Characteristics of precipitated calcium carbonate by hydrothermal and carbonation processes with mega-crystalline calcite using rotary microwave kiln

In the conventional kiln, mega-crystalline calcite (m-CC) breaks apart easily during calcinations, and cannot be easily converted to CaO due to that it requiring a lot of heat. In this study, m-CC was calcined to CaO of around 1 mm using the rotary microwave kiln. Furthermore, CaCO₃ was produced by the carbonation process and hydrothermal process, and the form of CaCO₃ was characterized.Calcination of m-CC using the rotary microwave kiln resulted in CaO (97 wt%) of relatively fine size.CaCO₃ of colloidal-shaped and 6 μm in size could be prepared by applying the carbonation process to Ca(OH)₂ using a bubble reactor at 25 °C. As the carbonation temperature increased from 25 to 80 °C, the shape of prepared CaCO₃ changed from a colloidal-type to spindle-type of 1 μm due to self-assembly. Also, hexagonal-shaped aragonite could be prepared by the hydrothermal process with the supersaturated Ca(HCO₃)₂ solutions.     

Long-term strength and durability parameters of lightweight concrete in hot regime: importance of initial curing

The paper presents results on strength development and durability of 35 and 50 MPa total lightweight concretes exposed to hot marine exposure conditions for a period of 7 years. An initial water curing of 7 days and subsequent seaside exposure was found more beneficial for the strength development of lightweight concrete. One day of initial curing and subsequent seaside exposure was not very conducive for the strength development. A marginal degradation in both the stiffness and the modulus of rupture of the concretes over the exposure period was observed. Likewise, the water penetrability of the two mixtures, for all the three initial curing regimes, increased over a period of 7 years. This establishes that the compressive strength of concrete is not synonymous with its durability. Overall, 3–7 days of initial water curing seems most desirable to enhance the durability of concrete exposed to hot salty marine exposure conditions.     

Emulation of ambient carbon dioxide diffusion and carbonation within nickel mining residues

The dynamic evolution of CO₂ sequestration using nickel mining residue (NiMR) was studied using a specially-designed dual-compartment differential fixed-bed diffusion–carbonation cell. Reactivity of NiMR material in terms of carbonation uptake was monitored as a function of time, initial CO₂ composition and liquid saturation. Dynamic tests exposed a positive correlation between CO₂ breakthrough time across the NiMR layer and liquid saturation. Low carbonation rates were observed under both dry and completely saturated conditions. The latter suffered from slow CO₂ diffusion in water whereas the former, despite the facilitated transport of gaseous CO₂ across NiMR samples, lacked a sufficient supply of leached magnesium and dissolved CO₂. However, the CO₂ uptake was substantially stimulated when partially saturated NiMR layers were carbonated. The carbonation kinetics was rationalized in terms of reaction medium and CO₂ dissolved species. Under partially saturated conditions, rapid gaseous CO₂ diffusion and dissolution in interstitial water along with leaching of magnesium were key processes in carbonation.     

Influence of nucleation seeding on the performance of carbonated MgO formulations

The continuation of the hydration and carbonation reactions within reactive MgO cement formulations is inhibited by the formation of hydrate and carbonate phases around MgO particles, resulting in a low MgO utility and limited mechanical performance. This study introduces carbonate seeds into the pore space of MgO-based concrete mixes to enable the nucleation and growth of carbonates on the seed surfaces. The influence of seeds on the hydration and carbonation capability, mechanical performance and microstructural development was evaluated through isothermal calorimetry, water absorption and compressive strength measurements, along with TGA, XRD and SEM analyses. The introduction of ≤1% seed within the initial mix design increased the carbonate phase content and improved carbonation degree by up to 96% by increasing the availability of Mg(OH)₂ for carbonation. The dense formation of carbonates in seeded samples enabled improved microstructures and 28-day strengths of 64 MPa, which were 33% higher than unseeded samples.     

Ex situ mineral carbonation for CO2 mitigation: Evaluation of mining waste resources,aqueous carbonation processability and life cycle assessment (Carmex project)

This article presents the main outputs from the multidisciplinary Carmex project (2009–2012), which was concerned with the possibility of applying ex situ mineral carbonation concepts to mafic/ultramafic mining wastes. Focus points of the project included (i) matching significant and accessible mining wastes to large CO₂ emitters through a dedicated geographical information system (GIS), (ii) analysis of aqueous carbonation mechanisms of mining waste and process development and (iii) environmental assessment of ex situ mining waste carbonation through life cycle assessment (LCA) methodology. With a number of materials associated with the mining sector, the project took a close look at the aqueous carbonation mechanisms for these materials and obtained unexpected carbonation levels (up to 80%) by coupling mechanical exfoliation and reactive carbonation. Results from this work support the possibility of processing serpentine-rich peridotites without applying the classical first step of heat activation. Perspectives are also given for the carbonation of Ni-pyrometallurgical slag available closed to ultramafic mining residues. LCA of the mining waste carbonation system as a whole made it clear that the viability of this CO₂ storage option lies with the carbonation process itself and optimisation of its operating conditions. By combining the body of knowledge acquired by this project, it is concluded that New Caledonia, with its insularity and local abundance of ‘carbonable’ rocks and industrial wastes coupled with significant greenhouse gas (GHG) emissions from world-class nickel pyro and hydrometallurgical industries stands out as a strong potential candidate for application of ex situ mineral carbonation.     

Integrated characteristics and performance of zero emission coal system

Before the commercialization of zero emission coal (ZEC), some technical hurdles must be settled and the performance of the whole system needs to be studied. The main technical hurdles of ZEC system are analyzed and some solutions are presented in this paper. A detailed ZEC system is setup and its characteristics are studied. High temperature solid oxide fuel cell (SOFC) is used to produce electricity and served as the heat source for the decarbonater. ZnO and NaHCO₃ are proposed to deeply eliminate H₂S and HCl. H₂ recycle ratio should be kept at 0.75 for the high conversion ratio of carbon in the gasifier and the high system efficiency. Calcium to carbon mole ratio (CTCR) should be kept around 0.6 to reach high CO₂ sequestration rate and acceptable economic penalty. Fuel utilization factor (U_f) of SOFC should be kept around 0.55 so as to supply enough heat to the decarbonater, and steam to carbon ratio (STCR) for the reformers in the system should be kept around 2.0. With these optimized parameters, the total efficiency of the system of 69.1% and the CO₂ sequestration ratio of 87% can be reached.     

Model for practical prediction of natural carbonation in reinforced concrete: Part 1-formulation

A model is proposed for prediction of natural carbonation in reinforced concrete (RC) structures, and is potentially applicable to existing and new RC structures. The major components of the model comprise mathematical functions applied to predict the influence of concrete composition, and environmental factors on natural carbonation.This paper introduces the model concept and explains its structure including derivation, optimization and calibration. Over 163 data sets taken from a 10-year carbonation study were used in the model development and calibration. Only the experimental data that were based on outdoor natural exposure environment were employed in this research. Also in this study, the proposed model is compared with fib-Model Code 2010 using carbonation predictions generated from 346 data sets involving real world, highway structures. It is shown that the proposed model is comparably accurate and involves mainly basic tests with no major anticipated costs.     

Multiscale characterization of carbonated wollastonite paste and application of homogenization schemes to predict its effective elastic modulus

This paper describes the multiscale characterization of the carbonated wollastonite paste using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and statistical nanoindentation (SNI, also known as ‘grid indentation’) methods as well as micromechanical homogenization models. Wollastonite (CaSiO₃) fibers are commonly used as filler in ceramics or plastics. However, wollastonite can also be regarded as non-hydraulic binder material since upon carbonation it forms a heterogeneous matrix with mechanical properties similar to those of the conventional hydrated cement pastes. Carbonation reaction of wollastonite results in the formation of two main products: calcium carbonate (CaCO₃) and amorphous silica gel (SiO₂). The SEM/EDS microanalysis performed on this system revealed that the average calcium to silica (Ca/Si) atomic ratio of the silica gel phase was around 0.40. Three individual carbonated wollastonite paste samples, each representing a different degree of carbonation were selected for nanoindentation tests. The obtained elastic moduli for silica gel, calcium carbonate, and unreacted wollastonite grains were, respectively, 41.7 GPa, 67.3 GPa, and 134.7 GPa. The micromechanical homogenization models were then utilized to predict the effective (also referred to as ‘homogenized’) elastic moduli of the carbonated wollastonite paste. The predicted values of the effective elastic moduli of carbonated wollastonite pastes were found to be in the range of corresponding values for hydrated high to ultra-high performance cement pastes. Additionally, the values of the effective elastic moduli of the carbonated wollastonite pastes were observed to increase with the increase in the degree of carbonation.     

碳酸二甲酯生产中碳化脱盐工艺优化的研究

研究了碳酸二甲酯装置生产过程中碳化脱盐工艺参数优化的问题。针对现有碳酸二甲酯装置碳化脱盐工艺存在的问题,从调整装置工艺参数和优化现有工艺流程入手,使得丙二醇的产品质量和产品收率都得到提高,从而使得装置经济效益大大提高。     

Reproducing ten years of road ageing — Accelerated carbonation and leaching of EAF steel slag

Reuse of industrial aggregates is still hindered by concern for their long-term properties. This paper proposes a laboratory method for accelerated ageing of steel slag, to predict environmental and technical properties, starting from fresh slag. Ageing processes in a 10-year old asphalt road with steel slag of electric arc furnace (EAF) type in the subbase were identified by scanning electron microscopy (SEM) and leaching tests. Samples from the road centre and the pavement edge were compared with each other and with samples of fresh slag. It was found that slag from the pavement edge showed traces of carbonation and leaching processes, whereas the road centre material was nearly identical to fresh slag, in spite of an accessible particle structure. Batches of moisturized road centre material exposed to oxygen, nitrogen or carbon dioxide (CO₂) were used for accelerated ageing. Time (7-14 days), temperature (20-40 °C) and initial slag moisture content (8-20%) were varied to achieve the carbonation (decrease in pH) and leaching that was observed in the pavement edge material. After ageing, water was added to assess leaching of metals and macroelements. 12% moisture, CO₂ and seven days at 40 °C gave the lowest pH value. This also reproduced the observed ageing effect for Ca, Cu, Ba, Fe, Mn, Pb, Ca (decreased leaching) and for V, Si, and Al (increased leaching). However, ageing effects on SO₄, DOC and Cr were not reproduced.