XRD and Unconfined Compressive Strength Study for a Qualitative Examination of Calcium–Arsenic Compounds Retardation of Cement Hydration in Solidified/Stabilized Arsenic–Iron Hydroxide Sludge

This study investigates the adverse effects of synthesized arsenic–iron hydroxide sludge, the by-product of arsenic removal by coagulation with ferric chloride, on unconfined compressive strength (UCS) and cement hydration of solidified/stabilized matrices. The results from both UCS tests and X-ray diffraction (XRD) implied that synthesized arsenic–iron hydroxide sludge might not be chemically inert in a cementitious environment, which could account for the retardation of cement hydration. The culprit for this retardation is likely to be the multiphase formation of calcium arsenic compounds suggested by the strong peak at 7.90?? (11.2°?2θ). This peak appeared when more than 20 and 13% of arsenic–iron hydroxide sludge were added to the solidification/stabilization (S/S) process of cement–water and cement-hydrated lime–water systems, respectively. The proposed mechanisms for the retardation of cement hydration by calcium–arsenic compounds are calcium complexation and, subsequently, surface precipitation due to the interaction between desorbed arsenate and hydration by-products in a cement porewater environment. The extent of the hydration retardation is qualitatively determined by the semiquantitative comparison of Ca3SiO5 and Ca2SiO4 remaining after 28?days of hydration between the control S/S samples and that with various doses of the sludge added. When 20 and 33% of the sludge were added into the S/S matrices, the remaining Ca3SiO5 and Ca2SiO4 were more than that of the control sample by factors of 2 and 3.2, respectively.