Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO₂. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)₂ and C-S-H takes place. Carbonation gives rise to almost complete disappearance of C-S-H gel, while Ca(OH)₂ remains in appreciable amount. This may be associated with the CaCO₃ precipitation, forming a dense coating around partially reacted Ca(OH)₂ crystals. The way in which CO₂ is fixed in carbonated samples is studied. The results indicate that CO₂ is chemically bound as CaCO₃, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO₃ is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO₃ are preferentially associated with C-S-H carbonation.     

Measurement methods of carbonation profiles in concrete: Thermogravimetry, chemical analysis and gammadensimetry

This paper deals with two experimental methods to determine carbonation profiles in concrete. Gammadensimetry is a non-destructive test method able to measure the total penetrated CO₂ and to monitor the carbonation process during laboratory accelerated tests. The second method is thermogravimetric analysis (TGA) supplemented with chemical analysis (CA): as TGA is performed on a small mortar sample not representative of the whole tested concrete, CA is needed to proportion the sample cement content, the sand content and to correct the TGA results becoming thus representative of the concrete mix. Consequently, TGA-CA gives accurate quantitative profiles in carbonated cementitious materials. Results are reported for an ordinary Portland cement paste, and three concrete mixes, containing siliceous or calcareous aggregates. The CO₂ mass loss due to carbonation occurs from 530 to 950 °C, which overlaps the temperature range of the calcareous aggregate dissociation. To solve the problem, the origin of CaCO₃ is carefully analyzed. Calcium carbonate ensuing from C-S-H carbonation dissociates in a lower temperature range than the more stable one ensuing from portlandite carbonation and from limestone, which enables C-S-H carbonation to be distinguished from calcareous aggregates. Therefore, TGA-CA allows the CaCO₃ ensuing from C-S-H carbonation to be measured and to calculate the portlandite degraded by carbonation. Thus, the total calcium carbonates profiles can be deduced even when calcareous aggregates is present in the concrete mix.     

Preparation of ultrafine calcium carbonate particles with micropore dispersion method

Ultrafine particles of CaCO₃ were synthesized by dispersing the mixture of CO₂ and N₂ into the Ca(OH)₂/H₂O slurry with a micropore-plate. Because the micropore is micrometers scale, process of momentum transfer, mass transfer and reaction was significantly enhanced. The carbonation process of Ca(OH)₂/H₂O system was monitored with pH and conductivity. Operation conditions were investigated on the specific surface area of particles, such as initial slurry concentration and volume, gas flowrate and concentration, and temperature. The crystal structure of particles was characterized with BET, IR, TEM, SEM, etc. Results showed ultrafine particles were calcite with general shape of cube, whose size was about 40 nm and specific surface area was more than 25 m²/g. This preparation method is easy to operate.     

Cyclic CO2 capture behavior of KMnO4-doped CaO-based sorbent

This study examines the CO₂ capture behavior of KMnO₄-doped CaO-based sorbent during the multiple calcination/carbonation cycles. The cyclic carbonation behavior of CaCO₃ doped with KMnO₄ and the untreated CaCO₃ was investigated. The addition of KMnO₄ improves the cyclic carbonation rate of the sorbent above carbonation time of 257 s at each carbonation cycle. When the mass ratio of KMnO₄/CaCO₃ is about 0.5-0.8 wt.%, the sorbent can achieve an optimum carbonation conversion during the long-term cycles. The carbonation temperature of 660-710 °C is beneficial to cyclic carbonation of KMnO₄-doped CaCO₃. The addition of KMnO₄ improves the long-term performance of CaCO₃, resulting in directly measured conversion as high as 0.35 after 100 cycles, while the untreated CaCO₃ retains conversion less than 0.16 at the same reaction conditions. The addition of KMnO₄ decreases the surface area and pore volume of CaCO₃ after 1 cycle, but it maintains the surface area and pores between 26 nm and 175 nm of the sorbent during the multiple cycles. Calculation reveals that the addition of KMnO₄ improves the CO₂ capture efficiency significantly using a CaCO₃ calcination/carbonation cycle and decreases the amount of the fresh sorbent.     

Carbonation of fly ash in oxy-fuel CFB combustion

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO₂ ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO₂ partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO₂ concentrations of 80-85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO₂ concentration set at 80% and H₂O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H₂O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ? 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H₂O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H₂O. For T ? 400 °C, no fly ash carbonation occurred without the presence of H₂O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ? 300 °C, small amounts of Ca(OH)₂ were found in the final product alongside CaCO₃. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.     

Carbonation reaction of lime, kinetics at ambient temperature

The uptake of carbon dioxide due to the carbonation reaction of Ca(OH)₂ in ambient temperature of approximately 20 °C has been studied. Different types of lime have been used and the CO₂ concentration has been varied to identify the influence of different variables on the kinetics of the reaction. A closed loop system has been developed and validated that allows measurement of the carbonation progress directly from monitoring CO₂ uptake. Thermal analysis (TA) was used to verify the degree of carbonation that reached up to 83%. Factor analysis on the data set has demonstrated that reaction speed is not dependent upon the CO₂ concentration within the limits tested. Carbonation speed depends on the specific surface of the lime. The results of this study contribute to research carried out on lime mortar carbonation models and on the carbonation process in general.     

Properties of biomineralized (CaCO3) PVP-CMC hydrogel with reference to its cytotoxicity

The authors focus on properties of biomineralized (CaCO₃) PVP-CMC hydrogel (designated as I–X) including cytotoxicity assay using primary mouse embryonic fibroblasts. The biomineralized samples (VII–X) showed >80% cell viability, was selected for further characterizations. FTIR and XRD indicate deposition of CaCO₃ within the PVP-CMC hydrogel matrix, SEM shows changes in morphology and pore diameter (VII and VIII: 1–12 µm; IX: 10–70 µm; X: 70–170 µm), TGA determines the decomposition scenario of CaCO₃, and tensile strength of samples (VII–X) ranged between 0.04 and 1.0 GPa, which practically corresponds to the modulus of cancellous bone.     

Rapid Carbonation for Calcite from a Solid-Liquid-Gas System with an Imidazolium-Based Ionic Liquid

Aqueous carbonation of Ca(OH)₂ is a complex process that produces calcite with scalenohedral calcite phases and characterized by inadequate carbonate species for effective carbonation due to the poor dissolution of CO₂ in water. Consequently, we report a solid-liquid-gas carbonation system with an ionic liquid (IL), 1-butyl-3-methylimidazolium bromide, in view of enhancing the reaction of CO₂ with Ca(OH)₂. The use of the IL increased the solubility of CO₂ in the aqueous environment and enhanced the transport of the reactive species (Ca²⁺ and CO₃²⁻) and products. The presence of the IL also avoided the formation of the CaCO₃ protective and passivation layer and ensured high carbonation yields, as well as the production of stoichiometric rhombohedral calcite phases in a short time.     

CaCO3 addition effect on the hydration and mechanical strength evolution of calcium aluminate cement for endodontic applications

Calcium aluminate cement (CAC) hydrates conversion can be inhibited by adding CaCO₃, leading to C₃A·CaCO₃·11H (3CaO·Al₂O₃·CaCO₃·11H₂O) formation. However, despite its benefits, the stability of this monocarbonate hydrate is not fully understood, especially when the samples are kept in contact with liquid during the curing step. Thus, taking into account the increasing interest in the CAC application as a biomaterial in the endodontic area, this work addresses the evaluation of the mechanical strength and phase transformations of a commercial cement (Secar 71) containing 15 or 20 wt% of CaCO₃. Compressive strength, apparent porosity, dimensional linear changes, X ray diffraction and thermogravimetric tests were carried out to evaluate samples immersed in water and kept at 37 °C between 1 and 30 days of curing. According to the collected results, CAH₁₀ and C₂AH₈ formation were inhibited in CaCO₃ containing compositions and the presence of the C₃A.CaCO₃.11H phase led to a significant cement mechanical strength increase. Nevertheless, the partial decomposition of this monocarbonate hydrate was detected at 37 °C in the range of 1-7 days and the continuous hydration of CA and CA₂ also affected the compressive strength behavior of the evaluated samples.     

Predicting Ca(OH)2 content and chemical shrinkage of hydrating cement pastes using analytical approach

A semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)₂ content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)₂ amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 °C, respectively. Hydration degree and Ca(OH)₂ content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method.The comparison reveals a good consistency between modelled and experimental data at early age of hydration.     

Facile synthesis and flame retardant performance of NaAl(OH)2CO3 whiskers

The regular and well-dispersed NaAl(OH)₂CO₃ whiskers were successfully synthesized via facile hydrothermal route using aluminium isopropoxide and NaHCO₃. The effects of reactants and reaction temperature on the structure and morphology of NaAl(OH)₂CO₃ whiskers have been investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermo-gravimetric analysis (TGA) were employed to characterize the synthesized samples. Most NaAl(OH)₂CO₃ whiskers possess an average diameter of about 500 nm and lengths ranging from 10 to 30 μm. NaAl(OH)₂CO₃ whiskers exhibit excellent flame retardant performance and mechanical strength in poly(ethylene-co-vinyl acetate) matrix (EVA).     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

The effect of water on the sulphation of limestone

A series of tests was conducted in a thermogravimetric analyzer (TGA) to study the sulphation behaviour of limestone in the presence of water over the temperature range of 800-850 °C. Four different Canadian limestones, all with a particle size range of 75-425 μm, were sulphated using a synthetic flue gas with a composition of 15% CO₂, 3% O₂, 0% or 10% H₂O, 1750 ppm SO₂ and the balance N₂. Water was shown to have a significant promotional effect on sulphation, especially in the diffusion-controlled stage. However, the effect of water during the kinetic-controlled stage appeared to be much less pronounced. Based on these results, it is proposed that the presence of water leads to the transient formation of Ca(OH)₂ as an intermediate, which in turn reacts with SO₂ at a faster rate than CaO does. Alternatively stated, it appears that H₂O acts as catalyst for the sulphation reaction of CaO.     

Pyrolysis of pine wood in a slowly heating fixed-bed reactor: Potassium carbonate versus calcium hydroxide as a catalyst

Catalytic pyrolysis of pine wood was carried out in a fixed-bed reactor heated slowly from room temperature to 700 °C under a stream of purging argon to examine the effects of the physically mixed K₂CO₃ or Ca(OH)₂ on the pyrolysis behaviors. K₂CO₃ demonstrated a stronger catalysis for decomposition of hemicellulose, cellulose and lignin constituents, leading to the reduced yield of liquid product in conjunction with the increased yields of gaseous and char products because of the promoted secondary reactions of liquid product. With the addition of 17.7 wt.% of K₂CO₃, none of saccharides, aldehydes and alcohols was formed and the formation of acids, furans and guaiacols was substantially reduced, whereas the yields of alkanes and phenols were increased. Potassium led to an increase in the cumulative yields of H₂, CO₂ and CO at 700 °C. Ca(OH)₂ somewhat promoted the decomposition of cellulose and lignin constituents, and the effect of Ca(OH)₂ on the yields of liquid and char was opposite to that of K₂CO₃. With the addition of 22.2 wt.% Ca(OH)₂, some groups of liquid product such as acids and aldehydes disappeared completely and the yields of saccharides, furans and guaiacols were somewhat reduced, while the yield of alcohols was remarkably increased in contrast to the result of K₂CO₃. The addition of Ca(OH)₂ did not significantly change the total yield of gaseous product at 700 °C but enhanced the yield of H₂.     

Effect of Nano-CaCO3 on Mechanical and Thermal Properties of Polyamide Nanocomposites

Polyamide-CaCO₃ nanocomposites were prepared by melt intercalation on twin-screw extruder. Various particle sizes (23, 17 and 11 nm) of CaCO₃ were synthesized by in-situ deposition technique. The shape and sizes of nano-CaCO₃ particles were confirmed by transmission electron microscopy (TEM). Nano-CaCO₃ was added from 1 to 4 wt% in the polyamide. Properties such as Tensile strength, Elongation at break, Hardness, and Flame retardency were studied. These results were compared with commercial CaCO₃ filled composites. Nano-CaCO₃ filled in polyamide shows, 3 fold improvement in Young's modulus in comparison to commercial CaCO₃ and 4–7 folds to virgin polyamide. Besides that, a polyamide nanocomposite shows 2 times improvements in flame retarding and vicat softening properties compared to commercial CaCO₃. Moreover, thermal degradation was studied on TGA and found to be improved compared to commercial CaCO₃. This was due to uniform dispersion of nano-CaCO₃ with greater surface area in comparison to commercial CaCO₃ in the polyamide matrix. Extent of dispersion of nano-CaCO₃ was studied along with microcracks generated during tensile testing using scanning electron microscope (SEM).     

Moving carbonation fronts in concrete: A moving-sharp-interface approach

We present a new modeling strategy for predicting the penetration of carbonation reaction fronts in concrete. The approach relies on the assumption that carbonation reaction concentrates macroscopically on an a priori unknown narrow strip (called reaction front) moving into concrete gradually changing its mechanical and chemical properties. We propose a moving-interface model to forecast the maximum penetration depth of gaseous CO₂ in the porous concrete matrix. The main questions driving this research are: How fast does the carbonation front move? and How long does it take until the front reaches the reinforcement?. As model output, we determine simultaneously the position of the carbonation front and the profiles of the active concentrations. The model equations are solved using a specially tailored finite element scheme and are validated relying on experimental data from the Ph.D. thesis by D. Bunte Zum Karbonatisierungsbedingten Verlust der Dauerhaftigkeit von Außenbauteilen aus Stahlbeton, Ph.D. thesis, TU Braunschweig (1994). Our approach should be viewed as an alternative to the standard carbonation models.     

铵基循环碳酸化固定CO2

引言由煤等化石燃料燃烧产生的温室气体CO2的捕集与封存已引起国际社会的广泛关注[1-2];其中,模仿自然界钙镁硅酸盐矿物风化过程的碳酸化固定是实现大规模封存CO2的重要途径,与其他封存技术相比,碳酸化固定CO2环境风险性小,并可     

MICROPOROUS SOLIDS FROM SiO2-Al2O3

Polymeric Si-Al sols were prepared from Al(OBu^s)₃ and Si(OEt)₄ (TEOS). Acetylacetone and isoeugenol were employed separately to chelate the Al precursor in order to slow down the chemical reactivity, avoiding precipitation. The characterization techniques were Fourier Transform Infrared Spectroscopy (FTIR), nitrogen sorption measurements, Thermal Gravimetric (TGA) and Differential Thermal Analyses (DTA). The Si-O-Al bond vibration was detected by FTIR in the 1056-1074 cm^m1 region. From the N₂ adsorption, isotherms the surface area results obtained range between 15.8 to 408.4 m²/g. Corresponding average pore diameter are 3.397 to 1.417 nm. It is found that the porous texture of the materials mainly depends to the chelating agent in the sols. A strong influence of the molecular structure of the chelating agents in the sols was observed in the oxides by TGA and DTA studies.     

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.