Mineralogical phase analysis of alkali and sulfate bearing belite rich laboratory clinkers

The activation of laboratory belite clinkers has been carried out by adding variable amounts of alkaline salts (K₂CO₃, Na₂CO₃), and/or SO₃ as gypsum in the raw materials but keeping almost constant the main elements ratios, Ca/Si/Al/Fe. Quantitative phase analyses by the Rietveld method using high resolution synchrotron and strictly monochromatic CuKα₁ laboratory X-ray powder diffraction data has been performed. Quantitative phase analysis results have been compared to validate the protocol using laboratory X-ray data. The agreement in the results is noteworthy, which indicates that good quantitative phase analyses can be obtained from laboratory X-ray powder data. Qualitative studies have confirmed that the addition of alkaline salts to raw mixtures promotes the stabilization, at room temperature, of the highest temperature polymorphs: α′_H-C₂S and α-C₂S. Quantitative studies gave the phase assemblage for ten different laboratory belite clinkers. As an example, an active belite clinker with 1.0 wt.% of K₂O and 1.0 wt.% of Na₂O (amounts added to the raw mixtures) contains 8.5(3) wt.% of β-C₂S, 21.2(3) wt.% of α'_H-C₂S, 24.1(2) wt.% of α-C₂S, 18.9(3) wt.% of total C₃S, 17.3(2) wt.% of C₃A and 10.0(2) wt.% of C₄AF. A belite clinker with 0.8 wt.% SO₃ (nominal loading) contains 60.7(1) wt.% of β-C₂S, 6.7(2) wt.% of α′_H-C₂S, 12.3(7) wt.% of C₃S, 9.1(2) wt.% of C₃A and 11.2(2) wt.% of C₄AF. Overall, quantitative phase analyses have shown that alkaline oxides stabilize α′_H-C₂S and α-C₂S, sulfur stabilizes β-C₂S, with a large unit cell volume, and the joint presence of alkaline oxides and sulfur promotes mainly the stabilization of the α′_H-C₂S polymorph.     

XRD, EDX and IR analysis of solid solutions between thaumasite and ettringite

Solid solutions between thaumasite and ettringite were prepared by methods analogous to those well established for the preparation of thaumasite and ettringite. The extent of immiscibility in this system is investigated by varying the Al:Si and SO₄²⁻:CO₃²⁻ ratios in reactant mixtures. The solids produced were analysed by quantitative X-ray diffraction, with Rietveld refinement also providing accurate unit cell dimensions, energy-dispersive X-ray analysis and infrared spectroscopy. The compositional and unit cell variations in the solid solution are discussed. A wide variety of solid solution compositions were produced with both the thaumasite and ettringite structures, but all preparations were considerably diluted by secondary amorphous products.     

Influence of chemical and physical characteristics of cement kiln dusts (CKDs) on their hydration behavior and potential suitability for soil stabilization

The interaction of CKDs with a given soil depends on the chemical and physical characteristics of the CKDs. Hence, the characterization of CKDs and their hydration products may lead to better understanding of their suitability as soil stabilizers. In the present article, four different CKD powders are characterized and their hydration products are evaluated. A detailed chemical (X-ray diffraction), thermogravimetric and morphological (scanning electron microscope) analyses of both the CKD powders and the hydrated CKD pastes are presented. In general, high free-lime content (~ 14–29%) CKDs, when reacted with water produced significant amounts of calcium hydroxide, ettringite and syngenite. These CKDs also developed higher unconfined compressive strength and higher temperature of hydration compared to CKDs with lower amounts of free-lime. An attempt was made to qualitatively correlate the performance of CKD pastes with the chemical and physical characteristics of the original CKD powders and to determine their potential suitability as soil stabilizers. To that effect a limited unconfined compressive strength testing of CKD-treated kaolinite clays was performed. The results of this study suggest that both the compressive strength and the temperature of hydration of the CKD paste can give early indications of the suitability of particular CKD for soil stabilization.     

Thaumasite-ettringite solid solutions in degraded mortars

The thaumasite form of sulfate attack (TSA) has been observed in mortar prisms made from Portland-limestone cements after laboratory storage in 1.8% magnesium sulfate solution at 5 °C for 5 years. The prisms all showed evident signs of degradation, which increased with increasing limestone content. X-ray powder diffraction indicated that a solid solution was formed in all the prisms, which was based on the crystal structure of thaumasite, but extended towards the chemical composition of ettringite. The prism made from the cement with the highest level (35%) of limestone replacement gave the greatest amount of the thaumasite solid solution and had a composition close to the thaumasite end member, whereas those cements with lower (15%, 5% and 0%) levels of replacement gave reduced amounts of the solid solution and had greater lattice parameters. The solid solutions that have been observed are compared with those reported by Barnett et al. for synthetic thaumasite samples grown from sucrose solution.     

Mineralogical and chemical properties of FGD gypsum from Florina,Greece

BACKGROUND: The aim of this work is to define the chemical and mineralogical composition of the fuel gas desulphurization (FGD) gypsum produced from the Meliti thermal power plant in the region of Florina in North West Greece, in order to investigate potential uses in the cement industry. Mineralogical and microprobe analyses were carried out on FGD gypsum samples collected from the Meliti 330 MW lignite‐fired power plant. RESULTS: Results show that the main component of the FGD gypsum is pure mineral gypsum (CaSO₄·2H₂O). The particle size of the gypsum ranges from 5 to 50 µm and the crystals are mainly of rhomboid shape. Microprobe analysis shows that the concentration of CaO and SO₃, which are the main components, range from 31.9%–32.5% and from 45.90–46.40%, respectively. CONCLUSION: This FGD gypsum can easily substitute the natural gypsum used in the production of cement. Copyright © 2007 Society of Chemical Industry     

Internal deterioration of concrete by the oxidation of pyrrhotitic aggregates

This paper presents research results on the causes of a severe concrete deterioration, which occurred in many building foundations approximately 2 years after construction. Concrete samples were investigated with X-ray diffraction (XRD) analysis, a scanning electron microscope (SEM) and a petrographic examination performed with a stereomicroscope. It was found that the early cracking of concrete stemmed from the oxidation of the pyrrhotite found in the anorthosite aggregates used to produce the concrete. The oxidation process led to the precipitation of iron hydroxides having a higher volume than the original pyrrhotite does. The presence of micas (biotite) close to the pyrrhotite seemed to promote and accelerate the oxidation process.     

Zeolite formation in alkali-activated cementitious systems

Autoclaved aerated concrete (AAC) is a unique building material. Because of its cellular nature, it is lightweight, self-insulating, sound- and fireproof, as well as insect and mold resistant. Furthermore, AAC is free of VOCs and various fibers associated with wood and glass wool construction. In an attempt to toughen AAC and make it less prone to on-site damage, a conventional fly-ash-based AAC formulation is being supplemented with sodium hydroxide (NaOH). The introduction of sufficient alkali promotes the growth of crystalline zeolites in the tobermorite matrix during autoclave curing. It is postulated that in situ grown zeolites will serve the same purpose as added fibers. Inasmuch as fly-ash-based AAC reactions often do not go to completion, a phase study of the development of tobermorite and zeolites from a gel-like slurry made from reagent grade chemicals was undertaken. Mixtures were studied as a function of time and temperature. Phase development depends on bulk composition and curing conditions. Longer curing at higher temperatures causes the Na-P1 that forms initially to change to analcime. Whereas Na-P1 is bladelike in habit and is seen to intermingle with the slightly larger blades of tobermorite, the Na-P1 gradually undergoes a phase change to analcime that forms very large cubes. This change has the potential to disrupt the AAC matrix.     

Effect of pressure on the crystal structure of ettringite

X-ray diffraction and infrared data have been collected from a sample of ettringite from ambient pressure to 6.4 GPa. The sample was found to reversibly transform to an amorphous phase at 3 GPa. The isothermal bulk modulus of ettringite was found to be 27(7) GPa and the incompressibilities of the lattice parameters were found to be 71(30) GPa along a and 108(36) GPa along c.     

Synthesis and characterization of two PbO-chromium oxides

PbCrO₄ and Pb₂CrO₅ have been selectively synthesized via a chemical gas-liquid precipitation process by adjusting the pH value. The structure, size and morphology of the as-synthesized products are characterized by X-ray diffraction and transmission electron microscopy. Differences in UV-Vis spectra are related to differences in size and morphology.     

X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products

Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO₂] and three calcium-bearing compounds: hannebachite [CaSO₃·1/2H₂O], gypsum [CaSO₄·2H₂O] and ettringite [(Ca₆(Al(OH)₆)₂(SO₄)₃·26H₂O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement.     

Characterization of cement minerals, cements and their reaction products at the atomic and nano scale

Recent advances and highlights in characterization methods are reviewed for cement minerals, cements and their reaction products. The emphasis is on X-ray and neutron diffraction, and on nuclear magnetic resonance methods, although X-ray absorption and Raman spectroscopies are discussed briefly.     

Natural 1.1 and 1.4 nm tobermorites from Fuka, Okayama, Japan: Chemical analysis, cell dimensions, Si NMR and thermal behavior

A natural specimen of tobermorite, which contained both 1.1 and 1.4 nm phases, was studied by optical microscopy, X-ray powder diffractometry (XRD), EPMA, analytical TEM, ²⁹Si NMR and TG-DTA, and the results were compared with those for a specimen from Crestmore, California. The sample, occurring as a vein in contact with metamorphic rocks, could be divided into three zones. In two of the zones containing only the 1.1 nm phase, about 10% of the Si were substituted by Al. The third zone was an intergrowth of 1.1 and 1.4 nm tobermorites and contained almost no Al. The basal spacings of the 1.1 and 1.4 nm tobermorites were changed mainly to 0.9 nm by heating at 300 °C for 24 h but in some parts remained at 1.1 nm. ²⁹Si NMR results showed that silicate anions in the 1.1 nm tobermorite were double chains, which changed mainly into single chains, on heating at 300 °C. XRD results indicated that natural tobermorites are more highly crystalline than the synthetic ones of calcium silicate products.     

A study on the cementlike properties of municipal waste incineration ashes

This paper describes the origin of various ash wastes arising from municipal solid waste (MSW) incinerators, especially bag filter precipitator (BP) residue [called fly ash (FA)] and scrubber residue [called reaction product (RP)], which has cementlike compositions. This makes them suitable for immobilization for cement. The cementlike compositions are potential reuse as well as for stabilization treatments. The relations of ash wastes' alkalinity and salinity with lead leachability are correlated with composition.     

Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD

Synchrotron radiation X-ray powder diffraction has been used to monitor the evolution of ettringite in C3A-gypsum synthetic mixture and in commercial cement systems during the first hours of the hydration process. The hydration of the paste was achieved using a remote controlled system, in order to collect data as soon as water is added to the system. The use of full-profile Rietveld method during the analysis of the diffractograms collected allowed us to monitor the evolution of phases weight fraction. The rigorous measurement of the lattice parameters and of the diffraction peak shape proved to be very useful to obtain information on the structural evolution of ettringite and on the mean grain size of the crystal phases. Depending on the admixture added to the system, the precipitation of well crystalline ettringite takes some hours. During this “induction” period we observe a significant variation of a and c lattice parameter values for ettringite. In particular a increases from 11.8 Å to 11.24 Å, the value for pure ettringite. The c parameter decreases from 22 Å to 21.48 Å. The lattice parameter variation could be related to small crystallite size effect, but the large variation more likely reflects also crystallographic changes, such as defect re-organization during the nucleation and growth process or also changes in the SO₃ and H₂O content in the ettringite channel. Not surprisingly the amount and the grain dimensions of crystalline ettringite are affected by the chemistry of the system. We observed the same evolution trend during ettringite formation also in shrinkage-compensating commercial cements (composed by mixture of Ca-Al cements, Portland cement and bassanite), in which ettringite is the main hydrous phase present.     

Crystallized alkali-silica gel in concrete from the late 1890s

The Elon Farnsworth Battery, a concrete structure completed in 1898, is in an advanced state of disrepair. To investigate the potential for rehabilitation, cores were extracted from the battery. Petrographic examination revealed abundant deposits of alkali silica reaction products in cracks associated with the quartz rich metasedimentary coarse aggregate. The products of the alkali silica reaction are variable in composition and morphology, including both amorphous and crystalline phases. The crystalline alkali silica reaction products are characterized by quantitative X-ray energy dispersive spectrometry (EDX) and X-ray diffraction (XRD). The broad extent of the reactivity is likely due to elevated alkali levels in the cements used.     

Structural,electrical and magnetic behavior of mechanothermally synthesized multidoped bismuth ferrite

Some lead-free compounds of a general formula (Bi_1-xSr_x)(Fe_1-xMn_x)O₃ (x?=?0–0.15 with the interval of 0.05) were prepared by a mechanical alloying followed by sintering process. Structural, electrical and magnetic characteristics of multi-doped elements (Sr-Mn) in bismuth ferrite have been examined at different field frequencies and temperatures. X-ray diffraction studies suggest the rhombohedral phase for x?≤?0.1 and the orthorhombic phase for x?=?0.15. Study of frequency-dependent dielectric properties showed the enhancing trend of dielectric constant with increasing co-doping concentration. Detailed analysis of impedance data at different frequencies and temperature estimated the contribution of grains and grain boundaries in the capacitive and resistive properties of the studied samples. The study of magnetic properties exhibits the weak ferromagnetism in co-substituted samples with maximum saturation magnetization (M_s = 0.121?emu/gm) for higher concentration of doping (x?=?0.15). The magneto-electric coefficient (α_ME), measured with the varying DC magnetizing field under fixed AC magnetic field, is found to be 15.368?Oe.     

Composite systems fluorgypsum-blastfurnance slag-metakaolin, strength and microstructures

The hydration and properties of composite cementitious pastes with 75% fluorgypsum were investigated; blastfurnace slag and metakaolin were the complementary cementitious materials. The pastes were cured under water at 20 °C for 360 days. All pastes developed and maintained strength under water, except those of commercial gypsum. The addition of metakaolin had a positive effect, after 360 days compressive strengths of 13.4, 13.8 and 14.6 MPa were registered for systems with 0%, 5% and 10% of metakaolin, respectively. The microstructure of the composite pastes was formed of a framework of gypsum crystals, which formed in the initial stages; the matrix was later densified by the formation of C-S-H and ettringite, as a result of the slag and metakaolin reactions. The fluorgypsum reacted rapidly in the first days, however it was still present after one year; the slag reacted in a slower fashion, and the metakaolin was very reactive and contributed with the ettringite since the early ages, which enhanced the strength.     

Adsorption of pharmaceuticals from aqueous solutions on synthetic zeolites

The objectives of this study were to measure the sorption capacity of a dilute solution of drugs in water on hydrophobic zeolites and to quantify aspects of their adsorption mechanisms for potential use in wastewater remediation. The removal of three drugs, erythromycin (ERY), carbamazepine (CBZ) and levofloxacin (FLX) from water using three organophilic zeolites (Y, mordenite (MOR), ZSM-5) was investigated. Adsorption isotherms show that these three drugs are adsorbed in remarkable amounts by Y. These data were confirmed by thermogravimetric analysis. The presence of these drugs inside the Y cage was revealed by unit cell parameter variations and structural deformations obtained by X-ray structure analyses carried out using the Rietveld method on zeolite after adsorption. Our results indicate that the adsorption properties of zeolitic materials do not only depend on micropore size and that zeolite shape selectivity also depends on structural features which are difficult to determine “a priori”. In order to test the ability of organophilic zeolites in removing drugs from wastewater, the adsorption of ERY, FLX and CBZ on zeolite Y from water samples collected at the outlet of a wastewater treatment plant located in Northern Italy was determined. These drugs were almost completely adsorbed by Y zeolite, confirming that this material is suitable for removing drugs from wastewater.     

Stability and reactivity of thaumasite at different pH levels

Thaumasite (CaCO₃·CaSO₄·CaSiO₃·15H₂O) has been reported to form at low temperatures (below 15 °C) during sulfate attack. Reactions between calcium silicate hydrate (C-S-H) and Ca⁺², CO₃⁻², SO₄⁻², CO₂ and water, or between ettringite and C-S-H, CO₃⁻² and/or CO₂ and water, result generally in the formation of thaumasite. In some instances, thaumasite may be affected by the presence of other chemicals in the surrounding environment (i.e., phosphates and ammonia in agricultural soil). There are insufficient data regarding the stability of thaumasite at different pH levels in the presence of other chemical ions. Understanding this issue might help in the detection of the thaumasite form of sulfate attack, and, therefore, in one's choice of the appropriate protection technique. This work reports the reactivity of thaumasite with phosphate, carbonate and bicarbonate ions at different pH levels ranging from 6.00 to 12.00, as well as the stability of thaumasite at high pH levels (greater than 12.00). Thaumasite was found to react with these ions at pH levels at and below 12.00; however, thaumasite was stable with minimal reactivity at pH levels greater than 12.00.