10 %). The reference samples exhibited noticeably higher compressive strength relative to their effective porosity compared to the GWS-modified mixtures, indicating that the presence of GWS altered the typical strength-porosity relationship observed in pervious concrete. However, due to increased capillary pore content, the immobilization efficiency of GWS-modified concrete increased twofold (from 27 % to approx. 54 %) in static test conditions compared to the reference series. Conversely, in dynamic flow tests, pollutant contact was restricted to the outer cement matrix layer, yielding more modest gains in immobilization performance. The interfacial transition zone (ITZ) between the cement matrix and internal curing agent (GWS grains) was identified as a factor significantly influencing the immobilization potential of pervious concrete.