Sulfoaluminate Cement Clinker Research Articles

Hydration mechanism of anhydrite and calcium sulfoaluminate co-activated slag cement: insights into the role of composition

Utilization of Portland cement to initiate the dissolution of blast furnace slag (BFS) in super-sulfated cement is limited by low slag substitution level and poor early strength. In this study, a new activator composed of anhydrite and calcium sulfoaluminate cement clinker was proposed to enhance the utilization of BFS in super-sulfated cement. Tests were conducted on the compressive strength, autogenous shrinkage, hydration heat, pH and conductivity of the composites. XRD, 29Si NMR, BSE and MIP were employed. Results reveal that ettringite, gypsum and C-S-H are the primary hydration products. The composites with 20% activator display the highest compressive strength and shrinkage accompanied by the highest hydration heat and degree, as well as the highest quantity of monomer silica tetrahedrons and the largest volume of fine pores. Thermodynamic modeling confirms the critical value for the activator-substitution level. Additionally, the activator-BFS composite exhibited significantly lower carbon emissions compared to Portland cement.

Numerical Simulation of Pre-Reduction for a New Process of Acid Production from Phosphogypsum by Gas Sulfur Reduction

The gas sulfur reduction of phosphogypsum in the acid co-production of sulfoaluminate cement clinker is a new process for treating phosphogypsum. The reduction furnace of this system was studied and analyzed by combining computational fluid dynamics (CFD) and experimental validation. The effects of n(CaSO4)/n(S2), particle residence time, and kiln tail flue gas temperature on the performance of the reduction furnace were obtained. A second-order response model based on the response surface methodology was developed using a three-factor Box–Behnken design (BBD). The results show that the comparison error between the simulation and test data of the reduction furnace is acceptable. The above three conditions arranged in order of significance in terms of their effect on the performance of the reduction furnace is n(CaSO4)/n(S2) > particle residence time > kiln tail gas temperature. Finally, by optimizing the response surface model, the predicted optimal operation parameter combination is n(CaSO4)/n(S2) = 3.04, with the particle residence time and flue gas temperature at the kiln end given as 8.90 s and 1265.39 K, respectively.

Effect of Iron Phase on the Formation of Barium Calcium Sulphoaluminate Clinker

In this paper, analytically pure chemical reagents, as raw materials, were fired in barium calcium sulfoaluminate cement clinker. The effect of the iron phase on the calcination of barium calcium sulfoaluminate cement clinker was studied. The content of f-CaO in the sample was determined using the ethylene glycol method. The raw meal’s heat absorption and heat release were tested with an integrated thermal analyzer TG-DSC, and XRD and SEM measurements were used to characterize the composition and microstructure of the clinker. The results showed that the iron phase could lower the decomposition temperature of the calcium carbonate. When the calcination temperature increased, the lattice spacing of the mineral changed. The XRD pattern showed that a substitution reaction had occurred. Ba2+ replaced Ca2+ and formed a sulfoaluminate barium calcium mineral. The SEM images showed hexagonal plates or dodecahedral barium calcium sulfoaluminate minerals.

Effect of Al2O3/SiO2 ratio on the chroma and phase compositions of white sulfoaluminate cement clinker

Fe element is the main factor affecting the whiteness of white sulfoaluminate cement (W·SAC). With the reduction of high-quality raw materials, the Fe content of the raw materials used to produce W·SAC has been increasing. In this study, increasing the Al2O3/SiO2 ratio (N) was found to increase the whiteness of W·SAC, which can help to alleviate the scarcity of resources for manufacturing W·SAC. The effect of N on the phase compositions, chroma and hydration of clinker was studied by the Rietveld method, scanning electron microscopy–energy dispersive spectrometry (SEM–EDS), spectrophotometry and calorimetry. The whitening mechanism for W·SAC by increasing N was elucidated. With increasing N, Fe was more dispersed into ye’elimite, and for N above 1.5, 0.5% Fe2O3 completely dissolved into other phases without forming C4AF, which increased the brightness (L*) and greenness (a*) and reduced the yellowness (b*) of the clinker, thereby significantly improving the whiteness. With increasing N, the ye’elimite content increased, the C2S and unquantifiable crystalline phase (ACn) content decreased, and the content of gehlenite (which does not have hydraulic activity) decreased, which increased the overall early hydraulic activity of W·SAC and the accumulated heat of W·SAC hydration.

Effects of different alumina carriers on the phase formation and mechanical properties of sulfoaluminate cement clinker

Finding alumina carriers that can replace bauxite to produce sulfoaluminate cement (SAC) is a significant issue. The effects of bauxite (Ba), industrial aluminium hydroxide (I-AH), industrial aluminium oxide (I-AO) and analytically pure aluminium oxide (A-AO) on the phase formation and mechanical properties of SAC clinkers were studied by differential scanning calorimetry/thermogravimetry, Rietveld quantitative analysis, scanning electron microscopy/energy dispersive spectrometry and isothermal calorimetry. It was found that sintering SAC clinker with Ba and I-AH as alumina carriers accelerated the rate of the solid-phase reaction, promoted the formation of the target phase and led to the formation of ye'elimite with a small grain size. The compressive strength at each age was thus higher. However, when amorphous alumina was used as the alumina carrier to sinter SAC clinker, the solid-phase reaction rate was slow, which hindered the formation of the target phase, free alumina appeared and ye'elimite with a large grain size was formed, so the compressive strength at each age is lower. Aluminium oxide with finer particle size can be decomposed from I-AH and Ba during calcination, which can accelerate the solid-phase reaction rate and promote the formation of the target phase. The associated element in Ba (titanium) can lead to the formation of more α′H-C2S, but obstructed the formation of ye'elimite.

Preparation and Performance of Ternesite-Ye'elimite Cement.

Ternesite–ye’elimite (TCSA) cement is a new type of environmentally advantageous binder prepared by introducing ternesite, a reactive phase, into belite calcium sulfoaluminate cement clinker. This paper reports the laboratory production of TCSA cement by the addition of minor elements to achieve the coexistence of ternesite and ye’elimite. The influence of dopants on the mineralogical composition of clinkers and the clinkering conditions for the preparation of TCSA cement clinkers were investigated by X-ray powder diffraction and scanning electron microscopy. The mechanical properties and hydration products of the cement pastes were also studied. The results indicated that the addition of CaF2, P2O5 and Na2O can promote the coexistence of ternesite and ye’elimite, and that Na2O is the most effective candidate. TCSA cement clinkers could be successfully prepared at 1150 °C for 30 min by doping 0.3% Na2O. The TCSA cement clinkers exhibited shorter setting times than the BCSA cement clinkers. The later strength of TCSA cement showed a significant increase compared with BCSA cement. The effect of Na2O was different on the strength development for TCSA and BCSA cement. The dissolution of ternesite could promote the formation of ettringite. The reactivity of belite was higher in TCSA cement due to the formation of strätlingite.

The Role of Brownmillerite in Preparation of High-Belite Sulfoaluminate Cement Clinker

High-belite sulfoaluminate cement (HBSC) clinker containing brownmillerite was prepared using the industrial raw materials limestone, aluminum tailings, aluminum ore waste rock, and anhydrite. The effect of brownmillerite on clinker sintering and clinker minerals and the mechanical performance of HBSC was investigated using thermal analysis, petrographic analysis, and quantitative X-ray diffraction (QXRD). Results indicated that brownmillerite promoted the formation of clinker minerals and stabilized calcium sulfoaluminate (C4A3$) through the substitution of Fe3+ for Al3+ in C4A3$, which increased the actual C4A3$ content and decreased the brownmillerite content compared to that of the designed theoretical mineral composition. However, the early compressive strength of HBSC pastes decreased with the increase in brownmillerite content due to the decrease in the total amount of early-strength clinker minerals. Brownmillerite also influenced belite (C2S) structures and increased the γ-C2S content with poor hydration activity, thus inhibiting the strength development of HBSC pastes. The proper amount of brownmillerite in HBSC clinker can ensure the early strength and strength development of HBSC pastes.

Microstructural characteristics of belite–sulfoaluminate cement clinkers with bottom ash

The incorporation mechanism of impurities derived from raw materials into a belite-sulfoaluminate cement clinker with bottom ash was investigated in two raw mixtures. Alkalis lowered the sintering temperature by 50 °C. Ti stabilized β belite and prevented the formation of γ-belite. Sodalite solid solution showed an incomplete substitution of Si by Al. At a higher Ti content ferrite solid solution trended towards perovskite composition. Although calcium sulfoaluminate only fully developted at 1300 °C, the clinkers sintered at 1200 °C were the most reactive, as the presence of anhydrite at 1200 °C accelerated early hydration. Clinker reactivity decreased with an increase in sintering temperature.

Revealing the doping mechanism of barium in sulfoaluminate cement clinker phases

The doping mechanism of Ba in sulfoaluminate cement (SAC) clinker was systematically studied. Results from density functional theoretical simulations, backscattered electron diffraction and X-ray diffraction show that Ba ions prefer to incorporate into the mineral C4A3S¯ by substituting Ca atoms while little enter into the other two minerals, C2S and C4AF. Such doping preference was also certified by the results of individual mineral of C4A3S¯ and C4AF, respectively. Further analyses from partial density of states, electron density difference and local distortions indicate that the Ba-doping preference in C4A3S¯ is attributed to the efficiency of newly formed bonds, which is enhanced by electronic structure matching and small local distortions. The understandings are essential in guiding the production of Ba-bearing SAC clinker by utilizing industrial barium slags, thus should be very important in sustainable production of SAC clinkers, environmental governance and industrial collaborative disposal of waste.

Research on the Hydration Properties of C4A3S-CSH2 Cement System at Different Temperatures.

Sulphoaluminate cement has the advantage of low-temperature application performance, but its hydration mechanism at low temperatures is not yet clear. Anhydrous calcium sulfoaluminate (C4A3S) is the main mineral in the composition of sulfoaluminate cement clinkers. In this paper, C4A3S mixed with gypsum (CaSO4∙2H2O) to form a C4A3S-CSH2 cement system; X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), scanning electron microscopy (SEM) and mercury intrusion analysis (MIP) to clarify the effect of temperatures on the hydration properties of C4A3S-CSH2 cement system. The results showed that hydration of the C4A3S-CSH2 cement system could carry on at low temperatures, even at −15 °C. The main hydration product was ettringite. Low temperatures did not change the types of the hydration products, but the low temperature of 0 °C was more favorable for the formation of ettringite. The early hydration of the C4A3S-CSH2 cement system was inhibited by the decrease in temperature. However, hydration of the cement at 0 °C continued at a high rate after one day. Morphologies of the ettringite for the C4A3S-CSH2 cement system at −15 °C were needle-like structures, while they were of columnar structure at 0 °C. The compressive strength of samples at 0 °C reached 82 MPa, which is significantly higher than that at 20 °C.

Microstructure and Properties of Sulfoaluminate Cement‐Based Grouting Materials: Effect of Calcium Sulfate Variety

Effect of calcium sulfate variety on the microstructure and properties of cement‐based grouting materials made by sulfoaluminate cement clinker, calcium sulfate resource, and quicklime was investigated in this paper. Isothermal calorimetric, XRD, DTA‐TG, and SEM‐EDS were used to characterize the reaction process of cement‐based grouting materials. The mechanical property and expansion of the harden body were also tested. The results showed that (1) the formation of ettringite can be completed during the first 2 h when gypsum was selected, causing nonexpansion of the harden body, and ettringite tends to be precipitated near the clinker grain, which has a negative influence on the “skeleton effect” of ettringite, resulting in lower strength of the hardened body. (2) Ettringite can be continually and slowly precipitated when an anhydride was chosen, and the hardened body shows significant expansion characteristic. Besides, ettringite in anhydride paste tends to be precipitated in the form of complex interleaving, which can generate a positive influence on mechanical property of the hardened paste.

Retardation effect of elevated temperature on the setting of calcium sulfoaluminate cement clinker

The hydration of calcium sulfoaluminate (CSA) clinker at 5, 20 and 40 °C was characterized by a combination of XRD, DTA-TG and ESEM analysis (also with backscattered electron imaging, BEI). Meanwhile, the setting times and compressive strength development were studied. It was interesting to notice that elevated temperature retards the setting of CSA clinker and decreases compressive strength prominently, while it promotes the hydration heat releasement and hydration degree. With the temperature rising from 5 °C to 40 °C, the amount of ettringite formed after 1 d decreases by half, but alumina gel increases by more than 4 times, accompanying with a more inhomogeneous microstructure and higher porosity. ESEM observations indicate that such massive alumina gel covers on the unreacted particles and other hydrates, which could account for the retardation of elevated temperature on the setting of CSA clinker.

Enthalpy of formation of ye\u2019elimite and ternesite

Calcium sulfoaluminate clinkers containing ye’elimite (Ca4Al6O12(SO4)) and ternesite (Ca5(SiO4)2SO4) are being widely investigated as components of calcium sulfoaluminate cement clinkers. These may become low energy replacements for Portland cement. Conditional thermodynamic data for ye’elimite and ternesite (enthalpy of formation) have been determined experimentally using a combination of techniques: isothermal conduction calorimetry, X-ray powder diffraction and thermogravimetric analysis. The enthalpies of formation of ye’elimite and ternesite at 25 °C were determined to be − 8523 and − 5993 kJ mol−1, respectively.

Pilot scale production of novel calcium sulfoaluminate cement clinkers and development of thermal model

Pilot scale trials were successfully performed on the production of novel calcium sulfoaluminate (CSA) cement clinker in a direct natural gas heated rotary kiln at the IBU-tec facility in Germany. A raw meal throughput of ∼25kg/h was fed to the rotary kiln heated by co-combustion of natural gas and elemental sulfur, with the latter serving as both fuel substitute and reactant, to partially or wholly replace gypsum as the source of sulfur in CSA production. A well-mixed heat transfer kiln model was developed to predict the overall kiln heat flux and gas temperature profiles, to account for gaseous radiative properties. The predicted gas temperature inside the kiln varied from 1566K in the flame zone to 1019K at the feed zone, with peak temperature approaching 1724K. The combined emissivity of the CO2 and H2O gas mixture varied between 0.13 and 0.2 at these temperatures for partial pressure ratio PH2O/PCO2 of 1.7. The trial kiln has a low thermal efficiency of 3% of the total supplied energy.The simplistic model provided approximate performance predictions for the KDO kiln and also for different kiln sizes and can help to establish the effects of operating parameters on heat transfer trends.

Effect of Anhydrite on the Early Hydration Performance of Rapid Setting and Hardening Belite Sulfoaluminate Cement

In order to investigate the early hydration behavior of rapid setting and hardening belite sulfoaluminate cements, the methods of X-ray Diffraction, Scanning Electron Microscope, Compressive Strength test and Setting Times test were used to identify and quantify the hydration kinetics and microstructure of this new-found cements in China. The results showed that the main mineral compositions of high belite sulfoaluminate cement clinker included calcium sulfoaluminate (4CaO·3Al2O3·CaSO4), belite (2CaO·SiO2), ferrite phase, free gypsum and free lime. It was found that not only the setting time and compressive strength but also the composition of hydration products were influenced by anhydrite to some extent. Meanwhile, a mass of AFt and AFm generated along with the hydration process at different ages, overlapped, crossed and penetrated through calcium silicate hydrate gel and aluminum oxide to form a relatively dense structure which could contribute to the high strength of cement.

Production of belite calcium sulfoaluminate cement using sulfur as a fuel and as a source of clinker sulfur trioxide: pilot kiln trial

A pilot-scale trial was undertaken in a 7·4 m kiln to demonstrate the production of belite calcium sulfoaluminate cement clinker using sulfur as a raw feed. The sulfur was introduced in its elemental form as fuel to the burner, thus partially offsetting fuel costs while providing sulfur trioxide which is incorporated in the clinker. The study demonstrates that sulfur trioxide is readily sequestered into cement clinker in a standard rotary kiln; however, some scrubbing of the exit gas may still be required. As the products of scrubbing (anhydrite or gypsum) are usually required to optimise the cement formulation, the recovered sulfur product can find an immediate use in the final cement product. This trial demonstrates the successful production of targeted belite calcium sulfoaluminate clinkers at scale using sulfur as both a fuel and sulfur trioxide source.

Preparation of calcium sulfoaluminate-belite cement from marble sludge waste

Marble sludge waste was used as a major cement raw material in sintering sulfoaluminate cement clinker successfully in the laboratory. The influences of raw mix composition as well as different burning temperatures were investigated. Starting materials and prepared cement were characterized through different techniques including; Fourier transform infrared spectroscopy (FTIR); X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that calcium sulfoaluminate–belite cement can be produced by burning a raw mixture contains in weight percent (25% kaolin, 20% gypsum and 55% marble sludge waste) at firing temperature ranging between 1200 and 1250°C.

Effect of strontium oxide on the formation mechanism of dicalcium silicate with barium oxide and sulfur trioxide

Belite-barium calcium sulfoaluminate cement is a new type of cement characterised by low energy consumption and low carbon dioxde emissions, but the slow hydration of the belite phase leads to a slow mechanical property gain at early ages compared with ordinary Portland cement. A doping technique has been proposed as one option to promote its reactivity. Strontium slag, a kind of chemical industrial waste produced from the manufacture of strontium salt, has a high level of emissions in China. In this study, the influence of strontium oxide on the formation mechanism of dicalcium silicate with barium oxide and sulfur trioxide, which are both in excess in belite-barium calcium sulfoaluminate cement clinker, is investigated. Results show that strontium oxide accelerates the decomposition process of calcium carbonate, and its final decomposition temperature can be lowered by 67°C. Strontium oxide decreases the formation rate of dicalcium silciate at the same temperature, but the amount of total dicalcium silicate formed can still be as high as 94·5 wt% at 1350°C. Moreover, it is revealed that strontium oxide plays a significant role in stabilising β-dicalcium silicate, and reduces γ-dicalcium silicate by about 40%. This study could lead to environmental benefits and cement performance improvement.

Natural analogs of belite sulfoaluminate cement clinkers from Negev Desert, Israel

Ye’elimite-larnite rocks in the Hatrurim formation of the Negev Desert, Israel, are natural analogs of belite sulfoaluminate (BSA) cement clinkers. They have been produced by ultrahigh-temperature combustion metamorphism at ambient pressure of a calcareous sedimentary precursor. Their mineralogy consists of 35-50 vol% β-Ca2SiO4, 15-20 vol% ye’elimite, 7-15 vol% ferrites, and 15-20 vol% fluorapatite and/or fluorellestadite. A few grains of hatrurite (Ca3SiO5) and α′-Ca2SiO4 have been observed as well. The composition of α′- and β-Ca2SiO4 polymorphs by EPMA are near Ca1.96-1.98Na0.01-0.02 Si0.96P0.03Al0.01O4, whereas ye’elimite has an approximate composition by EPMA of Ca3.99Mg0.02Ba0.01 Na0.02K0.02Al5.73Fe3+0.16Si0.10S0.97P0.02O16. The Al content of brownmillerite Ca2(Fe1-xAlx)2O5 ranges from x = 0.20-0.27. Fe-analog of shulamitite (Ca3Fe2TiO8) contains up to 15.1 wt% TiO2. Ye’elimite-larnite rocks were derived from chalky sediments by burning of combustible gas with a Tmax at 1200-1350 °C. The mineral content, microstructure, and texture/fabric of the ye’elimite-larnite rocks imply that chalky and/or marly sediments with randomly distributed clay, phosphorite, and gypsum may be utilized as cheap naturally homogenized and pulverized mixtures for industrial production of BSA cement clinker, as an environment-friendly alternative to ordinary Portland cement (OPC).

Recycling the product of thermal transformation of cement-asbestos for the preparation of calcium sulfoaluminate clinker

According to recent resolutions of the European Parliament (2012/2065(INI)), the need for environmentally friendly alternative solutions to landfill disposal of hazardous wastes, such as asbestos-containing materials, prompts their recycling as secondary raw materials (end of waste concept). In this respect, for the first time, we report the recycling of the high temperature product of cement-asbestos, in the formulation of calcium sulfoaluminate cement clinkers (novel cementitious binders designed to reduce CO2 emissions), as a continuation of a previous work on their systematic characterization. Up to 29wt% of the secondary raw material was successfully introduced into the raw mix. Different clinker samples were obtained at 1250°C and 1300°C, reproducing the phase composition of industrial analogues. As an alternative source of Ca and Si, this secondary raw material allows for a reduction of the CO2 emissions in cement production, mitigating the ecological impact of cement manufacturing, and reducing the need for natural resources.