Comparison of solidification/stabilization effects of calcite between Australian and South Korean cements

The differences in the effect of calcite on the strength and stability of Pb-rich wastes solidified and stabilized using Australian and South Korean ordinary Portland cements are examined in this study. Pb-rich waste stabilized using Australian OPC has been shown to possess both substantially higher unconfined compressive strength and lead immobilization ability than South Korean OPC as a result of its higher C₃S content and the associated enhanced degree of precipitation of lead on the surfaces of silicate phases present. Calcite addition is observed to have an accelerating effect on the OPC-induced solidification/stabilization of Pb-rich wastes as gauged by the unconfined compressive strength and leachability of the solids formed. This effect is observed to be far more dramatic for South Korean OPC than for Australian OPC. Using scanning electron microscopy, waste stabilized with cement and calcite was observed to develop significantly greater proportions of hydrated crystals than wastes stabilized with cement alone. The results of X-ray diffraction studies have shown that the presence of calcite in South Korean OPC results in greater acceleration in the formation of portlandite than is the case for Australian OPC.     

Microstructural study of sulfate attack on ordinary and limestone Portland cements at ambient temperature

This paper presents an investigation on the mechanism of sulfate attack on Portland cements (PCs) containing limestone filler. It is based on the analysis of microstructure and composition of mortar specimens (ASTM C 1012) stored for 2 years in sodium sulfate solution (0.352 M). Microstructure was studied using quantitative X-ray diffraction (XRD) on samples taken from the surface to the core of the specimens. The profile of compounds formed by sulfate attack was determined millimeter by millimeter at 1 and 2 years. Results show that sulfate attack in mortars containing limestone filler is characterized by an inward movement of the reaction front leading first to the formation of ettringite, later to gypsum deposition, and finally to thaumasite formation when the decalcification of mortar leads to the breakdown of C-S-H.     

Sulfate attack on cementitious materials containing limestone filler — A review

This review summarizes the results of sulfate performance in laboratory and field tests where limestone is used as a constituent of cement (PLC) or as a sand replacement where it is particularly beneficial to the properties of self compacting concretes (SCC).Laboratory studies on paste, mortar or concrete specimens exposed to Na₂SO₄ and MgSO₄ solutions in a wide range of concentrations at different temperatures as well as mixtures with different compositions, cement compositions and limestone proportions are considered in a conceptual analysis as for the resistance to external sulfate attack and, especially, thaumasite sulfate attack.A detailed analysis of environmental aggressiveness (concentration, temperature and pH), mixture composition and cement composition used in each study are presented for PLC and SCC. Reported field studies are also shown, only a few cases have used limestone filler in their composition. A conceptual graphical analysis is then proposed to relate the degree of surface deterioration and mineralogical composition of attacked surface to the main variables of external sulfate attack: water/cementitious material ratio, limestone content and C₃A content of the cement. Observation of graphical analysis clearly shows that deterioration by ESA is mainly governed by effective w/c ratio and C₃A content of the cement. Surface damage is controlled when low effective w/c ratio and low C₃A are used. In MgSO₄ solution, low temperatures increase the degree of deterioration. Thaumasite is the last attack stage in the different sulfate environments.     

The influence of relative humidity on structural and chemical changes during carbonation of hydraulic lime

Studies monitoring the carbonation of NHL3.5 hydraulic lime are described. Weight-gain measurements, focused ion beam imaging, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to monitor changes in structure and composition occurring in lime pastes after exposure to 100% carbon dioxide at relative humidities of 65 and 97%. Lime paste exposed to a relative humidity (R.H.) of 97% indicated a higher carbonation rate compared to paste exposed to 65% R.H. Surface analysis showed that the sample exposed to a relative humidity of 97% was completely carbonated. No calcium hydroxide was detected. A small amount of calcium hydroxide was, however, present at the surface of the sample exposed to 65% R.H. These observations suggest that high humidity results in the formation of a thin layer of crystalline calcium carbonate covering silicate and hydroxide phases. The actual mass increase of the sample also indicated that uncarbonated calcium hydroxide remained beneath the surface.     

Studies on Mechanical,Thermal, and Flame Retarding Properties of Polybutadiene Rubber (PBR) Nanocomposites

An effect of nanosize CaCO₃ on physical, mechanical, thermal and flame retarding properties of PBR was compared with commercial CaCO₃ and fly ash filled PBR. CaCO₃ at the rate of 9, 15, and 21 nm were added in polybutadiene rubber (PBR) at 4, 8 and 12 wt.% separately. Properties such as swelling index, specific gravity, tensile strength, Young's modulus, elongation at break, modulus at 300% elongation, glass transition temperature, decomposition temperature, flame retardency, hardness, and abrasion resistances were determined. The swelling index decreased and specific gravity increased with reduction in particle size of fillers in PBR composites. There was significant improvement in physical, mechanical, thermal and flame-retarding properties of PBR composites due to a reduction in the particle size of fillers. Maximum improvement in mechanical and flame retarding properties was observed at 8 wt.% of filler loading. This increment in properties was more pronounced in 9 nm size CaCO₃. The results were not appreciable above 8 wt.% loading of nano fillers because of agglomeration of nanoparticles. In addition, an attempt was made to consider some thermodynamically aspects of resulting system. The cross-linkage density has been assessed by Flory-Rehner equation in which free energy was increased with increase in filler content.     

Effect of Nano CaCO<Subscript>3</Subscript> on thermal properties of Styrene Butadiene Rubber (SBR)

Styrene butadiene rubber (SBR) as matrix was reinforced separately with 9, 15 and 21 nm sizes of CaCO₃, which were synthesized by matrix mediated growth technique. The mixing and compounding was done on two-roll mill and sheets were prepared in compression molding machine. The effect of nature and loading of nano CaCO₃ on these rubber nanocomposites was investigated thoroughly by different characterizations such as DSC, TGA, XRD, and mechanical properties. An appreciable increase in glass transition temperature has been observed from DSC study. 9 nm sizes of CaCO₃/SBR composites show more increment in Tg as compared to pristine SBR as well as different sizes of CaCO₃ filled SBR. This increment in Tg is due to restricted mobility of nano CaCO₃ filled SBR nanocomposites. XRD study of nanocomposites showed that nano CaCO₃ dispersed uniformly throughout the matrix because of the small peak at lower 2θ. This uniform dispersion of nano CaCO₃ contributes towards the higher mechanical properties of rubber composites. From TGA study, it was observed that as the size of CaCO₃ reduces the thermal stability increases as compared to pristine SBR. The other results of these rubber nanocomposites were compared with commercial CaCO₃ filled SBR. Partly this research paper has been presented in International conference on ‘RubberChem 2006, Dec 5–6, 2006, Munich, Germany.     

Replacement of raw mix in cement production by municipal solid waste incineration ash

The feasibility of municipal solid waste incineration (MSWI) ash utilized as the replacement of raw mix in cement production is investigated. Result shows that sieving, self-grinding, and magnet separation processes are necessary to remove the debris, salt, and metallic contents that existed in the MSWI ash. By using the pretreated MSWI ashes, the produced cement specimens were in compliance with the unconfined compression strength (UCS) standard in Taiwan at small replacement percentage (<5%). When ash replacement percentage is large (more than 10%), the strength development of specimens would be hindered due to the deficient formation of the calcium silicate. Calculation on lime saturation factor (LSF) also shows a descending trend in consequence of the increase in replacement percentage. Thus, compositional effect should be taken into consideration for promoting the calcium silicate formation at the case of large ash replacement. In this research, adjustment of chemical composition was achieved by adding 183 g calcium oxide per kilogram of cement raw mixture with 15% ash replacement. After adjustment, the produced cement could develop seven- and fivefold increase on UCS compared with those without calcium oxide supplement at 3 and 7 days of curing, respectively. Results concluded that the MSWI ash was suitable in reuse for cement production under a well-conditioned situation.     

The contribution of class-F fly ash to the strength of cementitious mixtures

The contribution of fly ash to the physical properties of cementitious mixtures has received considerable attention since its inclusion as an essential ingredient of High Performance Concrete (HPC). However, the chemical contribution to the overall structure development has not been fully understood because of the masking of its hydration products by those of cement. In a mixture of class-F fly ash and lime (Ca/Si=2), portlandite diminishes and C₄AH₁₃ forms due to addition of Al to solution. The latter converts to hydrogarnet and C₃ASH₄. CSH is detected at 3 days and continues to increase in intensity. The ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the Al/Si ratio is 0.24 and the average chain length is 10 units. The presence of Al as approximately one-fifth of the Si in a chain length of 10 units suggests that Al tetrahedra may be present in bridging positions.     

Anomalous fluorescence in near-infrared Raman spectroscopy of cementitious materials

The results of near-infrared Fourier transform Raman spectral examination of cement minerals, several commercial Portland cements, and hydrating Portland cements are critically examined. It is shown that structured fluorescent effects dominate the observed spectra for C₃S, C₂S, and cement, and none of the bands generated by this mode of examination is a true Raman band. The apparent bands for the Portland cements are in different positions to those for the individual cement minerals. The fluorescence-derived spectra for different cements are similar to each other but vary enormously in intensity for different cements. Hydration progressively reduces the intensity of the bands, but does not generate bands at new locations. It is tentatively suggested that the fluorescence effect may be somehow associated with the status of the cement components as orthosilicates (i.e., composed of isolated silica tetrahedra. Samples of high-purity C₂S exhibit the fluorescence effect, but samples of CS (wollastonite) of similar purity do not. The latter are metasilicates (i.e., composed of linked chains of silica tetrahedra). They exhibit normal Raman Stokes and anti-Stokes bands.     

The experimental investigation of concrete carbonation depth

Phenolphthalein indicator has traditionally been used to determine the depth of carbonation in concrete. This investigation uses the thermalgravimetric analysis (TGA) method, which tests the concentration distribution of Ca(OH)₂ and CaCO₃, while the X-ray diffraction analysis (XRDA) tests the intensity distribution of Ca(OH)₂ and CaCO₃. The Fourier transformation infrared spectroscopy (FTIR) test method detects the presence of C-O in concrete samples as a basis for determining the presence of CaCO₃. Concrete specimens were prepared and subjected to accelerated carbonation under conditions of 23 °C temperature, 70% RH and 20% concentration of CO₂. The test results of TGA and XRDA indicate that there exist a sharp carbonation front. Three zones of carbonation were identified according to the degree of carbonation and pH in the pore solutions. The TGA, XRDA and FTIR results showed the depth of carbonation front is twice of that determined from phenolphthalein indicator.     

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.     

Studies on scaling of membranes in desalination by direct contact membrane distillation: CaCO3 and mixed CaCO3/CaSO4 systems

Scaling of membranes by CaCO₃ and CaSO₄-CaCO₃ is of considerable concern in membrane desalination processes. It is particularly relevant for porous crossflow hollow fiber-based membrane distillation (MD) processes which can achieve high water recovery and can encounter heavy precipitation of scaling salts. Therefore an analysis of the scaling potential for CaCO₃ and mixed CaSO₄-CaCO₃ systems is presented first in terms of the saturation index profiles throughout the crossflow hollow fiber membrane module as a function of the location in the module for feed solutions resulting from high water recovery. Scaling experiments during DCMD with tap water, CaCO₃ and mixed CaSO₄/CaCO₃ were conducted over a wide range of values of saturation index (SI) (10<SI_calcite<64, 1.1<SI_Gypsum<1.5) using porous fluorosilicone coated crossflow hollow fiber membrane desalination modules. The effects of flow rates, flow patterns (cross vs. parallel flow) and the nature of the membrane surface on possible scaling scenarios were further investigated for the scaling salt CaSO₄. Experimental results at high saturation indices show that even when the precipitation rate was fast in the CaCO₃ system at elevated temperatures or high concentrations, no significant loss in water vapor permeation was observed suggesting no effect of scaling on membrane flux. However, for a few of the mixed CaSO₄-CaCO₃ systems, the water vapor flux dropped somewhat. Possible explanations have been provided and a method to solve this problem has been illustrated. Fast feed flow rate resulted in a shortened induction period. Crossflow flow pattern and the nature of the hydrophobic porous coating on the membrane surface were proven to be helpful in developing the resistance to scaling. Results of modeling show that concentration polarization effects are far more important than temperature polarization effects.     

Microstructure of tricalcium silicate and Portland cement systems at middle periods of hydration-development of Hadley grains

The development of the microstructure of C₃S paste and a Portland cement paste was studied between 7 and 24 h by means of backscattered electrons in a field-emission SEM. The course of hydration was measured by isothermal calorimetry. While the abundant occurrence of Hadley grains (hollow-shells) in Portland cement systems is well documented from a number of SEM and other microscopy studies, some earlier reports have noted that Hadley grains do not form in C₃S or alite paste alone. This report shows evidence of Hadley grains in C₃S paste, and follows their development from middle to late hydration stages. At around 10 h the microstructure with respect to Hadley grains were seen to develop in a very similar manner in C₃S and cement. In both systems, a narrow gap often developed between the receding anhydrous cores and layer of reaction product enveloping the cores. By 1 day, Hadley grains had continued to develop only in the cement paste, where they became a prominent feature. Only small ‘hollowed-out’ hydration shells were observed in the C₃S paste by 1 day. These were presumably reminiscences of the small gapped Hadley grains seen at the earlier hydration stages.     

Elaboration and thermal study of iron-phosphorus-substituted dicalcium silicate phase

In the present work, we examine the simultaneous effect of iron and phosphorus additions on the calcium carbonate decomposition in CaCO₃, SiO₂, Fe₂O₃ and P₂O₅ mixtures at the molar ratio CaCO₃/SiO₂=2. The formation of the dicalcium silicate Ca₂SiO₄ is also studied. The temperatures of the decarbonation and the enthalpy evolution during the heating of the mixtures are measured. The additions of Fe₂O₃ and P₂O₅ oxides decrease the onset temperature of the CaCO₃ decomposition. The energy consumption of decarbonation at about 835 °C is lowered when the dopant concentrations increase. Synthesized solid solutions are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The free-lime quantity is determined by chemical analysis. The mineralogical analysis at room temperature of the products of the reaction shows the presence of iron-phosphorus-doped β, α′ and α-C₂S modifications.     

Co-utilisation of alkaline solid waste and compressed-or-supercritical CO2 to produce calcite and calcite/Se red nanocomposite

The coal combustion fly-ash and alkaline paper mill waste were previously used to sequester CO₂ via waste-water-CO₂ interactions. For this case, a solid mixture (calcite and un-reacted waste) was obtained after carbonation process. In the present study, we propose a solid-water separation of free lime (CaO) or free portlandite (Ca(OH)₂) contained in waste prior to carbonation experiments in order to produce pure calcite or calcite/Se⁰ red composite. The calcite and carbonate composite syntheses have been also independently studied, but for both cases, a commercial powdered portlandite was used as calcium source.For this study, the extracted alkaline-solution (pH = 12.2-12.4 and Ca concentration = 810-870 mg/L) from alkaline solid waste was placed in contact with compressed or supercritical CO₂ at moderate or high temperature, leading a preferential nucleation-growth of submicrometric particles of calcite (<1 μm) with rhombohedral morphology at 90 °C and 90 bar (9 MPa), whereas a preferential nucleation-growth of nanometric particles of calcite (<0.2 μm) with scalenohedral morphology at 30 °C and 20 bar (2 MPa) were observed. When, the extracted alkaline-solution was placed in contact with supercritical CO₂ (90 bar) at high temperature (90 °C) and in presence of unstable seleno-l-cystine compound, the nucleation-growth of calcite/Se⁰ red nano-composite taken place. The composite consisted predominantly of spherical, amorphous nanometric-to-submicrometric of elemental red selenium (<500 nm) deposited on the calcite matrix. Here, the calcite was constituted by nano- to microrhombohedral crystals (<2 μm) and micrometric agglomerates and/or aggregates (<5 μm). These results on the particle size and morphology of crystal faces are very similar to calcite produced using commercial powdered portlandite as alkaline reactant and calcium source. This study is a nice example of feasibility to obtain possible ecological and economical benefits from waste co-utilisation.     

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.     

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.     

A simple and efficient preparation of LaFeO3 nanopowders by glycine-nitrate process: Effect of glycine concentration

LaFeO₃ perovskites have been prepared by the glycine-nitrate process (GNP) at various glycine-to-nitrate molar ratios. The perovskites have been systematically characterized by X-ray diffraction, BET surface area, scanning electron microscopy, transmission electron microscopy and temperature-programmed reduction to study the effect of glycine concentration on various properties of LaFeO₃. The X-ray diffraction patterns of the as-prepared and calcined samples show the formation of orthorhombic phase without any impurities. The BET specific surface areas of various perovskites increased with an increase in glycine-to-nitrate ratio (GNR) of 2.0 but were nearly constant at higher ratios. The scanning electron microscopy indicates that the prepared material is flake-like at GNRs ≤1.5 and exists as agglomerated particles at GNRs ≥2.0. The particle size of the as-prepared samples was in the range of 30-130 nm depending on the GNR and the calcined samples exhibited particle size in the range of 60-160 nm. The samples that were prepared at GNR < 1.5 did not show any peaks in temperature-programmed reduction, but the samples prepared at a GNR of 2.0 and above showed the reduction of Feⁿ⁺.     

The effect of Cr2O3 and P2O5 additions on the phase transformations during the formation of calcium sulfoaluminate C4A3S¯

The study describes the influence of chromium and phosphorus additions on the phase transformations during the formation of the calcium sulfoaluminate Ca₄Al₆SO₁₆ from CaCO₃-Al₂O₃-CaSO₄ mixtures. The temperatures of decarbonation reaction were measured by means of differential thermal analysis. The enthalpy variations at different heating temperatures were determined by isothermal calorimetry. The results show that the Cr₂O₃ and P₂O₅ additions lower the onset temperature of decarbonation. The energy requirement at high temperature of the doped mixtures is less, compared to that of mixtures without dopants. The formation of solid solutions is followed by X-ray diffraction. The higher concentrations of additives hinder the formation of the C₄A₃S¯ phase.     

Calcium carbonate efflorescence on Portland cement and building materials

Whitish deposits of calcium carbonate, CaCO₃, frequently develop on Portland cement concrete and on masonry units, including brick and tile, which have been bonded with Portland cement. These surface deposits are termed efflorescence and are most frequently encountered in new or recent construction. While efflorescence is not normally damaging, except possibly to decorative coatings, it is aesthetically undesirable. The origin of efflorescence is explained and a physicochemical model is developed to explain and quantify the key features of its formation. Calculations and experiments highlight the important role of soluble alkalis in the formation of efflorescence. Mechanistic interpretations and calculations suggest ways in which efflorescence can be mitigated by interrupting one or more steps of the process in conjunction with improved materials selection.