Synthesis, formation mechanism and hydraulic activity of novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂

Abstract

This paper describes a new approach for synthesizing novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂ via the high-temperature reactions within the Ca₇ZrAl₆O₁₈–Al₂O₃ system and characterizes products using X-ray diffraction (XRD), Rietveld XRD quantification, infrared spectroscopy, scanning electron microscopy combined with energy-dispersive spectrometry and heat flow calorimetry techniques. Quantitative comparison of the phase transformation in the samples 25 mass% Al₂O₃–75 mass% Ca₇ZrAl₆O₁₈, 50 mass% Al₂O₃–50 mass% Ca₇ZrAl₆O₁₈ and 75 mass% Al₂O₃–25 mass% Ca₇ZrAl₆O₁₈ sintered in the temperature range 1300–1500 °C was presented. Detailed investigations of the reaction mechanism in the synthesis process in these samples show that calcium zirconium aluminate is unstable when it is heated with alumina, and it undergoes chemical reactions forming calcium aluminates with different CaO/Al₂O₃ molar ratios and Zr-bearing compounds (CaZrO₃ or ZrO₂). Probability of liquid-phase sintering of ceramics prepared from alumina and Ca₇ZrAl₆O₁₈ that provides particle rearrangement increases with increasing the CaO/Al₂O₃ mass ratio of composites. Specifically, this paper discusses composition design and microstructural characterization for achieving modern calcia–alumina–zriconia composite cements. A model that explains the chemical and microstructural changes within the Ca₇ZrAl₆O₁₈–Al₂O₃ system due to high temperature was proposed. This study extensively investigates the effects of the cement phase composition on the hydraulic activity in the Ca-aluminates and Zr-bearing compounds containing blended binder pastes.

Graphical Abstract

Introduction

Calcium monoaluminate (CaAl₂O₄, CA), calcium dialuminate (CaAl₄O₇, CA₂) and calcium hexa-aluminate (CaAl₁₂O₁₉, CA₆) are lime-alumina compounds involved in up-to-date ceramic technology and have attracted considerable attention due to their unique properties. CA (1606 °C) [1, 2] is the main chemically active hydraulic constituent of calcium aluminate cements (CACs) [3]. CA₂ is a compound which can be broadly used in modern refractories with a high resistance to thermal shock, especially due to its a high melting temperature (1759 °C) [1, 2] and very low or even negative coefficients of thermal expansion [4,5,6]. CA₆ is a non-hydraulic compound and more refractory than the other phases, having a melting point of 1850 °C [1, 2]. Calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) having incongruent melting point at 1550 °C [7] has recently been analyzed in terms of its hydraulic properties [8,9,10,11]. Considering the above mentioned, the CaO–Al₂O₃–ZrO₂ multiphase composite cements can be recognized as very promising candidates for high-temperature applications.

There are several methods used for production of calcia–alumina–zirconia ceramics. Lee et al. [12] synthesized Al₂O₃-modified calcia-stabilized zirconia materials from fine powders by sintering at 1450–1600 °C in air atmosphere of green pellets shaped via uniaxial pressing and then isostatically pressed at 200 MPa. Liu et al. [13] prepared porous alumina–zirconia ceramics by infiltrating porous alumina ceramics previously prepared with ultra-fine alumina powder via tert-butyl alcohol (TBA)-based gel-casting method. Ai et al. [14] fabricated zirconia matrix ceramics doped with 7.5 vol% nano-Al₂O₃ by two-step microwave sintering, two-step conventional sintering and one-step microwave sintering. According to a research paper discussing the formation mechanism of calcium aluminate compounds (Ca₃Al₂O₆, Ca₅Al₆O₁₄, Ca₁₂Al₁₄O₃₃, CaAl₂O₄, CaAl₄O₇ and CaAl₁₂O₁₉) based on high-temperature solid-state reaction within the temperature ranges 1100–1400 °C, CaO gradually diffuses into the Al₂O₃ core and then a layered distribution of calcium aluminates occurs [15]. Composites of ZrO₂–CaAl₄O₇ (CA₂) were synthesized by Bruni et al. [16] using the conventional sintering technique at 1400 °C from the mixture of monoclinic polymorph of zirconia with different amount of calcium aluminate cement (CAC). Calcium hexaluminate (CaAl₁₂O₁₉, CA₆, hibonite) in an elongated grain morphology was formed in situ during sintering of zirconia-toughened alumina ceramics with CaO at 1600 °C for 4 h [17]. The CA₆–CA₂–Al₂O₃ three-phase composite was also recently synthesized by the solid-state reaction process at an elevated temperature using the previously synthesized CA₂ compound and commercial corundum powder as starting materials [18]. They are also known the low thermal expansion ceramics based on calcium dialuminate (CaAl₄O₇, CA₂), calcium dialuminate-calcium zirconate (CaZrO₃, CZ) and calcium dialuminate–magnesium aluminate (MgAl₂O₄, MA) binary phases materials from the literature [4,5,6].

Thus, the present work was aimed at studying of the high-temperature reactions between Ca₇ZrAl₆O₁₈ and Al₂O₃ for obtaining of novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂ with favorable hydraulic properties by in situ synthesis method. This approach to the composition and microstructural designs strategies is also a somewhat novel concept for the materials scientist.

Experimental

Materials and cements preparation procedure

The materials used in this study were a commercially available α-Al₂O₃ (PFR 20 reactive alumina, supplied by Alteo) with the chemical and physical characteristic given in Table 1 and calcium zirconium aluminate, Ca₇ZrAl₆O₁₈ (C₇A₃Z; C≡CaO, A≡Al₂O₃, Z≡ZrO₂). High-purity commercial mono-modal reactive alumina powder was used as a raw material for the preparation of binary mixtures of Al₂O₃ and Ca₇ZrAl₆O₁₈. For this work, Ca₇ZrAl₆O₁₈ was obtained by a two-step synthesis procedure. In a first step, the calcium carbonate (99.81% CaCO₃, Chempur), α-Al₂O₃ (99.8% Al₂O₃, Across Organics) and zirconia (98.08% ZrO₂, Merck) powders were mixed with the 7:3:1 molar ratio of CaO, Al₂O₃ and ZrO₂ oxides, and then, the mixture was homogenized for 2 h in a ball mil and pressed into cylinders having a diameter of 2 cm. All green pellets were calcined at 1200 °C for 10 h. In a second step, solid-state sintering of the pellets made from the calcined powder at 1500 °C for 30 h resulted in a single-phase orthorhombic Ca₇ZrAl₆O₁₈ structure. An intermediate grinding/mixing stage in order to improve homogeneity was necessary. Nevertheless, some trace impurities, in the form of CaZrO₃ and Ca₁₂Al₁₄O₃₃ were detected.

Table 1 Chemical and physical characteristics of reactive alumina micropowder [19]

As a next step, novel calcia–zirconia–alumina pellets were prepared by a high-temperature reactive sintering of before synthesized Ca₇ZrAl₆O₁₈ and reactive Al₂O₃ micropowder. The initial compositions of the Ca₇ZrAl₆O₁₈–Al₂O₃ mixtures were located within the Ca₇ZrAl₆O₁₈–CaAl₂O₄–CaZrO₃, CaAl₂O₄–CaAl₄O₇–CaZrO₃ and CaAl₄O₇–CaAl₁₂O₁₉–ZrO₂ composition triangles of a ternary system CaO–Al₂O₃–ZrO₂. They contained 25, 50 or 75% by mass of Al₂O₃ and 75, 50 or 25% by mass of Ca₇ZrAl₆O₁₈, respectively. Homogenization of the mixtures proceeded for 2 h. Densification of the mixed powders was conducted by pressureless sintering of pellet samples at temperatures of 1300–1500 °C under air atmosphere (Table 2). The aluminates CaAl₂O₄ (CA), CaAl₄O₇ (CA₂) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇) were also synthesized by two-cycle heating and grinding stoichiometric mixtures of CaCO₃ and Al₂O₃ for use as reference material for calorimetric studies.

Table 2 Percentage composition (% by mass) of ceramic pellet samples and sintering condition

Methods

X-ray diffraction analysis (XRD) was carried out using the X’Pert Pro PANalytical X-ray diffractometer. The XRD patterns of previously synthesized calcium zirconium aluminate phase, reactive alumina, calcium aluminates and calcia–zirconia–alumina composite cements were collected by step scanning with step of 0.02° over the range 2Θ 5°–90° at room temperature.

The phases developed during sintering were compared and confirmed using search–match reference ICDD database. Multiphase Rietveld phase quantification was also done using standard ICSD files supported by the PANalytical HighScore Plus software [20]. Goodness of fit (GOF) values lower than 2 proved a good quality Rietveld refinement for all samples. Various functional groups present in the prepared composite cements were identified by FT-IR. A Bruker Vertex 70v spectrometer was employed for this purpose. The FT-IR spectrometer was operated in the mid-IR region (4000–400 cm⁻¹) with a resolution of 4 cm⁻¹. Each spectrum represents the average of 128 scans. Samples were prepared by the standard KBr pellet method. Moreover, scanning electron microscopy (SEM, Nova NanoSEM 200 FEI) combined with energy-dispersive X-ray spectroscopy (EDS) was used to examine the microstructure development. For this purpose, resin-embedded samples were polished, coated with carbon to be conductive and then examined by SEM.

For microcalorimetric measurements, the pellets were ground and milled to obtain fine powder. Specific surface and grain size distribution of samples were measured by a laser diffraction analyzer (the Master Sizer 2000 Ver. 5.60 apparatus of Malvern (UK)). Heat of hydration of the novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂ (Tables in Fig. 1a–c) and the reference aluminate phases was determined with a TAM Air microcalorimeter (TA Instruments) by integrating the continuous heat flow signal during the 20-h hydration process. Calorimetric measurements of synthetic cements were performed using the in situ mixing treatments of the binder pastes under the following conditions: temperature of 25 °C and a water/solid mass ratio w/s = 1.0. Tests were performed by mixing the pastes inside the calorimeter to allow the study of cement early hydration processes. For this purpose, two grams of dry powder was poured into a glass vial, and water weighed into mounted syringes. Admix ampoule was introduced into the calorimeter prior mixing, thus equilibrating to isothermal environment. Mixing was done after equilibration inside the calorimeter to enable the quantitative access to the heat of the early hydration process. A second glass vial containing sand (providing the same heat capacity that the mass of binder paste) was used as an inert reference in the twin channel of calorimeter to ensure the best stability of the baseline. Heat flow curves were normalized to the mass of total dry binder and reported in terms of power (mW g⁻¹ dry binder) versus time (hours).

Fig. 1

The results of quantitative Rietveld analyses made with HighScore Plus (PANalytical) program for the samples 25A–75C₇A₃Z (A), 50A–50C₇A₃Z (B) and 75A–25C₇A₃Z (C) sintered in the temperature range 1300–1500 °C; C≡CaO, A≡Al₂O₃ and Z≡ZrO₂

Results and discussion

Phase transformation

Phase transformation of samples 25A–75C₇A₃Z, 50A–50C₇A₃Z and 75A–25C₇A₃Z sintered in the temperature range 1300–1500 °C was examined by XRD analysis, and the crystalline phase content in the samples as determined by Rietveld refinements of the powder patterns is summarized in the tables presented in Fig. 1a–c.

Analysis of the quantitative phase changes based on the Rietveld XRD quantification shown in Fig. 1a–c is suitable to draw some reliable conclusions related to the mechanism of the reaction within the Ca₇ZrAl₆O₁₈–Al₂O₃ system in question. Now let us consider 25A–75C₇A₃Z sample (Fig. 1a). Calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) reacted with Al₂O₃ to form calcium monoaluminate (CaAl₂O₄) and calcium zirconate (CaZrO₃) following sintering at 1300 °C for 1 h. The same applies for the samples 50A–50C₇A₃Z and 75A–25C₇A₃Z where the content of CaAl₂O₄ also increases and the contents Ca₇ZrAl₆O₁₈ and Al₂O₃ accordingly decrease (Fig. 1b–c). The reaction proceeds according to Eq. 1.

$${\text{Ca}}_{7} {\text{ZrAl}}_{6} {\text{O}}_{18} + 3{\text{Al}}_{2} {\text{O}}_{3} \to 6{\text{CaAl}}_{2} {\text{O}}_{4} + {\text{CaZrO}}_{3}$$

(1)

On the other hand, in the sample 25A–75C₇A₃Z calcium monoaluminate (CaAl₂O₄) reacted simultaneously with Al₂O₃ to form unstable calcium dialuminate (CaAl₄O₇) under the same sintering condition. After sintering at 1400 °C, CaAl₄O₇ disappeared, the amount Ca₇ZrAl₆O₁₈ decreased and the amount of CaAl₂O₄ and CaZrO₃ increased. A quantitative approach based on XRD results in proposed mechanism of the chemical reaction 2:

$${\text{Ca}}_{7} {\text{ZrAl}}_{6} {\text{O}}_{18} + {\text{CaAl}}_{4} {\text{O}}_{7} + 2{\text{Al}}_{2} {\text{O}}_{3} \to 7{\text{CaAl}}_{2} {\text{O}}_{4} + {\text{CaZrO}}_{3}$$

(2)

Following sintering at 1500 °C for 1 and 3 h, it was found that Ca₇ZrAl₆O₁₈ completely disappeared due to high-temperature eutectic reactions and formation of Ca₁₂Al₁₄O₃₃. The phase formation at each stage of sintering arising when several different reactions occur at different sintering condition will be discussed in detail later when discussing the microstructure evolution.

In 50A–50C₇A₃Z composite, a larger content of Al₂O₃ was enough to remain CaAl₄O₇ as a stable phase. At the temperature range between 1300 and 1500 °C the increment in CaAl₄O₇ content and the decrease in amount of both Al₂O₃ and initially formed CaAl₂O₄ phase were observed. Thus, the following reaction is proposed:

$${\text{CaAl}}_{2} {\text{O}}_{4} + {\text{Al}}_{2} {\text{O}}_{3} \to {\text{CaAl}}_{4} {\text{O}}_{7}$$

(3)

It was also found that evolution with temperature of the amount of crystalline phases present in the 75A–25C7A₃Z material occurred. So, Fig. 1c shows a remarkably strong increase with CaAl₁₂O₁₉ content and the accompanying decrease in content of both CaAl₄O₇ and Al₂O₃. The results of quantitative analysis of diffraction data support the proposed mechanism of reaction 4. In the alumina-rich part of the phase diagram calcium zirconate (CaZrO₃) not exists in equilibrium and then it must be converted in zirconia according to the chemical reaction 5.

$${\text{CaAl}}_{4} {\text{O}}_{7} + 4{\text{Al}}_{2} {\text{O}}_{3} \to {\text{CaAl}}_{12} {\text{O}}_{19}$$

(4)

$${\text{CaZrO}}_{3} + {\text{Al}}_{2} {\text{O}}_{3} \to {\text{CaAl}}_{2} {\text{O}}_{4} + {\text{ZrO}}_{2}$$

(5)

The final composition of investigated samples is presented as X-ray diffractograms in Figs. 2–4 with JCPDS standard cards used for phase identification.

Fig. 2

X-ray diffraction pattern of the 25Al₂O₃–75C₇A₃Z composite sintered at 1500 °C for 3 h

Fig. 3

X-ray diffraction pattern of the 50Al₂O₃–50C₇A₃Z composite sintered at 1500 °C for 3 h

Fig. 4

X-ray diffraction pattern of the 75Al₂O₃–25C₇A₃Z composite sintered at 1500 °C for 3 h

Spectroscopic results

Figure 5 shows the IR spectrum of sintering products of Al₂O₃ and Ca₇ZrAl₆O₁₈ powders. FT-IR is suitable for analyzing the calcium aluminate phases and final characterization of CaO–Al₂O₃–ZrO₂ materials. According to paper presented by Tarte [21] concerning the infrared spectra of inorganic aluminates and characteristic vibrational frequencies of AlO₄ tetrahedra and AlO₆ octahedra, two different regions can be identified within the infrared spectra of calcium aluminates: the first one in the region of 700–900 cm⁻¹ that corresponds to the stretching vibrations of a lattice of interlinked AlO₄ tetrahedra; and the second one appears in the region of 400–500 cm⁻¹, which can be particularly useful for determining the presence of bending mode of the AlO₄ lattice [22, 23]. In the spectrum of the main phase present in considered ceramic composite, CaAl₂O₄, the relevant signals are centered at 417, 447, 542, 573, 638, 687, 721, 764, 779, 790, 804, 820, 841 and 868 cm⁻¹. The structural arrangement considered as the mentioned groups was also found in a minor phase of the 25Al₂O₃–75C₇A₃Z sample, i.e., Ca₁₂Al₁₄O₃₃. Despite the fact that dodecacalcium hepta-aluminate gives the spectrum with a very strong and broad absorption band with a maximum at 835–850 cm⁻¹ and the two strong bands at 573–580 cm⁻¹ and at 457–465 cm⁻¹ [23, 24], the coincidence of positions of some bands in the IR spectra of the two phases occurred. The 50A–50C₇A₃Z sample gives a spectrum dominated by the absorption bands of calcium dialuminate CaAl₄O₇ positioned at about 420, 442, 538, 573, 810, 834, 864, 918 and 943 cm⁻¹. The results are in very good agreement with the data presented in Ref. [22, 23]. These bands appear to coincide with some bands seen in the infrared spectrum of CaAl₂O₄. Similarly, IR-active fundamental band of calcium zirconate [25] is not clearly seen in this spectrum only showing up as a inflection point at 523 cm⁻¹.

Fig. 5

FT-IR spectra of the 25Al₂O₃–75C₇A₃Z, 50Al₂O₃–50C₇A₃Z and 75Al₂O₃–25C₇A₃Z composites obtained via sintering at 1500 °C for 3 h

The 75A–25C₇A₃Z sample gives an extremely complicated spectrum (Fig. 5) due to the coincidence of positions of some bands in the IR spectra of CaAl₁₂O₁₉, α-Al₂O₃ and CaAl₄O₇. Thus, the appearance of intense absorption bands at 606 and 638 cm⁻¹ may serve to identify the corundum being the representative compound, since its structure is built up by AlO₆ octahedra only [21]. In contrast, structure of calcium hexa-aluminate is based upon the structural arrangements where Al is distributed over three symmetrically independent octahedral sites, one tetrahedral site, and one trigonal bipyramidal site [26]. Thus, by following the approach given in the paper [21] that the characteristic frequency ranges are as follow: “condensed” AlO₄ tetrahedra 700–900 cm⁻¹, “isolated” AlO₄ tetrahedra 650–800 cm⁻¹, “condensed” AlO₆ octahedra, 500–680 cm⁻¹ and “isolated” octahedra AlO₆ 400–530 cm⁻¹, the specific feature of the IR spectrum of CaAl₁₂O₁₉ is the presence of IR bands at about 400, 467, 530, 553, 619, 656, 702, 741, 788 and 945 cm⁻¹ marked in the 75A–25C spectrum (Fig. 5). The peak positions of these bands are in very good agreement with those presented in Ref. [26]. However, it is important that some peak positions of CaAl₁₂O₁₉ coincide with the peak positions observed for the CaAl₄O₇. Characteristic IR absorption frequencies of CaAl₄O₇ were found within a very broad absorption peak located in the region between about 800 and 900 cm⁻¹. IR spectrum recorded for the prepared 75A–25C₇A₃Z sample is dominated by characteristic infrared absorption bands of functional groups belonging to some calcium aluminates, and it shows no significant band distinctive of cubic, tetragonal and monoclinic polymorphs of zirconia having their strong maxima at 480, 510 and 515 cm⁻¹, respectively [27].

Microstructural evolution

The mechanism of the high-temperature reactions between Ca₇ZrAl₆O₁₈ and Al₂O₃ was investigated based on the microstructure observations and chemical analysis with SEM–EDS. The BSE images of the polished cross sections of 25A–75C₇A₃Z and 75A–25C₇A₃Z composites sintered at 1300, 1400 and 1500 °C are shown in Figs. 6, 7. From these figures, it can be seen that during the first reaction stage at 1300 °C an attack of the alumina ions along the grain boundaries of calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) occurred and calcium monoaluminate (CaAl₂O₄) and calcium zirconate (CaZrO₃) (Figs. 6 and 7, point 2) were then formed. This process proceeds according to the general balanced chemical reaction 1. Generally, high-temperature calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) decomposition in the presence of Al₂O₃ and the further phase transformations can be explained by the phase diagram of the ternary system CaO–Al₂O₃–ZrO₂ [7, 9]. Let us consider the line joining the points represented by the composition of Ca₇ZrAl₆O₁₈ (CaO 47.8 mass%, Al₂O₃ 37.2 mass%, ZrO₂ 15.0 mass%) and Al₂O₃ 100 mass%. Thus, when coarse-grained Ca₇ZrAl₆O₁₈ is combined and sintered with Al₂O₃ micropowder, the original Ca₇ZrAl₆O₁₈ grains will be rimmed by a reaction zone. The composition of this reaction zone could be expected by any point of the line Ca₇ZrAl₆O₁₈–Al₂O₃ within the ternary system CaO–Al₂O₃–ZrO₂. This means that the starting powders will go through a series of chemical reactions before the final stable compounds are formed, with the intermediate compounds being non-equilibrium liquid or solid transient phases. This process would start with reaction 1 and proceed according to the general balanced chemical reactions 3–5.

Fig. 6

SEM micrographs of the studied 25A–75C₇A₃Z composite sintered at 1300 °C (1 h), 1400 °C (1 h) and 1500 °C for 3 h (polished cross sections and fractured cross section). EDS analysis in spots: 1—Ca₇ZrAl₆O₁₈, 2—CaZrO₃ embedded in CaAl₂O₄ matrix, 3—Al-rich CaAl₂O₄, 10—Ca₁₂Al₁₄O₃₃, 11—CaAl₂O₄ and 12—coarse-grained CaZrO₃

Fig. 7

SEM micrographs of the studied 75A–25C₇A₃Z composite sintered at 1300 °C (1 h), 1400 °C (1 h) and 1500 °C (1 and 3 h). EDS analysis in spots: 1—Ca₇ZrAl₆O₁₈, 2—CaZrO₃ embedded in CaAl₂O₄ matrix, 3—Al-rich CaAl₂O₄, 4—CaAl₄O₇, 5—Al₂O₃, 6—CaZrO₃ (coarse-grained) or ZrO₂ (fine-grained) embedded in CaAl₄O₇, 7—ZrO₂ embedded in CaAl₄O₇ matrix, 8—CaAl₄O₇ and 9—CaAl₁₂O₁₉

Due to the diffusion phenomena, the calcium zirconium aluminate grain was altered, having become mainly enriched in zircon and reduced in calcium. At the same time, Zr⁴⁺ ions may be considered rather as immobile during this process. Calcium zirconate, CaZrO₃, formed during reaction 1 tends to agglomerate in large oval grains embedded in calcium aluminate matrix (xCaO·yAl₂O₃). Let us consider 25 mass% Al₂O₃–75 mass% Ca₇ZrAl₆O₁₈ sample where zirconium migration comes rather from the presence of liquid transient phase that produced the fully reacted ceramic composite. Another is to use starting powders wherein the proportion of ingredients in the starting dry mixture is as follows: 75 mass% Al₂O₃ and 25 mass% Ca₇ZrAl₆O₁₈ that will pass through a solid stage, rather than liquid, before the final microstructure is reached (Fig. 7, 1500 °C 3 h). Simultaneously, aluminum ions Al³⁺ move against a concentration gradient to increase their concentration inside Ca₇ZrAl₆O₁₈ grain core. As a result of an intense counter-diffusion of calcium ions and aluminum ions through reaction zone, the chemical equilibrium could be reached. Nevertheless, the final composition of the three phases in equilibrium, i.e., Ca₇ZrAl₆O₁₈–CaZrO₃–CaAl₂O₄ and CaAl₄O₇–ZrO₂–CaAl₁₂O₁₉ was not received as it was expected for both 25A–75C₇A₃Z and 75A–25C₇A₃Z, respectively. The investigation of the microstructural evolution within the 75A–25C₇A₃Z sample confirmed by XRD and FT-IR analysis has shown that the thermodynamic equilibrium was not reached due to the presence of both Al₂O₃ and CaAl₄O₇ non-equilibrium phases that could undergo of transition according to reaction 4. Hence, final microstructure of the 75A–25C₇A₃Z ceramics achieved at 1500 °C and 3 h sintering consist of ZrO₂ particles embedded in the CaAl₄O₇ phase (Fig. 7, point 7) occurring in the initial region of Ca₇ZrAl₆O₁₈ grain and CaAl₁₂O₁₉ (Fig. 7, point 9). Additionally, the 75A–25C₇A₃Z composite cement consists of alumina as checked by XRD and FT-IR.

Calorimetric studies

The heat of hydration was measured for the grounded cementitious materials (reference compounds and composite cements) whose particle size distribution known as the median diameter or the medium value of the particle size distribution was about 30 μm. A reference measurement of hydration heat of single component, i.e., CaAl₂O₄ (CA), CaAl₄O₇ (CA₂), Ca₁₂Al₁₄O₃₃ (C₁₂A₇) and Ca₇ZrAl₆O₁₈ (C₇A₃Z) (Fig. 8) using isothermal heat flow calorimeter at 25 °C was a useful tool for both studying and assessing the hydration behavior of these cementitious compounds.

Fig. 8

Heat flow curves of reference materials, i.e., CaAl₂O₄-, CaAl₄O₇-, Ca₇ZrAl₆O₁₈-, Ca₁₂Al₁₄O₃₃-based cementitious pastes prepared with w/s = 1.0 hydrated at 25 °C. Heat flow curves normalized to mass of binder paste (with in situ mixing)

The heat flow curves for the 25A_75C₇A₃Z, 50A_50C₇A₃Z and 75A_25C₇A₃Z hydrating multiphase cement pastes prepared with a water/solid ratio of 1.0 at 25 °C are shown in Fig. 9a–c, respectively. Looking at the overall results, some relationships have been noticed in observations. Firstly, the heat flow peaks of all composite cements are strongly enhanced by the presence of Ca-containing phases (Fig. 9a–c). It means an increase in the peak intensity of heat flow of hydration with decreasing the Al₂O₃/Ca₇ZrAl₆O₁₈ mass ratio of the starting mixtures, as shown in Fig. 9a–c. Interestingly, intensity of the heat flow peak of composite cements belonging to the system CaO–Al₂O₃–ZrO₂ hydration is also strongly reduced with the degree of conversion of Ca₇ZrAl₆O₁₈ within the temperature range from 1300 to 1500 °C. As may be also seen, the hydration of all multiphase composite cements is accelerated with increasing the Al₂O₃/Ca₇ZrAl₆O₁₈ mass ratio of the starting mixtures. The induction period was shortened, and the second exothermic heat effect was accelerated considerably as the Al₂O₃ amount increased. The influence of alumina on the hydration of Ca-aluminates is a phenomenon well known [28].

Fig. 9

Heat flow curves of 25A_75C₇A₃Z (a), 50A_50C₇A₃Z (b) and 75A_25C₇A₃Z (c) pastes hydrated at 25 °C for w/s = 1.0, normalized to mass of binder paste (with in situ mixing). Cements obtained at 1300, 1400 and 1500 °C

Going into details, presented figures (Fig. 9a–c) illustrate the hydraulic activity of each cements, especially, the hydration kinetics of individual constituent phases and the mutual interaction of these constituents. Test results indicated that the phase composition of particular cements expressed in mass percent (Fig. 1a–c) of the possible constituent hydraulic phases, i.e., CaAl₂O₄ (CA), CaAl₄O₇ (CA₂), Ca₇ZrAl₆O₁₈ (C₇A₃Z) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇) and/or Al₂O₃ (A) strongly affects the microcalorimetric curve obtained in the isothermal conditions. Thus, these curves reflect the rate of heat evolution vs. time for each considered composite cements from the system CaO–Al₂O₃–ZrO₂. It is well known that some stages of reaction can be identified on the calorimetric curve, namely: the pre-induction (the heat of wetting effect), induction (II), hydration rate increase (III), hydration rate decrease (IV) and low hydration rate (V), respectively. Thus, calcium monoaluminate hydration (Fig. 8) can be considered as good model of cement reaction with water. It is clearly visible that the shape of the heat flow curves of hydrating cement pastes is characteristic and the observed differences relate mainly to the stages generically, but each one can have different duration depending on the cement mineral composition. The all calorimetric curves shown in Fig. 9a–c exhibit one major feature, an intense first heat peak which occurs immediately after injection and mixing the water with the dry cement powders. This quick release of heat is associated with the wetting and initial dissolution of the cement particles within a few minutes after adding water. Besides, the rate evolution during hydration of the samples heat treated at 1300 °C (samples 75A_25C₇A₃Z, 50A_50C₇A₃Z, 25A_75C₇A₃Z) and 1400 °C (sample 25A_75C₇A₃Z) for this peak is strongly enhanced by the presence of the highly reactive calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) phase (Figs. 1a–c). The highly reactive nature of Ca₇ZrAl₆O₁₈ has been extensively studied in recent years [9, 11].

Microcalorimetric curves of the 25A_75C₇A₃Z samples (Fig. 9a-a–c) are strongly dominated by the intense exothermic heat associated with CA (CaAl₂O₄) hydration. It is shown that C₇A₃Z (1300 °C_25A_75C₇A₃Z, 1400 °C_25A_75C₇A₃Z) exhibits a strong accelerating effect, more effective than C₁₂A₇ (1500 °C_25A_75C₇A₃Z), on the hydration of calcium monoaluminate (CA).

The hydration of 50A_50C₇A₃Z composite cements is characterized by a heat flow peak with dissimilar acceleration rate and intensity which follows the induction period. The hydration of 1300 °C_50A_50C₇A₃Z cement is characterized by a double heat flow peak with different rate and intensity. Thus, the different behaviors, as regards hydration kinetics, of CA and CA₂ are indicated. The hydration reactions of calcium monoaluminate (CA) precede the reaction of CA₂ with water which, on the other hand, has the ability to very slow and lengthy reaction. The length of the induction period of 50A_50C₇A₃Z samples hydration has been found to be related to both CA and CA₂ contents and increased with increasing CA₂ content within the temperature range from 1300 to 1500 °C. Therefore, it is appropriate to say that CA phase acts as an accelerating agent for the hydration of CA₂ phase.

Microcalorimetric curve of the 1500 °C_75A_25C₇A₃Z sample (Fig. 9c-c.) is strongly dominated by the first stage, i.e., the initial wetting of the cement particles and by the second not intense exothermic heat (a weak diffuse exothermic effect) associated with CA₂ (CaAl₄O₇) hydration. Analyzing the graphs of the composite cements heat treated for 1 h at 1300 and 1400 °C, I observe the well-known accelerating effect of other aluminate compound on the hydration of CA₂. Increase in the reactivity of CA₂ was accomplished by the presence of reactive phases combinations like CA/C₇A₃Z (Fig. 9c-a) and CA (Fig. 9c-b). Interestingly, the hydraulic behavior of both CA₂ and CA is strongly enhanced by the presence of C₇A₃Z.

Discussion

In this paper, a new approach for synthesizing composite-designed CaO–Al₂O₃–ZrO₂ cements via the high-temperature reactions within the Ca₇ZrAl₆O₁₈–Al₂O₃ system in the temperature range between 1300 and 1500 °C was proposed. The reaction products formed in these samples were well understood by looking at compatibility triangles across the line joining Al₂O₃ and Ca₇ZrAl₆O₁₈ of the CaO–Al₂O₃–ZrO₂ phase diagram [7]. The configurations of three-phase regions in equilibrium are the following: I. Ca₇ZrAl₆O₁₈–CaAl₂O₄–CaZrO₃, II. CaAl₂O₄–CaZrO₃–CaAl₄O₇, III. CaAl₄O₇–CaZrO₃–ZrO₂, IV. CaAl₄O₇–ZrO₂–CaAl₁₂O₁₉ and V. CaAl₁₂O₁₉–ZrO₂–Al₂O₃. The phase transformation in the prepared samples 25 mass% Al₂O₃–75 mass% Ca₇ZrAl₆O₁₈, 50 mass% Al₂O₃–50 mass% Ca₇ZrAl₆O₁₈ and 75 mass% Al₂O₃–25 mass% Ca₇ZrAl₆O₁₈ having the initial phase composition in I, II and IV triangles, respectively, sintered in the temperature range 1300–1500 °C was investigated with X-ray diffraction (XRD), Rietveld XRD Quantification, infrared spectroscopy (IR) and scanning electron microscopy combined with energy-dispersive spectrometry (SEM/EDS) techniques. Final results from the various analytical techniques revealed that phase composition of the calcia–zirconia–alumina composite cements prepared via the high-temperature reactions between Ca₇ZrAl₆O₁₈ and Al₂O₃ differs from the strategically designed sample composition due to sintering at different experimental conditions. It was concluded that the 75 mass% Al₂O₃–25 mass% Ca₇ZrAl₆O₁₈ specimen was probably not fully equilibrated at 1500 °C due to the predominant diffusion path in solid-state reactions. The 25 mass% Al₂O₃–75 mass% Ca₇ZrAl₆O₁₈ and 50 mass% Al₂O₃–50 mass% Ca₇ZrAl₆O₁₈ specimens were fully equilibrated at temperatures of 1400 and 1500 °C, respectively.

Calcium zirconium aluminate (Ca₇ZrAl₆O₁₈) was unstable when it was heated with alumina, and it was subjected to chemical reactions with the formation of calcium aluminates increasingly richer in aluminum oxide (Ca₇ZrAl₆O₁₈ → CaAl₂O₄ → CaAl₄O₇ → CaAl₁₂O₁₉) and Zr-bearing compounds (CaZrO₃ or ZrO₂). Investigations by SEM showed that calcium zirconate tends to be uniformly distributed and as rounded oval grains within the calcium aluminate matrix with a high CaO/Al₂O₃ molar ratio corresponding to C₁₂A₇ (C≡CaO, A≡Al₂O₃) in the final microstructure. On the other hand, fine-grained zirconia were concentrated in microareas of the initial Ca₇ZrAl₆O₁₈ grains, and it was embedded in the Al-richer calcium aluminate matrix. Based on theoretical considerations on the reaction mechanism in the CaO–Al₂O₃–ZrO₂ system and the presented results it may be supposed that the reactions at the single-grain level would proceed from the stage 0 to III, which is graphically presented in Fig. 10.

Fig. 10

Schematic diagram of phase distribution sequence in the reaction zone of the binary mixture like Ca₇ZrAl₆O₁₈–Al₂O₃

The hydration process of composite cements belonging to the system CaO–Al₂O₃–ZrO₂ was characterized by the different intensity exothermic reactions. The shape of the heat flow curves resulted from calorimetric measurements reflects the cement hydration process. The proportion of the selected CaAl₂O₄, CaAl₄O₇, Ca₁₂Al₁₄O₃₃, Ca₇ZrAl₆O₁₈ or Al₂O₃ phases changes the shape of the heat flow curve, so the hydraulic reactivity of cement phases and their mutual influence on hydration behavior can be quantified.

Conclusions

Some general conclusions can be drawn based on the results of this work:

• 1. Reactive sintering process of Ca₇ZrAl₆O₁₈ and Al₂O₃ micropowders is an effective way for production of novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂.

• 2. The designed compositions of these hydraulic binders can be achieved with factors such as: ratio by mass of Ca₇ZrAl₆O₁₈ to Al₂O₃, time and sintering temperature.

• 3. The shrinking core model in which the particle size of Ca₇ZrAl₆O₁₈ is being consumed by reaction with alumina can be applied to visualization of reactions involving changes in microareas of unreacted core.

• 4. Phase composition affects the hydration kinetics of hydraulic binders determined using isothermal calorimetry.

• 5. Due to this unique combination of properties, novel composite cements belonging to the system CaO–Al₂O₃–ZrO₂ can be used in many types of unshaped refractory materials including castables.