Key Engineering Materials Vol. 617

Abstract: A new low density mineral material has been synthesized via a simple, flexible, cheap and easy to control process. This material is a synthetic carbonate produced by carbonation of a solid phase composed of a calcic part and a magnesian part. Typically, its production process includes the calcination of a raw dolomite (general formula CaCO₃.MgCO₃) into the oxide form, followed by an at least partial hydration of this oxide and a subsequent carbonation step. This process is thus close to the well-known process used for the production of Precipitated Calcium Carbonate (PCC), a common filler and pigment in plastic, paper and rubber, except that the raw material is a dolomite instead of a limestone. It has to be pointed out that flue gases from different industries can be used as a source of CO₂ for the carbonation. Many parameters have been studied, such as the hydration conditions or the carbonation conditions, allowing the production of a whole range of products. The products obtained via this process are composed of a calcic part, mainly as calcite and aragonite (CaCO₃), and a magnesium containing part, mainly as hydromagnesite (4MgCO₃.Mg (OH)₂.4H₂O). Compared to standard PCC, this material has a particularly low density (100-250 kg/m³ instead of 300-600 kg/m³ for standard PCC), a rather high BET specific surface area (15-35 m²/g instead of 4-15 m²/g for standard PCC) and a specific particle morphology and structure. The particles are indeed commonly spherical particles composed of a core of calcite/aragonite covered by a layer of hydromagnesite. This structure can be described as a core-shell structure. Due to these special features, this product could be used in multiple applications, such as raw material for the manufacture of insulation material, flame retardant, filler in plastics, paints, paper, rubber, and many more.

Abstract: In this study, a flat plate limestone of which surface area was accurately defined was dissolved into acid in an agitated vessel. Dissolution rates of limestone were measured under various agitation speeds, temperatures and acid concentrations. As a result, it was found that the rates were controlled by the diffusion of particular chemical species depending on acid concentration.

Abstract: This paper investigates the effect of siliceous, limestone and plastic aggregates on the compressive behavior of concrete. Firstly, the matrix/aggregates interface is characterized using a scanning electronic microscope. Compressive test is then simulated on a 3D numerical concrete using finite element method. The influence of the matrix/aggregates interface quality is quantified. The results give a new insight and help on the understanding of the contribution of the nature of the aggregate on the overall mechanical behavior of concretes.

Abstract: As heavy metal elution control material which controls the lead compound eluted from municipal waste incineration fly ash, we examined aluminum hydroxide and various adsorbent minerals, such as activated clay. Low crystallinity aluminum hydroxide can control the lead compound elution with a smaller addition as compared with various adsorbent minerals as a result of an examination. In more lead content fly ash, only the low crystallinity aluminum hydroxide fitted the lead compound elution standard limit in waste landfills site of Japan. We considered that the following two reasons contributed to reduce the amount of lead compound elution. Aluminum hydroxide reacted to a part for the calcium of fly ash, and then it reduced the solubility of the lead compound due to the fall of the pH of an eluate. The cement hydrate which is a reaction product of aluminum and calcium fixed the lead compound. We report the test result of the amount of lead compound elution from fly ashes which were obtained from some waste incineration plant.

Abstract: Hydrocalmite-like hydrate is one of calcium aluminate hydrated products. This hydrate has a layer structure and the principal layer, which is similar to calcium hydroxide define alternate with interlayer including divalent anions and water molecules. Their anions are able to react with outer anions by anion-exchange reaction. In this study, anion-exchange reaction between hydrocalmite-like hydrate including chloride anion (Ca₄Al₂(OH)₁₂·Cl₂·4H₂O, Cl-HC) and chromate anion (CrO₄^2-) is examined. CrO₄^2- is immobilized in various hydrated forms, which are varied with an increasing in the additive ratio of CaCrO₄ to Cl-HC. As the additive ratio of CaCrO₄ to Cl-HC is below 1.0 in molar, Ca₄Al₂(OH)₁₂·CrO₄·nH₂O (n=6 or 8, CrO₄-HC) is formed. In this case, the concentration of chromate ion in solution is less than 1.0 ×10^-3 mol·dm^-3 and over 99% of chromium immobilize in solid phase. However, when the additive ratio of CaCrO₄ is over 1.0, ettringite type hydrate including CrO₄^2- (Ca₆Al₂(OH)₁₂·3CrO₄ ·26H₂O) is generated and over 10 % of chromium is eluted from the solid phase.

Abstract: This study evaluated the fluorine removal capacity of half-burnt dolomite and examined the removal mechanism. As the result of examining the burning conditions of dolomite, it was found that dolomite burnt at 1023 K for 4 h has the highest performance. It turned out the fluorine removal mechanism that CaCO₃ does not contribute to reaction and that fluorine is sorbed with hydration of MgO. As the result of comparing removal capacity by changing the initial pH of fluorine solutions, it was found that performance is reduced in the strong alkaline solution.

Abstract: Tested super sulfated cements (SSC) are composed with various amounts of ground granulated blast furnace slag, gypsum or activated gypsum and Portland cement. Our initial results on the optimization of the composition and of the curing conditions of such cements are presented herein. Tests are conducted on the basis of standard mortar with two different gauging ratios according to old and new standards for SSC. Four types of curing are used. Mechanical performances are assessed at 2, 7, 28, 60 and 90 days on standard mortar samples (binder/sand mass ratio = 1/3). This study shows that, thanks to an optimization of the Portland cement content, it is completely possible to obtain super sulfated cement with a strength class 32.5N, and always in conformity with the new European standard. For a given SSC formulation, it is clearly demonstrated that the origin of the calcium sulfate addition that affects the development of sulfo-aluminous compounds in these mixtures constitutes a key point in the phenomena of setting and hardening. The use of heat-activated gypsum as a sulfate activator appears preferable.

Abstract: The purpose of this study is to evaluate the performances of Super Sulfated Cement (SSC) foams, focusing on structural, thermal and mechanical characteristics. The studied set of SSC foam samples is obtained with the same slurry. The chosen foaming method allows an interesting density variation: from 489 kg/m³ to 1793 kg/m³. Thanks to a CCD camera, the visual study of foam pore structure reveals two kinds of bubbles distribution and associated connectivity. This may partially explain the obtained thermal and mechanical behavior. Results show that SSC foams with low density (< 550 kg/m³) are usable as thermal insulator for non-loadbearing walls. Mean density SSC foams (550 kg/m³ < - < 640 kg/m³) can be used as slight-loadbearing and thermal insulating products in housing and SSC foams with high density (640 kg/m³ < - < 1200 kg/m³) as loadbearing products staying within lightweight class.

Abstract: Various problems related to pollution of soil and groundwater caused by toxic oxyanion have been actualized in recent years. In particular, the enlargement of the polluted area by diffusion of chromate ions has been a worry in Japan, and arsenic pollution of well water is also a serious problem in South Asia nations. Removal process of chromate and arsenic in aqueous solution using 14CaO·5Al₂O₃·6ZnO, which is an easy-to-use method technically, has been investigated in our laboratory. The results obtained are summarized as follows: 1) Oxyanion of chromium and arsenic such as CrO₄^2-, AsO₃^3- and AsO₄^3- ions are able to be removed from aqueous solution by addition of 14CaO·5Al₂O₃·6ZnO, however, the removal rate of those arsenic ions are much lower than that of CrO₄^2-. 2) Removal of CrO₄^2- from aqueous solution is caused by the immobilization to 3CaO·Al₂O₃·CaCrO₄·12H₂O, on the other hand, both AsO₃^3- and AsO₄^3- mainly occurred by the adsorption to unhydrated 14CaO·5Al₂O₃·6ZnO. 3) Simultaneous use of 14CaO·5Al₂O₃·6ZnO and calcium hydroxide is an effective method in the removal of CrO₄^2-, AsO₃^3- and AsO₄^3- from aqueous solution.