Blended Cements with kaolinitic calcined clays: Study of the Immobilization of Cr (VI)

Abstract

Portland and blended cement pastes have been demonstrated to be suitable to immobilize some hazardous heavy metals such as Cr(VI), one of the most dangerous for people's health. The aim of the this investigation is to study the capability to the immobilization of Cr(VI) of blended cement made with different calcined kaolinitic clays. In previous study, it was found that the reactivity of kaolinitic calcined clays used as partial replacement of Portland cement (15% and 30% by mass) largely depends on the cristallinity of kaolinite in the clay. Calcined clays obtained from raw materials containing kaolinite with disordered structure presents a very high pozzolanic activity allowing high replacement levels (30%) in blended cements. In this study, blended cement pastes containing 0, 15% and 30% by mass of two different kaolinitic calcined clays (ordered and disordered structure) were elaborated using a solution of 5000 ppm of K2Cr2O7 and, solution/blended cement of 0.50. Pastes were sealed curing until 28 days and the Cr(VI) immobilization in pastes was measured as the difference between added and leached chromium, using spectrophotometric determination of Cr(VI) water-soluble. Complementary, the hydration phases were determined by XRD and FTIR analyses, and morphological aspect of pastes was observed by SEM/EDS. Results show that both kaolinitic calcined clays have a quit different behavior respect to Cr(VI) immobilization. Blended cement with 15% of calcined clays from kaolinite with ordered structure presents a very high ( 90%) immobilization of Cr(VI) and it was higher than that achieved by PC. On the other hand, blended cements containing calcined clays from kaolinite with disordered structure presents the best performance from pozzolanic activity and compressive strength (up to 30% replacement) but they have a lower immobilization of Cr(VI) ( 60%). XRD analysis shows that PC and blended cements with high immobilization of Cr(VI) present an intense peaks assigned to the Cr-ettringite phase, and the SEM/EDS reveals the presence of chromium in C-S-H and in very small crystals surrounded by C-S-H. For pastes with low Cr(VI) immobilization, the Cr-ettringite is detected by XRD, but the small crystals encapsulated by C-S-H were only observed by SEM/EDS. The low immobilization in the calcined clay from kaolinitic clay with disordered structure is attributable to a high rate of pozzolanic reaction that consume the CH and could impede the immobilization of a large amount of Cr(VI). For kaolinitic clays with ordered structure, the lower replacement level (15%) attains a similar mechanical efficiency to the PC and increased the immobilization of Cr(VI).