Examination of hydrated and accelerated carbonated cement-heavy metal mixtures
Cement -based solidification/stabilisation (s/s) has been applied to the disposal of heavy metal bearing contaminated soil and wastes for approximately 50 years. This work studies the interactions of cement and heavy metals and provides further insight into encapsulation of heavy metals in cement matrices. The pastes and suspensions of calcium oxide, calcium hydroxide, pure cement phases ( 38, C}A, C4AF, Ci 2A7 and CA) and Portland cement with or without heavy metals (Zn2+ , Pb2+, Cu2+ and Cr3+) were examined by a number of analytical techniques. These techniques were X-ray powder diffraction (XRD), solid state magic angle spinning/nuclear magnetic resonance (MAS/NMR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), differential thermal analysis (DTA) and thermogravimetry (TG). Thermodynamic modelling using a geochemical code, PHREEQC, and the edited database, was carried out to elucidate the chemical reactions occurring in cement/heavy metal systems. Heavy metals acted as accelerators for hydration of CaO, CaS and Portland cement except that Zn2+ retarded the early-age hydration of Cfi and Portland cement. This work confirmed that the precipitation of portlandite was retarded due to the hydrolysis of heavy metals. Calcium ions resulting from the decomposition of cement phases combined with heavy metals to form calcium-heavy metal double hydroxides, including CaZn2(OH)6.2H2O, Ca2(OH)4Cu(OH)2.mH2O and Ca2Cr(OH)7 .3H2O. The carbonation of CaS and Portland cement resulted in the formation of calcium carbonate and the condensation of silicates from single tetrahedra to branching sites and three-dimensional frameworks (low Ca/Si ratio C-S-H gel). The polymerisation of C-S-H gel, and the polymorphism conversion and decomposition temperature of calcium carbonate were influenced by heavy metals. The incorporation of heavy metal cations in C-S-H gel is similar to that seen in glass. Heavy metals acted as network modifiers or network intermediates. In hydrated Portland cement pastes, aluminium was partitioned in ettringite or calcium carboaluminate. After carbonation, this work revealed that aluminium was in the tetrahedral form, forming mixed AlCVSiC^ branching or three-dimensional networks. This thesis presents the new structural models for C-S-H gel and the chemical mechanisms of 38 reactions with water and carbon dioxide in the presence or absence of heavy metals. In the absence of gypsum, the reaction products detected in the pastes of C3A, C4AF, Ci2A? and CA were gehlenite hydrate, calcium carboaluminate, C4AH X and hydrogarnet. Heavy metals, especially Zn 2+ , inhibited the formation of hydrogarnet and promoted the conversion of C-A-H to calcium carboaluminate and calcium carbonate. In the presence of gypsum, the major hydration product of C^A was ettringite. During carbonation, COs'" substituted for SO 4 2 " and formed calcium carboaluminate, and eventually transformed into calcium carbonate and gibbsite. The conversion of metastable calcium carbonate polymorphs (aragonite and vaterite) to calcite through Ostwald ripening occurred very slowly in the carbonated pastes containing gypsum. The reactivity of C 3 A, C^Ay, CA and C4AF during carbonation was much lower than seen during hydration. Heavy metals influenced the rates and products of hydration or carbonation of CsA, Ci2A7, CA and C4 AF and were completely incorporated in the reaction products of these phases. Thermodynamic modelling confirmed that accelerated carbonation could be beneficially employed to cement-based s/s to improve its effectiveness. Calculations of solubility and equilibrium phase assemblage are consistent with the experimental examination obtained in this work.