The effect of SiO2 additions on barium aluminate cement formation and properties

Barium aluminate cements have been synthesized by barium carbonate, alumina, kaolin and colloidal silica as starting materials. The effects of the source of SiO₂ and of firing temperature on phase formation and physical properties of the fired cements have been studied. Cement samples were characterized using XRD, SEM, EDX. The setting time and heat of hydration of cements were also evaluated. The barium aluminate cements were mixed in castables. Cold crushing strengths evaluated, and values compared to those obtained using calcium aluminate cement (Secar 71). Mixtures of BaCO₃ and Al₂O₃ were targeted to produce BaAl₂O₄; which had fast set time, expansive behavior and lower strength compared to samples with SiO₂ additions. SiO₂ additions, regardless of source, resulted in BaAl₂Si₂O₈ (celsian) formation. The prepared samples had short setting times and higher mechanical properties in comparison with standard calcium aluminate cement.     

A new aluminium-hydrate species in hydrated Portland cements characterized by Al and Si MAS NMR spectroscopy

Recent ²⁷Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ_iso = 5.0 ppm and the quadrupole product parameter P_Q = 1.2 MHz, has been characterized in detail by ²⁷Al MAS and ²⁷Al{¹H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH)₆³⁻ or O_xAl(OH)_6-x^(3+x)− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70-90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al³⁺ for Ca²⁺ substitution in the principal layers and interlayers of the C-S-H structure are discussed.     

Effect of High‐Energy Ball Milling and Silica Fume Addition in BaCO3–Al2O3. Part I: Formation of Cementing Phases

The effects of high‐energy ball milling and subsequent calcination on the formation of barium aluminate cementing phases using mixtures of Al₂O₃ and BaCO₃ were investigated. Silica fume was further added in the raw mixtures to observe its role on the cementing phase formation. Results indicated that the decomposition temperature of BaCO₃ lowered remarkably with the increase in milling time. Barium aluminate cements with grain size in nanometer range were obtained from high‐energy ball‐milled raw mixtures. X‐ray diffraction (XRD) results confirmed several crystalline barium‐silicate and barium aluminate phases present. Formation of crystalline BaO·Al₂O₃ phase was observed between 1000°C and 1100°C in the raw mixtures, which were obtained in amorphous state after milling for 5 h. This temperature is at least 300°C lower than that used in the traditional solid‐state method. Fume SiO₂ additions resulted in BaO·Al₂O₃·2SiO₂ (celsian) formation which acted as a retarder, provides more workability and mechanical strength.     

Calcium silicate/calcium aluminate composite biocement for bone restorative application: synthesis,characterisation and in vitro biocompatibility

Pure β-dicalcium silicate and monocalcium aluminate powder were prepared by Pechini method. A series of calcium silicate/calcium aluminate cements (CSC/CAC) were prepared. The setting time, crystalline phases, microstructures, compressive strength, cells attachment and silicon release of the cements were investigated. The results indicate that the setting time of CSC/CAC was shorter than that of either CSC or CAC. The hydration products in CSC/CAC composite are gehlenite (Ca₂Al₂SiO₇·8H₂O), calcium aluminate hydrate (Ca₃Al₂O_6?×?H₂O), and katoite (Ca₂Al₂O₆·6H₂O). Platelike crystals were found in the microstructure. The liquid to powder ratio has a significant effect on the porosity and the strength of CSC/CAC. The MC3T3 cells attached well to the surfaces of CSC/CAC. However, the cells proliferation on the surface of 7S3A was better than that of 3S7A due to its higher silicon release. In general, CSC/CAC exhibits good biocompatibility and relative high strength, and may be suitable for some non-load bearing bone restorative applications.     

Utilization of aluminum sludge and aluminum slag (dross) for the manufacture of calcium aluminate cement

Four calcium aluminate cement mixes were manufactured from aluminum sludge as a source of calcium oxide and Al₂O₃ and aluminum slag (dross) as a source of aluminum oxide with some additions of pure alumina. The mixes were composed of 35–50% aluminum sludge, 37.50–48.75% aluminum slag (dross) and 12.50–16.25% aluminum oxide. The mixed were processed then sintered at different firing temperatures up to 1500 °C or 1550 °C. The mineralogical compositions of the fired mixes investigated using X-ray diffraction indicated that the fired mixes composed of variable contents of calcium aluminate (CA), calciumdialuminate (CA₂), calciumhexaaluminate (CA₆) in addition to some content of magnesium aluminate spinel (MA). Sintering parameters (bulk density, apparent porosity and linear change) and mechanical properties (cold crushing strength) of the fired briquettes were tested at different firing temperature. Refractoriness of the cement samples manufactured at the optimum firing temperature was detected. Cementing properties (water of consistency, setting time and compressive strength as a function of curing time up to 28 days of hydration) of pasted prepared from the manufactured cement mixes at the selected optimum firing temperatures (1400 °C or 1500 °C) were also tested. Cement mixes manufactured from 45 to 50% aluminum sludge, 37.50–41.25% aluminum slag (dross) with 12.50–13.75% alumina were selected as the optimum mixes for manufacturing calcium aluminate cement since they satisfy the requirements of the international standard specifications regarding cementing and refractory properties as a result of their content of CA (the main hydraulic phase in calcium aluminate cement) and CA₂(the less hydraulic but more refractory phase). Although the recognized high refractoriness of CA₆, its formation affect badly the cementing properties of the other non-optimum mixes.     

Advances in alternative cementitious binders

There is a burgeoning interest in the development, characterization, and implementation of alternatives to Portland cement as a binder in concrete. The construction materials industry is under increasing pressure to reduce the energy used in production of Portland cement clinker and the associated greenhouse gas emissions. Further, Portland cement is not the ideal binder for all construction applications, as it suffers from durability problems in particularly aggressive environments. Several alternative binders have been available for almost as long as Portland cement, yet have not been extensively used, and new ones are being developed. In this paper, four promising binders available as alternatives to Portland cement are discussed, namely calcium aluminate cement, calcium sulfoaluminate cement, alkali-activated binders, and supersulfated cements. The history of the binders, their compositions and reaction mechanisms, benefits and drawbacks, unanswered questions, and primary challenges are described.     

Recycling of bone ash from animal wastes and by-products in the production of novel cements

The purpose of this research is to demonstrate the utilization of animal wastes and by-products in the production of low-energy and low-CO₂ clinkers and cements in order to preserve natural resources, such as limestone, while reducing CO₂ emissions released from the cement manufacturing process and reducing potential health risk to the world population (such as bovine spongiform encephalopathy or other health issues…). Pure calcium sulfoaluminate clinker was produced with calcium hydroxide, aluminum hydroxide, and calcium sulfate hemihydrate; followed by additional clinkers produced from substituting calcium hydroxide with bone ash (from 0 to 100% of the calcium hydroxide replaced). The final clinkers contained various amounts of ye'elimite, calcium aluminate phases, as well as tricalcium phosphate, depending on the firing temperature. Finally, some preliminary results on the hydration process and compressive strength are provided for the production of these binders.     

Raman Spectroscopy of Anhydrous and Hydrated Calcium Aluminates and Sulfoaluminates

Recent investigations have revealed the great potential of Raman spectroscopy for the characterization of clinker minerals and commercial Portland cements. The usefulness of this technique for the identification of anhydrous, hydrated, and carbonated phases in cement‐based materials has been demonstrated. In the present work, the application of micro‐Raman spectroscopy for the characterization of the main clinker phases of calcium aluminate cements and calcium sulfoaluminate cement is explored. The main stable hydrated phases as well as several important carbonated phases are investigated. Raman measurements on the following phases are reported: (i) pure, unhydrated phases: CA, C₁₂A₇, CA₂, C₂AS, cubic‐C₃A, C₄AF, and C₄A₃; (ii) hydrated phases: ettringite, monosulfoaluminate, and hydrogarnet (C₃AH₆); (iii) carboaluminate phases: hemicarboaluminate and monocarboaluminate. The present results, which are discussed in terms of the internal vibrational modes of the aluminate, carbonate, and sulfate molecular groups as well as stretching O–H vibrations, show the ability of Raman spectroscopy to identify the main hydrated and unhydrated phases in the aluminate and sulfoaluminate cements. The Raman spectra obtained in this work provide an extended database to the existing data published in the literature.     

Durability of Alkali‐Activated Materials: Progress and Perspectives

In the last decade, there has been rapid growth in interest in alternative binders, as part of the toolkit of cement technologies needed to mitigate the carbon footprint associated with the construction industry. Alkali‐activated materials (AAMs), including geopolymer binders and other related systems, have been identified as a key component of this move to lower CO₂ cements and concretes. These are clinker‐free cements which can exhibit comparable performance to conventional portland/blended cements, when they are adequately formulated and cured. However, AAMs have a somewhat limited record of durability in service, and this is one of the main limitations facing their commercial adoption at present. To provide the best possible answers to the question of long‐term durability within an experimentally accessible timeframe, standardized accelerated degradation testing methods have been widely adopted, in an attempt to simulate natural processes. It has been identified that the interactions between material and environment, which take place on microstructural and nanostructural levels, have a very significant influence on the outcomes of the durability tests. Here, we present an overview of the results obtained when AAMs are exposed to aggressive testing conditions such as elevated concentrations of CO₂, sulfates or chlorides. The key outcome of this article is a broader synthesis of the available data regarding the interactions between these new materials and their surrounding environment, which is then available to be used in the design, development, and implementation of environmentally sustainable, high‐performance cements and concretes for the 21st century.     

Elucidating the Role of the Aluminous Source on Limestone Reactivity in Cementitious Materials

When limestone (CaCO₃) is present in ordinary portland cement (OPC), carbonate‐AFm phases (i.e., hemi‐ and/or mono‐carboaluminate) are stabilized at the expense of the sulfate‐AFm, which is more commonly found in cement systems. In OPC, the quantity of AFm hydrates formed is often limited by the availability of aluminum. Therefore, as a means of enhancing AFm phase formation, this study elucidates the role of aluminous sources including: calcium aluminate cements, metakaolin, and a hydratable alumina to determine if their addition would enhance limestone reactions and carbonate‐AFm formation in cement systems. The results of a detailed study including: X‐ray diffraction, strength measurements, thermogravimetric analysis, and thermodynamic calculations are used to quantify solid phase constitutions, and the extent of limestone reacted. The results suggest that, the amount of limestone reacted and the specific carbonate‐AFm formed is sensitive to both, the nature of the aluminous source and limestone content. Pozzolanic reactions which occur when metakaolin is used as an aluminous source are noted to be especially beneficial in offsetting the effects of OPC replacement. It is noted that although the different aluminous materials react with different quantities of CaCO₃ during hydration, enhanced carbonate‐AFm formation alone is insufficient to ensure strength equivalence, when OPC is replaced by limestone.     

Aspects of making a superconducting yttrium ceramic by self-propagating high-temperature synthesis

Macrokinjetic features are considered for the combustion in oxygen of BaO₂–Cu–Y₂O₃ mixtures, which produces the high-temperature superconductor yttrium barium cuprate YBa₂Cu₃O_7–x. Studies have been made on how the initial temperature affects the combustion temperature and rate, and the critical temperature for self-ignition has been determined. Ultrasonic activation of the initial powders has an advantageous effect on the product quality. Thermal analysis has been applied to the activated mixtures, which indicates the reason for the rise in burning wave propagation rate and increase in conversion to the superconducting phase.Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 62–67, March–April, 1993.     

The impact of mechanical grinding on calcium aluminate cement hydration at 30 °C

Calcium aluminate cement (CAC) was ground for 1 and 2 h to investigate the impact of mechanical grinding on CAC hydration at 30 °C and CAC-bonded castable strength. Phase composition and microstructure of unground and ground cements after hydration for predetermined times and terminated by the freeze-vacuum drying were compared. The results indicate that the particle size and particle size distribution of CAC were reduced and narrowed, respectively by grinding, thereby favoring the hydration rate and the conversation of C₂AH₈ to C₃AH₆. Then enhanced cement hydration also increases the strengths of castables bonded with milled CAC after drying and firing.     

Does a tiny amount of dispersant make any change to refractory castable properties?

A growing demand for refractory castables with specific behaviors has given rise to a continuous technological evolution, mainly due to the broad knowledge of hydraulic binders available nowadays. The high alumina cements remain as the most important hydraulic binders for castables. Nevertheless, calcium aluminate bound castables still show a characteristic drop of strength at intermediate temperatures, which could also be affected by the castable chemical additive. Thus, this paper aims to highlight the influence of dispersants on the refractory castable properties with the firing temperature. It was noticed that the hydrates formed during the curing process of castable depends on the dispersing additive used. The FS60, a polycarboxylate ether, induced the AH₃ formation and its decomposition resulted in a more stable hydrate (AH), which increased the splitting strength with the thermal treatment temperature. At a high temperature, the CA₂ and CA₆ formation is also favored in the presence of this additive. However, it did not bring benefits to the castables creep behavior, resulting in a less tough structure.     

Development,characterization and optimization of a new bone cement based on calcium–strontium aluminates and chitosan-glycerin solution

Bioceramic bone cements are increasingly studied, developed, and improved to become a viable alternative to polymethyl methacrylate (PMMA)-based cements. In this regard, we aimed to develop a new cement composed of calcium aluminate (C₁₂A₇) and strontium aluminate (S₃A) powders obtained via solution combustion synthesis (SCS) and chitosan/glycerin solution. The cement properties were optimized through a design of experiments. The approach used in the optimization process was the 2^k factorial experimental design with insertion of 3 repetitions of the central point, which resulted in 11 compositions. All compositions were tested to determine the liquid/powder ratio (L/P), final setting time (ST), maximum hydration temperature (T_max), compressive strength and radiopacity. The results were statistically evaluated by analyzing the effects, Pareto diagram, ANOVA analysis and response surface plot. The models obtained in this study could precisely predict three responses: T_max, compressive strength, and radiopacity. An optimized composition for possible application as bone cement had an average T_max of 40.34 °C, compressive strength of 7.75 MPa and radiopacity of 3.76 mm Al, all above the standard requirements.     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

Dispersing (Deflocculating) Aluminas

The rheological properties of aqueous suspensions of a reactive alumina and high-alumina cements have been studied, and a method for evaluating the thinning effect due to agents used to deflocculate ceramic casting systems has been developed; a classification of thinning agents is proposed. Alumina dispersants make it possible to decrease the moisture content of Al₂O₃ suspensions by 50 – 60 rel.% and increase their volume concentration from 0.50 to 0.65. The characteristics of dispersing aluminas used for the thinning (deflocculation) of new refractory castables containing Al₂O₃ (76 – 96%) and organic deflocculants are given. Suspensions based on four types of dispersants (with a moisture content of 19 – 25%) exhibiting a thixotropic flow behavior are considered.     

Tensile strength of a water-setting (hydration-hardening) zirconium dioxide based concrete in the 300–2300 k range

Conclusions During primary heating of the modified zirconium dioxide-based water-setting concrete (TsGBM), after attaining the desired temperature level, _tns increases up to approximately 1h if the temperature is below 2000 K (due to sintering of the finely dispersed barium aluminate separated during the decomposition of the crystallohydrates and due to the transformation of the concrete into a two-phase ceramic) and up to approximately 5 h if the temperature exceeds 2000 K (owing to sintering of the particles of the electromelted zirconium dioxide filler) and, thereafter, it remains constant:the level of the stationary strength of TsGBM decreases monotonically with increasing temperature from 3.5 N/mm² (under normal conditions) up to 0.25 N/mm² (at a temperature exceeding 2100 K) and corresponds to the strength of a granular and porous zirconium dioxide ceramic;up to 1200 K, the stationary strength of the concrete is determined by the quantity and the crystallohydrates of barium aluminate and their bond strength; up to 2100 K, it is determined by the degree of sintering and the intrinsic strength of barium aluminate; and above 2100 K, it owes to the formation and the strength of the zirconium dioxide-zirconium dioxide type bonds.Translated from Ogneupory, No. 5, pp. 4–8, May, 1991.The authors gratefully acknowledge the help rendered by T. A. Melekhin and T. A. Evdokimov in carrying out specimen preparation and Yu. I. Chubarov in the operation of the testing unit.     

Early age hydration of rice hull ash cement examined by transmission soft X-ray microscopy

Early age hydration of barium-doped β-Ca₂SiO₄ cement, produced from rice hull ash (RHA), is examined by transmission soft X-ray microscopy. Use of low-energy cements produced from by-product materials, such as the cement considered here, may be economically and environmentally advantageous. However, the hydration kinetics and morphology and composition of the products of RHA-based β-Ca₂SiO₄ cements have not been investigated. Observation of the early age cement hydration shows evidence of cement dissolution and hydration product formation, including the formation of Hadley grains. The rates of the reaction and amount product formed appear to be related to the hydrothermal processing temperature and the chemical composition of the cement. That is, more rapid hydration is observed for barium-doped RHA cements produced at higher temperatures and for cements produced with higher barium contents, within the ranges examined.     

Resistance of blast furnace refractories to the action of aggressive reactants under conditions of an oxidizing gaseous atmosphere

Conclusions Investigations have been made of the resistance of ShPD-41, ShPD-39, and ShUD-37 chamotte refractories to the action of K₂CO₃, Fe₂O₃, blast furnace dust, and initial and final blast furnace slags under conditions of an oxidizing atmosphere. The investigation results showed that iron oxides and slag break down these refractories at 1400–1500°C. Dense ShPD-41 refractory is more resistant to the action of the reactants.The most resistant to the action of slags and iron oxides at 1400–1500°C are silicon carbide refractories with binders of silicon nitride and oxynitride.Translated from Ogneupory, No. 7/8, pp. 24–27, July–August, 1992.     

Examination of ye’elimite formation mechanisms

Ye’elimite is the main constituent of calcium sulfoaluminate (CSA) cement and one of the major constituents of belite sulfoaluminate or belite sulfoaluminate ferrite cements. The main objective of this work is to describe precisely the formation mechanisms of ye’elimite by solid-state reaction. Mineralogical composition development was monitored using XRD analysis, while microstructural monitoring was conducted using BSE-SEM coupled to EDS analysis. The results show that CaAl₂O₄ and CaAl₂O₇ are the main intermediate products during ye’elimite formation. At the microstructural scale, ye’elimite forms on calcium aluminate phases. Finally, Avrami’s model was suggested to discuss the ye’elimite formation rate according to sintering temperature and duration.