System CaO Al2O3 Research Articles

Thermodynamic descriptions of the CaO–Al2O3 and CaO–Al2O3–SiO2 systems over the whole composition and temperature ranges

AbstractThe CaO–Al2O3–SiO2 system is a fundamental system in aluminosilicate materials, and its thermodynamic description provides key information for ceramic design, cement production, slag tapping, and geological prediction. However, the existing thermodynamic calculations for the CaO–Al2O3 system show inaccurate heats of formation for the compounds and consequently a decomposition of the CA2 (CaO⋅2Al2O3) phase at a low temperature. Besides, the solid solutions in anorthite and cristobalite have never been considered in the previous thermodynamic assessments for the CaO–Al2O3–SiO2 system. Therefore, thermodynamic reassessments of the CaO–Al2O3 and CaO–Al2O3–SiO2 systems are carried out in the present work using the CALculation of PHAse Diagram method, considering the solid solution in anorthite and cristobalite for the first time. The anorthite and cristobalite phases are modeled based on the individual mechanism of solid solution, with the anorthite described by (Ca+2, Va)1(Al+3, Si+4)2(Si+4)2(O−2)8 and the cristobalite described by (Va, Ca+2)1(Si+4, Al+3)2(O−2)4. The present assessment solves the above issues and provides consistent thermodynamic descriptions for the liquid and solid phases. Based on the established thermodynamic database, a Scheil solidification simulation is carried out for a cement clinker and the variations of mass fractions of the phases as well as the composition of liquid during Scheil cooling are calculated. The present work guides the design of relevant materials and lays an essential foundation for the establishment of the thermodynamic database of multicomponent aluminosilicate materials.

Researching the influence of the CaO/Al2O3 ratio on ettringite formation and obtaining the structure of a cement paste with special properties

The hypothesis of the dependence of the value of the Gibbs surface energy on the ratio of molecular masses mСаО/nAl2О3 (ΔG=f(mСаО/nAl2О3)) is confirmed, and it is proposed to determine the effectiveness of its influence on the hydration process of minerals of composite sulfoaluminate systems by means of the mineral coefficient KСА= Сm/An, where Сm/An is the Gibbs surface energy, KСmАn is the mineral coefficient of the system. This makes it possible to arrange the minerals of the CaO-Al2O3 system in the following order: СА2, СА, С12А7, С3А respectively with surface energy, kJ/mol – 24.7; 51.86; 141; 145 and later use the coefficients for researching multicomponent systems. Research on the influence of nanodisperse additives in the composition of the system (Al2O3-CaO-SO3-H2O) is aimed at managing the hydration. Formation of a strong structure is associated with the creation of a gel phase and hydroaluminates of cubic crystal system − С3АН6, as well as a stable ettringite phase. With the help of nanomodifiers, the formed blocks are freely joined in the cement matrix, which leads to a decrease in internal stresses in the not yet formed cement system. In addition, there is an assumption on doping of C3AŚH32 and C3AŚH12 crystals.

Using the zirconium­containing binders for obtaining refractory concretes

Now the creation of new heat­resistance and refractory binding materials contributing towards reliable and durable service of high­temperature plants is paid great attention. To develop the physical and chemical principles of such materials technology first of all it is necessary to study the high­melting oxide systems and as a consequence, to obtain refractory products in terms of ones. Taking into consideration all mentioned above, the multicomponent zirconium­oxide systems are of potential interest.
 As a result of our investigations new compositions of high­strength, heat­resistance, and refractory zirconium­containing binders on the basis of different sections of the quaternary system CaO—Al2O3—ZrO2—SiO2 are obtained. Refractory concretes in terms of zirconium­containing cement and various aggregates, both natural (corundum, baddeleyite) and artificial (calcium zirconate), have been elaborated. It is revealed that the kind and granulometric composition of aggregates, molding methods, as well as cement­aggregate relation influence the physical and mechanical properties of concrete developed. Concretes obtained are characterized by high strength (50—60 MPa) and service property stability: refractoriness — more than 1700 °C thermal shock resistance — more than 30 cycles; thermal linear expansion coefficient — 4.8×10–6 ÷ 5.6×10–6 deg.–1, initial strain point under load of 0.2 MPa — 1400—1500 °C Materials developed comply with requirements to refractory materials and can be applied for monolithic linings of heat density parts of high­temperature plants.

Описание химического взаимодействия в системе CaO-Al 2O3-SiO2

In this study the construction of a phase tree and the description of the chemical interaction for the ternary oxide system CaO-Al 2O3-SiO2 are given. Particular interest to a system consisting of oxides of calcium, aluminum and silicon is associated with the production of highly demanded functional materials with desired properties. Melts of the CaO-Al2O3-SiO2 system are of great theoretical and applied importance. This is due to the significant role of melts of these oxides and their mixtures in metallurgy, ceramics production, and other industries. Phase relationships in the system with the complete disappearance of liquid in the CaO-Al2O3-SiO2 system has made it possible to construct a phase tree of the system, which includes a linear part and two cycles. The construction of the phase tree is given taking into account the formation of four double compounds in the CaO-SiO2 system, five double compounds in the CaO-Al2O3 system, one double compound in the Al2O3-SiO2 system, and two ternary compounds. The stable complex includes fifteen secondary phase triangles, interconnected by sixteen stable secants. For mixtures corresponding to equivalence points (points of intersection of unstable and stable secants), the chemical interaction is described in accordance with the law of equivalents. It is concluded that for all mixtures corresponding to equivalence points, interactions are thermodynamically possible under standard conditions. The prediction of crystallizing phases is made.

Modeling viscosity of SiO2-Al2O3-CaO based slags using Arrhenius and VFT models as well as the CALPHAD method

The first step of modeling the viscosity of slags is to choose a temperature-related viscosity model which plays a significant role in determining the framework of the entire model. In the present work, two temperature-related viscosity models, the Arrhenius and the VFT (Vogel-Fulcher-Tammann) models are compared for calculating the viscosity of slag systems taking the experimental data for the SiO2-Al2O3-CaO system as basis. The results show that the VFT model shows a better performance in describing experimental data for the temperature dependence of dynamic viscosity among all the investigated systems compared to the Arrhenius model. Such a good performance by the VFT model is explained by the actual network structure of liquid oxide systems described well by this model. Moreover, a linear relationship between the two semi-empirical parameters of the VFT model (ln A and T0) is found for the first time, which can decrease the number of independent parameters when modeling the viscosity of slags by this model. In addition, the viscosities of the CaO-Al2O3 and SiO2-CaO systems are modelled by the combined VFT-CALPHAD (CALculation of PHAse Diagrams) model over wide temperature and composition ranges.

Industrial Trials on Preparation of Cerium‐Treated H13 Steel by Electroslag Remelting with Cerium‐Oxide Containing Slag

To realize an industrial production of a cerium‐treated electroslag remelted H13 steel, electroslag remelting (ESR) of H13 steel using a CaF2–CeO2–CaO–Al2O3 system slag combined with a Si–Ca reductant addition is studied. The influence of the Al2O3 content in the slag on the cerium treatment effect of cerium‐oxide‐containing slag is investigated to determine the appropriate slag. A 52.2 wt% CaF2–26.7 wt% CeO2–17.8 wt% CaO–3.3 wt% Al2O3 slag (RE‐Slag) is employed for the industrial ESR production of H13 steel. An H13 steel remelted by a 70 wt% CaF2–30 wt% Al2O3 slag (CA‐Slag) is used for comparison. Al2O3 can suppress the reduction of cerium oxide by changing the activities of slag components. ESR with RE‐Slag achieves a cerium treatment of H13 steel. However, an uneven distribution of Ce is obtained due to the limitation of the reductant addition method. The H13 steel remelted with RE‐Slag (RE‐Slag‐H13) has higher cleanliness, finer dendritic structures, and finer primary carbides than those of the steel remelted with CA‐Slag (CA‐Slag‐H13). Compared to the forged CA‐Slag‐H13 steel, the banded segregation of the RE‐Slag‐H13 steel is suppressed, and the impact toughness is increased by 30%.

Phase Relations in the CaO-Nd2O3-Al2O3 System in Application for Rare Earth Recycling

Phase relations in the CaO-Al2O3-Nd2O3 system were investigated in the temperature range between 1330°C and 1600°C using equilibration technique followed by characterization using x-ray diffraction and scanning electron microscopy combined with energy-dispersive x-ray spectroscopy (SEM/EDX). Melting relations were investigated using differential thermal analysis followed by microstructure study using SEM/EDX. It was found that CaNdAlO4 (τ1) compound melted congruently at 1808°C, while CaNdAl3O7 (τ2) compound melted incongruently at 1766°C. The sample in the stability fields adjacent to lowest eutectic in the CaO-Al2O3 system (CaAl2O4 + Ca3Al2O6) and τ2 as well as samples in the stability field τ2 + CaAl2O4 + CaAl4O7 and τ1 + τ2 + Ca3Al2O6 were completely melted at 1600°C. The Ca12Al14O33 phase considered as not stable in anhydrous atmosphere was found instead of Ca3Al2O6 phase after heat treatment at 1330°C. After melting Ca12Al14O33 formed glass. The obtained results are important for selection of conditions for process of rare earth recycling from waste permanent NdFeB magnets.

Stable compounds in the CaO-Al2O3 system at high pressures

Using evolutionary crystal structure prediction algorithm USPEX, we showed that at pressures of the Earth's lower mantle CaAl2O4 is the only stable calcium aluminate. At pressures above 7.0 GPa it has the CaFe2O4-type structure and space group Pnma. This phase is one of prime candidate aluminous phases in the lower mantle of the Earth. We show that at low pressures 5CaO * 3Al2O3 (C5A3) with space group Cmc21, CaAl4O7 (C2/c) and CaAl2O4 (P21/m) structures are stable at pressures of up to 2.1, 1.8 and 7.0 GPa respectively. The previously unknown structure of the orthorhombic 'CA-III' phase is also found from our calculations. This phase is metastable and has a layered structure with space group P21212.

CaO-Al2O3 System and Possibilities of Sol-Gel Synthesis

CaO-Al2O3 System and Possibilities of Sol-Gel Synthesis

ВЛИЯНИЕ МОДИФИЦИРУЮЩИХ ДОБАВОК НА ПРОЦЕССЫ СИНТЕЗА КЛИНКЕРНЫХ МИНЕРАЛОВ ВЫСОКОГЛИНОЗЕМИСТОГО ЦЕМЕНТА

The article presents the results of studying the effect of modifying additives CaAl2O4 (CA) and CaAl4O7 (CA2), as well as boric acid on the processes of phase formation of clinker minerals. The calculation of the content of raw materials in the charge is carried out taking into account the production of high-alumina clinker with the participation of Al2O3 71–72 wt.% and CaO 27–28 wt.%. In accordance with the state diagram of the CaO–Al2O3 system, the mass amount accumulates in two calcium aluminate phases: CA 64 wt.% and CA2 36 wt.%. Chalk grade M-90 and commercial alumina grade G-0 are used as feedstock. Heat treatment of powder mixtures with different concentrations of modifying additives is carried out in the temperature range of 1250 - 1350°C with isothermal holding for 1 and 2 hours. It has been established that when 0,5 % H3BO3 is introduced into the charge, the design ratio of the CA and CA2 phases appears after heat treatment at 
 1350 °C for 2 hours. An increase in the content of boric acid to 1,0 % allows to obtain a phase clinker composition close to the design one at 1250 °C and a holding time of 1 hour. It has been shown that the introduction of HAC additives into the mixture affects the quantitative phase composition of products. A correlation is established between the amount of HAC introduced and the decrease in the content of unreacted α-alumina in samples heat-treated at 1350 °C for 1 hour.

Study on the structural properties of refining slags by molecular dynamics with deep learning potential

The CaO-Al2O3 is the most important basis of the multicomponent slag in the steel making industry. The microstructure of molten CaO-Al2O3 systems affects the physical, chemical, and metallurgic properties of refining slags. It is significant to establish a computational method to accurately describe the microstructure of the slag at the atomic level and speculate the properties of large slag models. In this paper, the effect of compositions on the structure of molten CaO-Al2O3 systems was studied by ab initio molecular dynamics, deep learning theory, and deep potential molecular dynamics. The structures of various molten slags were simulated by ab initio molecular dynamics, which accurately describes the interactions between atoms. The potential functions were obtained by deep learning theory. The properties of the large slag models were simulated by molecular dynamics with deep learning potential. As the molar fraction of CaO increases from 0.5 to 0.7, the combination of O and Al mainly forms [AlO4]-5, [AlO3]-3, and [AlO5]-7. Ca presents mainly as free cations in the molten CaO-Al2O3 system. In addition, the diffusion coefficient of Ca decreases from 9.0 × 10-10 m2/s to 5.4 × 10-10 m2/s. The diffusion coefficients of Al and O slightly decrease and are close to 1.1 × 10-10 m2/s and 3.0 × 10-10 m2/s, respectively. It is deduced that the dissolution of CaO provides free O2– and promotes the formation of [AlOn]-b and increases the polymerization degree, which causes deterioration of the fluidity of molten CaO-Al2O3 slags as XCaO increases from 0.5 to 0.7.

Laboratory investigation on quantitative effect of ladle filler sands on the cleanliness of a bearing steel

Laboratory experiments were performed using two kinds of heating furnaces to investigate the quantitative effect of ladle filler sands (LFS) on the cleanliness of a bearing steel at the casting start of the first heat and non-first heats of a casting sequence. After the direct addition of LFS into the molten steel, which was the condition of the first heat, inclusions in steel changed from MgO · Al2O3 to Al2O3. As the LFS/steel ratio increased from 0 to 1:600, the number density of inclusions significantly increased from 5.3 #/mm2 to 14.5 #/mm2 and the area fraction of inclusions sharply increased from 7.8 ppm to 60.6 ppm, inducing the sharp increase of T.O to 16.1 ppm. After the reaction between the LFS contained tundish covering powder (TCP) and the molten steel, inclusions in steel were separated into a MgO-Al2O3 system and a CaO-Al2O3(-MgO) system. With the increase of LFS/TCP ratio to 1:12, the average MgO content in inclusions decreased to 19.1 wt% while the Al2O3 content increased to 77.0 wt%, meanwhile, the T.O content increased to 6.0 ppm and the [Al] content decreased to 50 ppm. Based on experimental results and the thermodynamic analysis, the detrimental effect of LFS on the steel cleanliness of non-first heats was much slighter than that of the first heat owing to the use of high-basicity TCP. Accordingly, more efforts should be made to prevent the LFS charging into the molten steel at the casting start of the first heat of a casting sequence during the industrial production.

Development of dense Sr-substituted CaAl12O19 (CA6) ceramics synthesized by sol-gel combustion method

ABSTRACT The most alumina-rich compound in the CaO-Al2O3 system is calcium hexaaluminate (CaAl12O19, CA6). CA6 ceramics were synthesized by the sol-gel auto combustion method. CA6 phase formation has been achieved at 1200°C, which is a much lower temperature than the convention process (>1600°C). The influence of Sr2+ substitution on the densification of Ca1-xSrxAl12O19 (x = 0, 0.1, 0.2, and 0.3) ceramics was investigated. The X-ray diffraction analysis and structural refinement indicated the substitution of Sr2+ ions into the lattice of CA6. The elongation of the “c” direction of the CA6 crystal structure was found with Sr2+ ion substitution. Surface morphology analysis showed a crystal growth with Sr2+ substitution in CA6 upto x = 0.2 composition. The 1600°C sintered specimen (with x = 0.2 composition) showed the bulk density and open porosity of 3.39 g/cm3 and 4.92%, respectively. Thus, auto combustion synthesis and Sr2+ substitution into CA6 ceramics are feasible and beneficial for the densification of the ceramics.

A heterogeneous catalytic process to mitigate the acidity of bio-oils caused by the presence of volatile organic acids

The removal of the acidity produced by the presence of volatile organic acids in bio-oils, such as acetic acid, assisted by heterogeneous catalysts was investigated. For this purpose, CaO and alumina-supported catalysts were prepared, and silver nanoparticles were used as promoter. The prepared catalysts, both fresh and post-reaction, were characterized by several techniques: N2 physisorption (BET), SEM-EDS, TEM, XRD, and TPR. All catalysts were tested in the elimination of acetic acid, employing two different reaction systems: batch microreactors and a pressurized batch reactor. An appreciable CH3COOH elimination of ~60%–70% was obtained at low temperature (150–200 °C) using the CaO-Al2O3 system. The addition of silver improved the removal percentage. Silver-supported catalysts were also tested in a pressure-controlled system, achieving an excellent percentage of acetic acid removed (~97.5%). Silver would promote the incipient reactions of catalytic ketonization and aldol condensation.

Synthesis and Characterization of 12CaO\xb77Al2O3 Slags: The Effects of Impurities and Atmospheres on the Phase Relations

Synthesis of crystalline slags of 12CaO·7Al2O3 phase from the corresponding melt compositions in different atmospheric conditions and different purities is investigated. Observations using a thermogravimetry coupled with differential thermal analysis showed that the dehydration of a zeolitic 12CaO·7Al2O3 phase occur at 770 °C to 1390 °C before it congruently melts at 1450 °C. The X-ray diffraction pattern of the slag showed that a single 12CaO·7Al2O3 phase is produced from a mixture, which has small SiO2 impurity with a 49:51 mass ratio of CaO to Al2O3. A scanning electron microscope and electron probe micro-analyzer showed that a minor Ca-Al-Si-O-containing phase is in equilibrium with a grain-less 12CaO·7Al2O3 phase. Moreover, 12CaO·7Al2O3 is unstable at room temperature when the high-purity molten slag is solidified under oxidizing conditions contained in an alumina crucible. On the other hand, a high-temperature in-situ Raman spectroscopy of a slag that was made of a higher purity CaO-Al2O3 mixture showed that 5CaO·3Al2O3 phase is an unstable/intermediate phase in the the CaO-Al2O3 system, which is decomposed to 12CaO·7Al2O3 above 1100 °C upon heating in oxidizing conditions. It was found that 5CaO·3Al2O3 is present at room temperature when the 12CaO·7Al2O3 dissociates to a mixture of 5CaO·3Al2O3, 3CaO·Al2O3, and CaO·Al2O3 phases during the cooling of the slag at 1180 °C ± 20 °C in reducing atmosphere. It is proposed that low concentrations of Si stabilize 12CaO·7Al2O3 (mayenite), in which Si is a solid solution in its lattice, which is named Si-mayenite. Regarding the calculated CaO-Al2O3-SiO2 diagram in this study, this phase may contain a maximum of 4.7 wt pct SiO2, which depends on the total SiO2 in the system and the Ca/Al ratio.

Fixation of borate-anions in lamellar AFm-phases in the system CaO–Al2O3–B2O3–H2O at 20 ​°C, 40 ​°C and 50 ​°C

Synthesis of boron containing lamellar calcium aluminum hydroxy salts were performed at different relative humidities and the behaviour and properties of the lamellar phases were investigated. The following AFm compound could be synthesized in pure form and their properties were determined:AFm-phase: 3CaOAl2O3CaHBO3.nH2O.With excess boron an Ettringite-compound AFt with the following composition is coexisting in the investigated system:Ettringite-phase: 3CaOAl2O32Ca[B(OH)4]2Ca(OH)2·30H2O.The incorporation of boron in the interlayer of LDH-phases under the influence of OH-ions at different temperatures were investigated. Carbonate was excluded by using a glove box to get definite lattice parameters. Different boron containing AFm-phases, using pure phase compositions in the system OH-lamellar AFm phase and the boron containing AFm phase, were synthesized. The solid solution between the so called compounds Monoborate and Monohydroxide at different temperatures and different relative humidities, with careful control of the different water contents of the lamellar phases, was investigated. Boron contents in solution and the uptake in the interlayer of the AFm-phases was studied [13]. The uptake of boron from pure solutions and the newly formed relevant phases with AFm structure and at higher boron contents of the ettringite structure were determined.

Analysis of the influence of the solid and liquid phases on steel desulfurization with slags from the CaO–Al2O3 systems using computational thermodynamics

Normally, CaO is used in steel desulfurization in slags from the systems: CaO–CaF2, CaO–Al2O3, and CaO–SiO2. Among these, the CaO–CaF2 slags are more efficient. This paper uses computational thermodynamics to study the influence of solid and liquid phases in the efficiency of desulfurizing slags in the CaO–Al2O3 system. Desulfurizing slags were prepared to produce steels with sulfur levels below 20ppm. Simulations using computational thermodynamics were made to determine the properties of the slags at 1600°C. The experimental tests were carried out in an induction furnace at 1600°C. The experimental results showed that slags from CaO–Al2O3 system can be as efficient as slags from CaO–CaF2 system, reaching final sulfur contents lower than 10ppm. The conclusion was that slags containing higher quantities of liquid phase are more efficient. However, the presence of some amount of solid CaO is necessary to keep the liquid phase of slag always saturated in CaO and its activity equal to unity. A Steel Desulfurization Factor was proposed as an easy way to estimate the desulfurization efficiency.

Formation and Evolution of Non-metallic Inclusions in Ti-Bearing Al-Killed Steel During Secondary Refining Process

Industrial and laboratory experiments were carried out to investigate the formation and evolution of the inclusions in Ti-bearing Al-killed steel, and thermodynamic calculations were also conducted to check the stability of inclusions in steel. It is found that the addition of Ti influences the inclusions evidently. Before Ti addition, the inclusions transform along the route of “Al2O3 → MgO·Al2O3 spinel → CaO-Al2O3(-MgO) system” with the generation of dissolved Mg and Ca in liquid steel. When there is no sufficient Ca in liquid steel, the addition of Ti during refining process would lead to the transformation of MgO·Al2O3 spinel inclusions into MgO-Al2O3-TiOx inclusions. When a trace of dissolved Ca is formed, CaO-Al2O3-TiOx system inclusions can be generated from MgO-Al2O3-TiOx inclusions or CaO-Al2O3 inclusions. Higher calcium content originated from calcium treatment would result in the formation of CaO·TiO2 inclusions, and then weaken the castability of Ti-bearing Al-killed steel grades. Calcium treatment could also increase the frequency of macro-inclusions in steel, and evidently decline the quality of the steel. Cancellation of calcium treatment is suggested to improve the cleanliness of Ti-bearing Al-killed steel grades.

Analysis of the influence of adding CaF2 and Na2O to steel desulfurizing mixtures using computational thermodynamics

Steel desulfurization mixtures of the CaO-Al2O3 system are the most industrially used. These mixtures require an amount of solid CaO to keep them always saturated in CaO. The aim of this paper is to study the influence of adding CaF2 and Na2O on the desulfurization efficiency of CaO-Al2O3 system mixtures. Desulfurizing steel mixtures capable of producing steels with sulfur content of less than 0.0020% were formulated. Desulfurizing mixtures based on CaO-Al2O3-flux were prepared with fluxes CaF2 and Na2O. All mixtures presented 95% of liquid phase and 5% of solid CaO, which, according to the literature, are the most efficient slags of this system. The equilibrium conditions were simulated using the composition of the mixtures and the steel by means of computational thermodynamic software. In these simulations, the activities of the slag components, the percentage of solids and liquids, the slag viscosity, and the equilibrium sulfur content were determined. The experimental tests were carried out in an induction furnace at 1600 °C. In addition, data on sulfide capacity (Cs), sulfur partition (Ls) and optical basicity (ʌ) of the initial mixtures, were calculated. The results of the simulations show that all mixtures have thermodynamic potential to reach the target of sulfur content set for this work. The mixtures with CaF2 proved to be the most efficient. However, it is possible to obtain sulfur contents of 0.0020% using mixtures without CaF2.

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

AbstractPolycrystalline material of a novel phase in the system CaO–Al2O3–Cr2O3 has been obtained by solid‐state reactions. Chemical analysis indicated the composition CaAl2Cr2O7. Single‐crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl2.13Cr1.87O7 yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) Å3, Z = 3. Structure determination and subsequent least‐squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl2.13Cr1.87O7 or CaAl2Cr2O7 represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high‐temperature single‐crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.