Heavy Metal Removal Using Alkali Activated Kaolinite in the Cao-Al2O3-Sio2-H2O System

Most of calcium silicate hydrate or calcium aluminosilicate hydrate, CS(A)H, and related phases are relevant to Cement industry and have potential as geothermal well sealants as well as autoclaved construction materials. The CS(A)H system is highly complex and is comprised of crystalline to amorphous phases with variable composition that build up a wide family of phases, which are interesting for the variety of their structural arrangements, the peculiarity of the transformation processes in which they are involved and the relationships with compounds which form during the hydration of the Portland cement [1]. Different studies have been carried out on the hydrothermal transformation of kaolinite or metakaolinite into CS(A)H phases, with particular attention on tobermorite. The most widely studied aluminium-bearing materials and their effects on various aspects of tobermorite formation include: γ-Al2O3, Al(OH)3, kaolinite, metakaolinite, zeolite and solid waste materials. The raw materials used in the synthesis of tobermorite usually contain aluminium-bearing impurities and therefore the role of aluminium in the stabilization of tobermorite has been reported in several studies [2-10]. A chronology of the main results from these studies is presented as follows. [11-12] undertook a microstructural study on autoclaved clinker and slag-lime pastes in the presence and absence of silica sand using scanning electron microscopy, determining the formation of several hydration products, such as C-S-H, hydrogarnet and tobermorite. The formation of hydrogarnet for ratios of Al/(Si +Al) in the range of 0.12–0.50 when kaolinite is used as the source of aluminium was reported by [13]. Mitsuda & Taylor [3] studied the effect of substitution of Si by Al in tobermorite formed in the CaO-SiO2H2O system. Van Aardt & Visser [14] investigated the reaction between Ca(OH)2 and minerals containing alumina, which forms C4A-hydrates and/or hydrogarnet (calcium-alumina-silicate hydrate), depending on temperature. Serry et al. [15] studied metakaolin-lime hydration products, determining that gehlenite hydrate (C2ASH8) was the main hydration product and its amount increased with reaction time, whereas hydrogarnet crystallized at the early stages of hydration in the low-lime content mixtures, increasing with curing time. Atkins et al. [16] studied the solubility properties of ternary and quaternary compounds in the CaOAl2O3-SO3-H2O system, with ettringite (C6A3H32) and hydrogarnet (C3AH6) dissolving congruently, whereas monosulphate (C4AH2) and tetra-calcium aluminate hydrate (C4AH13) dissolved incongruently. The reaction mechanism of the hydrothermally treated CaO-SiO2-Al2O3 and CaO-SiO2-Al2O3-CaSO4 systems was conducted by Al-Wakeel & El-Korashy [17], using mixtures of CaO, amorphous SiO2 and kaolinite in the presence or absence of SO4 ions (added as CaSO4·2H2O), which were treated in suspension under hydrothermal conditions at temperatures of 80-200 °C. The results indicate the occurrence of several C-S-H phases, which are then transformed into Al-substituted 11 Å tobermorite. In both systems excess Al2O3 appeared as a hydrogarnet phase. [18] First reported the occurrence of hydrogarnet-type cubic crystals in polymer-modified mortars. [19] investigated the hydrothermal conversion of kaolinite with calcium hydroxide and dissolution of reaction products in hydrochloric acid, obtaining different reaction products, such as hibschite, omisteinbergite, tobermorite and jaffeite. Klimesch and Ray investigated the effects of quartz particle size on hydrogarnet formation and the nature of hydration products in unstirred autoclaved metakaolin-lime-quartz [20-22] and kaolin-lime-quartz [23] slurries. Hydrogarnet was always among the first phases formed and invariably appeared before 11 Å tobermorite and with increasing reaction time the amount Volume 1 Issue 4 2017