F. S. Hashem

Kinetic and thermal studies of removal of CrO 4 2− ions by ettringite

Ettringite was prepared in the presence of various dosages of CrO42− ions at three different temperatures. X-ray diffraction (XRD) and differential scanning calorimetery (DSC) results showed shift in the peaks characterized to ettringite as the result of formation of Cr-substituted ettringite. Substitution of SO42− by CrO42− ions in ettringite crystal resulted in delay in the rate of its formation, as well as reduction in the crystal diameters. However, the crystal diameters and rate of formation of both ettringite and Cr-substituted ettringite were increased by increasing temperature of preparation, and this was confirmed by scanning electron microscope (SEM) micrographs. The thermodynamic parameters: the enthalpy, entropy, and Gibbs free energy changes for substitution process were studied. It is concluded that the removal of chromate ions by ettringite is an endothermic spontaneous process. The positive value of ΔS° suggests increasing in randomness during the removal process.

A new method to create one-part non-Portland cement powder

Non-Portland cement or alkali-activated slag is regarded as non-friendly for users, due to the corrosive nature of alkaline solution. This negatively affects the mass production and commercial viability of this cement. In this work, user- and eco-friendly one-part non-Portland cement (NPC) was prepared by blending ground-granulated blast-furnace slag (GGBFS) with stable dry activator (SDA). SDA was synthesized by mixing two moles of NaOH with one mole of MgCO3 or dolomite, followed by drying and pulverizing to a fixed particle size (namely, SDA-M and SDA-D, respectively). The Mg source was chosen to prepare active magnesia with no firing. So, this process was considered as eco-friendly method for this purpose. Ordinary Portland cement (OPC) was used for comparison. The SDA-M content plays a circular role on the reduction of drying shrinkage of one-part NPC. The SDA-M has a potential impact on the compressive strength development of one-part NPC compared to SDA-D. NPC containing 12 mass% SDA-M (NPC-12) gave compressive strength values superior to those of OPC at all curing ages, suggesting that the NPC-12 can be beneficially used as an alternative to OPC as confirmed by X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM).

Chromium (III) Immobilization and Its Influence on C3 S Hydration

The hydration characteristics of C3 S were studied in the presence of chromium (III) for up to 7 days. Chromium was introduced by either direct incorporation (as Cr2 O3 ) during C3 S preparation or during the subsequent hydration with a chromium ( CrCl3 ) solution. Levels of Cr(III) used were 1 and 3% by wt. of C3 S . The chemically combined water content and free lime content were determined after 3 and 6 h and 1, 3, and 7 days of hydration. The results showed a retardation effect for the hydration of C3 S in the presence of Cr, especially during the first hours of hydration. This retardation effect was more evident in C3 S–Cr incorporated mixes than in C3 S mixes hydrated in Cr-containing solutions. X-ray diffraction examination was performed on selected samples. The immobilization percentage of Cr(III) by C3 S during hydration was determination by atomic absorption spectroscopy. A high degree of Cr immobilization (93.27–99.39%) was found after 7 days of hydration.

Utilization of OPC-Pumice composites for efficient heavy metals removal

ABSTRACT The immobilization effect of 1 and 3 mass % of Ni2+ or Co2+ ions on the hydration reaction of ordinary Portland cement (OPC) and ordinary Portland cement blended with 5 and 15 mass % pumice was investigated. This was done by studying the bulk density, total porosity, compressive strength as well as identifying the phase composition of the formed hydrates using X-ray diffraction (XRD) technique. In addition, leaching behaviour of Ni2+ and Co2+ ions was examined up to 90 days using static leaching technique. The results indicated that the incorporation of these ions causes a notable retardation for the early hydration of the OPC. This effect is more noticeable in presence cobalt ions. Besides, the retarding effect was increased by increasing the percentage of heavy metal ions incorporated in the hydrated matrix. However, for OPC-pumice blended cement pastes, the existence of Ni2+ or Co2+ ions resulted in an enhancement in the compressive strength values during the hydration period from 3-28 days to be comparable with the values of neat OPC pastes. This enhancement is attributed to the interaction of these ions with pumice which increases the heavy metal fixation.