Structure, microstructure and thermal stability characterizations of C₃AH₆ synthesized from different precursors through hydration

Abstract

In this paper, the influence of precursors (CA, C₇A₃Z, C₁₂A₇, C₆SrA₃Z, C₇A₂FZ and commercial calcium aluminate cement ‘Gorkal 70’) on the structure, microstructure and thermal stability characterizations of C₃AH₆ through hydration was investigated. The materials were characterized by X-ray diffraction, differential thermal analysis–thermogravimetric analysis–evolved gas analysis and scanning electron microscopy combined with energy-dispersive spectroscopy 24 h and 72 h after starting the hydration process. Results of investigation confirmed the influence of precursor on shape and grain size of C₃AH₆. The CaO/Al₂O₃ mass ratio of precursors before the hydration process affects the size of C₃AH₆ crystals: the higher the CaO/Al₂O₃ value, the larger the size of the crystals of C₃AH₆. Moreover, the presence of Sr and Fe affects the formation of stable C₃AH₆ crystals.

Introduction

The main phases in calcium aluminate cements are CA*, CA₂ and C₁₂A₇. The most predominant hydration products are CAH₁₀, C₂AH₈, C₃AH₆ and AH₃. However, the hydration of CAC strongly depends on the conditions of the process. At ordinary temperature, the only stable hydrates are C₃AH₆ and AH₃. At temperatures below 50 °C, the metastable phases are formed [1,2,3,4].

Some authors noted that CAH₁₀ and C₄AH_(11–19) are formed at low hydration temperatures (below 15 °C). In the temperature range of 15–27 °C, CAH₁₀ can coexist with C₂AH₈ and AH₃. When the hydration temperature is raised above 27 °C, C₂AH₈ and AH₃ dominate. As the temperature exceeds 50 °C, C₃AH₆ and AH₃ predominate. Moreover, it was found that elevated humidity is favorable to the formation of C₃AH₆ and Al(OH)₃ [2, 3, 5,6,7,8,9,10,11,12].

The conversion reactions are shown schematically below:

$$3{\text{CAH}}_{10} \to {\text{C}}_{3} {\text{AH}}_{6} + 2{\text{AH}}_{3} + 18{\text{H}}$$

(1)

$$3{\text{C}}_{2} {\text{AH}}_{8} \to 2{\text{C}}_{3} {\text{AH}}_{6} + {\text{AH}}_{3} + 9{\text{H}}$$

(2)

$${\text{C}}_{4} {\text{AH}}_{19} + {\text{C}}_{2} {\text{AH}}_{8} \to 2{\text{C}}_{3} {\text{AH}}_{6} + 15{\text{H}}$$

(3)

The formation of metastable hydrates in the form of hexagonal platelets is unfavorable because they have a tendency to transform into C₃AH₆-equiaxed crystals. During conversion, the lower density hydrates: CAH₁₀ (density: 1.74 g cm⁻³), C₄AH₁₉ (density: 1.81 g cm⁻³) and C₂AH₈ (density: 1.95 g cm⁻³) convert to denser C₃AH₆ (density: 2.52 g cm⁻³) [13, 14].

Some authors have found that it is possible to obtain C₃AH₆ with a specific structure. The cubic form was produced from C_1−xSr_xAl₂O₄ (the product of hydration was Sr-doped C₃AH₆), Ca₇ZrAl₆O₁₈ (formation of C₃AH₆ was strongly dependent on particle size and surface area), C₂AH₈ (in the presence of polycarboxylic acid type water-reducer), in the presence of two doped nano-additives Fe–TiO₂ and V–TiO₂, C₄AF, mixture of Portland cement and CAC, industrial CAC containing CA and CA₂, C₂AH₈ and CAH₁₀ with different amount of Al₂O₃. Fine nodules of C₃AH₆ were obtained from C₂AH₈; the granular-shaped crystals from C₂AH₈ (in the presence of alumina powder) and 10–100 nm layered nano-particles [4, 8, 15,16,17,18,19,20,21,22,23]. Moreover, some authors reported that the presence of gypsum, deflocculants on the basis of polycarbonate ethers and the carbonation process was a favorable factor to give C₃AH₆ [24,25,26,27,28].

The aim of this study was to gain C₃AH₆ from different precursors. CA, C₁₂A₇ and commercial calcium aluminate cement ‘Gorkal 70’ were widely used and selected as reference materials. Three phases containing Zr were tested: C₇A₃Z and C₇A₃Z were modified in two ways: C₆SrA₃Z (in which 1 mol of CaO was replaced by 1 mol of SrO) and C₇A₂FZ (in which 1 mol of Al₂O₃ was replaced by 1 mol of Fe₂O₃).

The precursors were selected in terms of potential property for cubic C₃AH₆ formation and to test the influence of foreign ions on the properties of C₇A₃Z. Moreover, the authors aimed to recognize the tendency of each precursor to form stable hydrates and explain the tendency of precursors to form C₃AH₆ with different grain sizes and shapes (qualitative and quantitative aspects).

Various analytical methods and scanning electron microscopy were used to investigate the progress of phase generation in six systems at 50 °C during hydration by stopping the chemical reaction after 24 and 72 h.

*Cement chemist’s abbreviation notation is used; thus, C = CaO, A = Al₂O₃, H = H₂O, Sr = SrO, Z = ZrO₂, F = Fe₂O₃, ${\bar{\text{C}}}$ = CO₂.

Experimental

Samples preparation

CA, C₇A₃Z and C₁₂A₇ were synthesized from stoichiometric mixtures of calcium carbonate (Chempur 98.81%), aluminum oxide (Acros Organic 99.7%) and zirconium dioxide (Merck 98.08%). C₆SrA₃Z was synthesized from the stoichiometric mixtures of strontium carbonate (Merck 99.0%), calcium carbonate (POCH 99.5%), aluminum oxide (Acros Organic 99.0%) and zirconium dioxide (Acros Organic 98.5%). C₇A₂FZ was synthesized from the stoichiometric mixtures of aluminum oxide (Acros Organic 99.0%), calcium carbonate (POCH 99.999%), zirconium dioxide (Acros Organic 98.5%) and iron (III) oxide (Chempur 96.0%). In either case, substrates in the form of powders were mixed with appropriate molar ratio of CaO, Al₂O₃, ZrO₂, Fe₂O₃ and SrO oxides. A test sample of commercial calcium aluminate cement ‘Gorkal 70’ was approved.

The prepared mixtures were homogenized in a zirconium ball mill for 2 h, then formed roller-shaped samples with a diameter of 20 mm under pressure 80 MPa and calcined at 1200 °C for 10 h (CA, C₇A₃Z and C₁₂A₇), at 1300 °C for 10 h (C₆SrA₃Z) and at 1000 °C for 10 h (C₇A₂FZ). The samples were cooled in the furnace, ground down in agate mortar to a grain size below 63 μm, homogenized in a zirconium ball mill for 2 h, then formed in roller-shaped samples with a diameter of 20 mm under pressure 80 MPa and fired at 1400 °C for 10 h (CA), at 1500 °C for 30 h (C₇A₃Z), at 1350 °C for 10 h (C₁₂A₇), at 1420 °C for 15 h (C₆SrA₃Z) and at 1300 °C for 10 h (C₇A₂FZ). After, firing samples were ground down again in agate mortar and milled in zirconium ball mill for 2 h to a grain size below 63 μm.

Methods of the investigation

The precursors before the hydration process were investigated by X-ray diffraction analysis in terms of chemical composition. Hydration products and their thermal stability were investigated by X-ray diffraction analysis, differential thermal analysis–thermogravimetric analysis–evolved gas analysis (DSC–TG–EGA(MS)) and scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS). In order to that, the neat cement pastes were composed. Each sample was homogenized with water by hand with w/c ratio 1.0, in a glass beaker. Samples in the form of pastes were placed in a sealed polyethylene bag and cured up to 72 h in a climatic chamber with the relative humidity maintained at 95% and the temperature of 50 °C. Subsequently, samples were dried by acetone quenching after 24 h and after 72 h since homogenization. The conditions for the hydration (temperature of 50 °C and w/c ratio of 1.0) of the precursors were adopted to create the most favorable conditions for gaining C₃AH₆. Dry powders were then analyzed by X-ray diffraction (X’pert ProPANalytical X-ray diffractometer) and simultaneous DSC–TG–EGA(MS) method (NETZSCH STA 449 F5 Jupiter coupled to QMS 403 D Aëolos) at a heating rate of 10 °C min⁻¹ under a flow of AR (50 mL min⁻¹). The containers for samples were corundum crucibles. The initial sample mass was 25 mg.

The description of microstructure evolution of cement pastes by SEM was carried out on the freshly broken surfaces of the hydrated cement pastes after 24 h and 72 h of curing duration. The samples were coated by carbon, which enables to remove any charge.

Results

The X-ray diffraction pattern of precursors before the hydration process is presented in Fig. 1. The results of X-ray diffraction analysis of the hydrated cement pastes 24 and 72 h after starting the hydration process are presented in Figs. 2–7. The TG, DSC and EGA curves as a result of the simultaneous DSC–TG–EGA(MS) method of 24-h and 72-h samples are presented in Figs. 8–11, respectively. Figures 12, 14, 16, 18, 21, 24 show the morphologies of 24-h cement pastes, whereas Figs. 13, 15, 17, 19, 22, 25 show the SEM images of their fracture surface after 72 h. The EDS spectrum of selected samples (C₆SrA₃Z, C₇A₂FZ, ‘Gorkal 70’) is presented in Figs. 20, 23, 26–28, respectively. The decomposition temperatures of CAC hydration products are presented in Table 1. Crystallographic data of hydrates detected by XRD are presented in Table 2. The obtained results have been extended to study the evolution of phase composition during the hydration process of different precursors.

Fig. 1

Results of X-ray diffraction analysis of precursors before hydration process

Fig. 2

X-ray diffraction pattern of CA after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Black circle: CaAl₂O₄ [JCPDS card No. 98-018-0997 (2θ = 30.043°, d = 2.97207 Å)], black diamond: C₃AH₆ [JCPDS card No. 98-009-4630 (2θ = 17.262° (the highest intensity XRD peak), d = 5.13291 Å)], plus sign: C₃A·CaCO₃·11H₂O [JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å)], white triangle: AH₃ [JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å)

Fig. 3

X-ray diffraction pattern of C₇A₃Z after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Asterisk: C₇A₃Z [JCPDS Card No. 98-015-7989), black diamond: C₃AH₆ [JCPDS card No. 98-006-2704 (2θ = 39.208°, d = 2.29587 Å)], white triangle: AH₃ (reference data: JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å)], plus sign: C₃A·CaCO₃·11H₂O [JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å)], degree sign: CZ [JCPDS card No. 98-009-7465 (2θ = 31.513°, d = 2.83666 Å)]

Fig. 4

X-ray diffraction pattern of C₁₂A₇ after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Modifier letter down arrow head: Ca₁₂Al₁₄O₃₃ [JCPDS card No. 98-024-1001 (2θ = 18.058°, d = 4.90102 Å)], black diamond: C₃AH₆ [JCPDS card No. 98-009-4630 (2θ = 17.262°, d = 5.13291 Å)], plus sign: C₃A·CaCO₃·11H₂O [JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å)], white triangle: AH₃ [JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å)]

Fig. 5

X-ray diffraction pattern of C₆SrA₃Z after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Black diamond: C₃AH₆ [JCPDS card No. 98-009-4630 (2θ = 17.262°, d = 5.13291 Å)], degree sign: Sr-doped CZ (similar to JCPDS Card No. 98-009-7465), asterisk: Sr-doped C₇A₃Z (similar to JCPDS Card No. 98-015-7989), white triangle: AH₃ (reference data: JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å)], plus sign: C₃A·CaCO₃·11H₂O [JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å)], circled times: C₄AH₁₉ [JCPDS card No. 00-042-0487 (2θ = 10.64350°, d = 8.301 Å)] and C₂AH₈ [JCPDS card No. 00-011-0205 (2θ = 10.7000°, d = 8.257 Å)]

Fig. 6

X-ray diffraction pattern of C₇A₂FZ after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Black diamond: C₃AH₆ [JCPDS card No. 98-006-2704 (2θ = 39.208°, d = 2.29587 Å)], degree sign: CZ [JCPDS card No. 98-009-7465)

Fig. 7

X-ray diffraction pattern of ‘Gorkal 70’ Cement after 24 h (a) and after 72 h (b), prepared with w/c = 1 at 50 °C. Plus sign: C₃A·CaCO₃·11H₂O [JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å)], white square: CaAl₄O₇ [JCPDS card No. 00-046-1475 (2θ = 25.318°, d = 3.51500 Å)], black diamond: C₃AH₆ [JCPDS card No. 98-009-4630 (2θ = 17.262°, d = 5.13291 Å)], white triangle: AH₃ [JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å)]

Fig. 8

TG profiles of the cementitious binder samples 24 and 72 h after starting hydration process, measured in situ by online coupled DSC–TG–EGA(MS) system (Ar flow 50 mL min⁻¹, heating rate 10 °C min⁻¹)

Fig. 9

DSC profiles of the cementitious binder samples 24 and 72 h after starting hydration process, measured in situ by online coupled DSC–TG–EGA (MS) system (Ar flow 50 mL min⁻¹, heating rate 10 °C min⁻¹)

Fig. 10

Gas evolution curves for representative mass spectroscopic fragments of H₂O (m/z = 18) vapor during the thermal decomposition of the cementitious binder samples in flowing argon 24 and 72 h after starting hydration process, measured in situ by online coupled DSC–TG–EGA(MS) system (Ar flow 50 mL min⁻¹, heating rate 10 °C min⁻¹)

Fig. 11

Gas evolution curves for representative mass spectroscopic fragments of CO₂ (m/z = 44) vapor during the thermal decomposition of the cementitious binder samples in flowing argon 24 and 72 h after starting hydration process, measured in situ by online coupled DSC–TG–EGA(MS) system (Ar flow 50 mL min⁻¹, heating rate 10 °C min⁻¹)

Fig. 12

SEM images of the fracture surface of 24-h hydrated CA

Fig. 13

SEM images of the fracture surface of 72-h hydrated CA

Fig. 14

SEM images of the fracture surface of 24-h hydrated C₇A₃Z

Fig. 15

SEM images of the fracture surface of 72-h hydrated C₇A₃Z

Fig. 16

SEM images of the fracture surface of 24-h C₁₂A₇

Fig. 17

SEM images of the fracture surface of 72-h hydrated C₁₂A₇

Fig. 18

SEM images of the fracture surface of 24-h hydrated C₆SrA₃Z

Fig. 19

SEM images of the fracture surface of 72-h hydrated C₆SrA₃Z

Fig. 20

EDS spectrum and quantitative composition analysis of point 1 region ((Ca,Sr)₃AH₆) in Fig. 19

Fig. 21

SEM images of the fracture surface of 24-h C₇A₂FZ

Fig. 22

SEM images of the fracture surface of 72-h hydrated C₇A₂FZ

Fig. 23

EDS spectrum and quantitative composition analysis of point 1 region (Fe-doped C₃AH₆) in Fig. 22

Fig. 24

SEM images of the fracture surface of 24-h hydrated calcium aluminate cement ‘Gorkal 70’

Fig. 25

SEM images of the fracture surface of 72-h hydrated calcium aluminate cement ‘Gorkal 70’

Fig. 26

EDS spectrum and quantitative composition analysis of point 1 region (C₃AH₆) in Fig. 25

Fig. 27

EDS spectrum and quantitative composition analysis of point 2 region (metastable hydrate—hexagonal platelets) in Fig. 25

Fig. 28

EDS spectrum and quantitative composition analysis of point 3 (Al(OH)₃) region in Fig. 25

Table 1 Decomposition temperatures of CAC hydration products in °C according to various studies, obtained by thermal analysis methods

Table 2 Crystallographic data of hydrates detected by XRD

Characteristics of precursors before the hydration process

The results of X-ray diffraction analysis of precursors before the hydration process are presented in Fig. 1. In the sample CA, pure-phase CaAl₂O₄ was identified. The sample C₇A₃Z included the main phase Ca₇ZrAl₆O₁₈ with a dope of CaZrO₃. The sample C₁₂A₇ contained pure-phase Ca₁₂Al₁₄O₃₃. The sample C₆SrA₃Z identified Ca₇ZrAl₆O₁₈—its peaks were moved toward lower values of 2θ after substitution for Sr and CaZrO₃. The sample C₇A₂FZ contained main phases: Ca₇ZrAl₆O₁₈ and Ca₂AlFeO₅ (brownmillerite) and dope of CaZrO₃. In the sample ‘Gorkal 70’, the identified phases were CaAl₂O₄ and CaAl₄O₇.

An overview of thermal analysis of the hydrated cement pastes

The thermal decomposition of hydrated cement pastes was characterized by two major temperature regions of mass loss (Fig. 8, Table 3): first an insignificant loss in the temperature range 25–200 °C (about 2.69–10.29% mass loss takes place in this region for all the samples) and, second, a sharp loss in the temperature range 200–350 °C (more than 10.45% mass loss takes place for all the samples). On the base of these data, the phases in the samples were identified.

Table 3 Mass loss in % in the temperature ranges 25–200 °C and 200–350 °C from heated hydrated samples determined using TG

The gel phases presented as a hydrated amorphous alumina gel-phase AH₃ or as a calcium aluminum hydrate (C-A-H)-type gel were dehydrated over a broad temperature range, probably in the temperature range of 25–200 °C and could partially overlap with dehydration of crystalline calcium aluminate hydrates. Mass losses at higher temperatures were associated with the loss of the structural water from the numerous calcium aluminate hydrates of the defined composition (Fig. 8). As can be seen from Figs. 9 and 10, the maxima of EGA peaks corresponded to the minima of DSC peaks.

Since the main reaction peaks could be clearly identified in the EGA curves, the temperature of EGA peaks of cement pastes was used for the peak assignment. For clarity of the figure, the temperature of EGA peaks of 72-h hydrated cement pastes is presented in Fig. 10 (marked in blue).

The MID curves for all the hydrated samples (Fig. 11) revealed an increase in ion intensity for m/z = 44 in the temperature ranges from ca. 250 to 300 °C and above 600 °C. This increase in ion intensity for m/z = 44 could be attributed to the carbon dioxide fragment ion CO₂⁺ of calcium aluminate carbonate hydrates, especially monocarboaluminate hydrate C₃A·CaCO₃·11H₂O [29].

Hydration of CA

24 h after starting the hydration process, the following products were identified (XRD, Fig. 2a): C₃AH₆, AH₃, C₄A${\bar{\text{C}}}$H₁₁ and unhydrated CA. On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 310 °C) and AH₃ (decomposition temperature: 284 °C) [23,24,25,26,27,28]; no effect from C₄A${\bar{\text{C}}}$H₁₁ was recorded. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 8 μm (Fig. 12).

72 h after starting the hydration process, XRD patterns (Fig. 2b) confirmed the presence of hydration products and the consumption of the CA during hydration. Results of gas evolution curves were not changed significantly (decomposition temperature of AH₃: 283 °C and the decomposition temperature of C₃AH₆: 307 °C) (DSC) [30,31,32,33,34,35]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 8–9 μm (Fig. 13).

Hydration of C₇A₃Z

24 h after starting the hydration process, the following products were identified on the basis of the strongest ‘100’ peaks (XRD, Fig. 3a): C₃AH₆, AH₃, C₄A${\bar{\text{C}}}$H₁₁, CZ, and C₇A₃Z. On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 309 °C) and AH₃ (decomposition temperature: 269 °C); no effect from C₄A${\bar{\text{C}}}$H₁₁ was recorded [30,31,32,33,34,35]. SEM observations confirmed the presence of C₃AH₆ in the shape of a truncated octahedron with a grain size of 20 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 14).

72 h after starting the hydration process, XRD patterns (Fig. 3b) confirmed the presence of C₄A${\bar{\text{C}}}$H₁₁ and CZ and the absence of C₇A₃Z. Results of gas evolution curves showed the increase in ionization current for C₃AH₆ (decomposition temperature: 312 °C) and AH₃ (272 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of C₃AH₆ in the shape of truncated octahedron with a grain size of 15–20 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 15).

Hydration of C₁₂A₇

24 h after starting the hydration process, the following products were identified the phases on the basis of the strongest ‘100’ peaks (XRD, Fig. 4a): C₃AH₆, AH₃, C₄A${\bar{\text{C}}}$H₁₁ and unhydrated C₁₂A₇. On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 299 °C), AH₃ (decomposition temperature: 272 °C) and C₄A${\bar{\text{C}}}$H₁₁ (decomposition temperature: 152 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 1–7 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 16).

72 h after starting the hydration process, XRD patterns (Fig. 4b) confirmed the consumption of the C₁₂A₇ during hydration, higher content of C₄A${\bar{\text{C}}}$H₁₁ and the same amount of AH₃. Results of gas evolution curves were not changed significantly (decomposition temperatures: of C₃AH₆: 300 °C; AH₃: 272 °C, C₄A${\bar{\text{C}}}$H₁₁: 152 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 1–7 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 17).

Hydration of C₆SrA₃Z

24 h after starting the hydration process, the following products were identified the phases on the basis of the strongest ‘100’ peaks (XRD, Fig. 5a): two types of C₃AH₆—Ca-rich and Sr-rich on the grounds of displaced XRD peaks, AH₃, C₄A${\bar{\text{C}}}$H₁₁, C₄AH₁₉, which can also be assigned to C₂AH₈ and probably Sr-doped C₇A₃Z and Sr-doped CZ. It was found that with the presence of SrO in cement a shift in the lines corresponds to a somewhat larger cell for the ‘pure’ undoped C₇A₃Z in comparison with Sr-doped C₇A₃Z solid solutions since the substitution of Sr²⁺ ion is larger than Ca²⁺ [28, 29].

On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 263 °C), Sr-rich (C,Sr)₃AH₆ (decomposition temperature: 343 °C) and probably C₂AH₈ (decomposition temperatures: 82 °C and 154 °C) rather than C₄A${\bar{\text{C}}}$H₁₁ (probability of multistage decomposition of C₂AH₈) [30,31,32,33,34,35]. SEM observations confirmed the presence of C₃AH₆ in the shape of a truncated octahedron with a grain size of 10 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 18).

72 h after starting the hydration process, XRD patterns (Fig. 5b) confirmed probably the presence of Ca-rich (C,Sr)₃AH₆, content of CZ Sr-doped, AH₃ and C₄A${\bar{\text{C}}}$H₁₁ (only by 2 peaks). Results of gas evolution curves confirmed the presence of AH₃ (decomposition temperature: 268 °C) and Ca-rich (C,Sr)₃AH₆ (decomposition temperature: 303 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of C₃AH₆ in the shape of a truncated octahedron with a grain size of 8–10 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 19).

The X-ray microanalysis was performed with SEM/EDS and gave direct evidence that Sr ions were presented in a significant amount in the chemical composition of the stable hydrate crystals (Fig. 20 EDS spectrum obtained from point 1). With respect to the results of the XRD and the shift of the main diffraction lines of C₃AH₆ toward the lower values of 2θ, the formation of a solid solution (C,Sr)₃AH₆ was confirmed [35,36,37].

Hydration of C₇A₂FZ

24 h after starting the hydration process, the following products were identified on the basis of the strongest ‘100’ peaks (XRD, Fig. 6a): C₃AH₆ and CZ. Phases contained Fe were undetectable. On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 310 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 20 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 21).

72 h after starting the hydration process, XRD patterns (Fig. 6b) confirmed the same composition than after 24 h after starting the hydration process. Phases contained Fe were undetectable. Results of gas evolution curves were not changed significantly—the main products of hydration were C₃AH₆ (decomposition temperature: 309 °C) [30,31,32,33,34,35]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 16–20 μm (Fig. 22), the presence of C₃AH₆ in the shape of a truncated octahedron with a grain size of 18–21 μm, hexagonal platelets of metastable hydrates and residue of AH₃. The X-ray microanalysis was performed with SEM/EDS and gave direct evidence that Fe ions could be incorporated in C₃AH₆ crystals (Fig. 23 EDS spectrum obtained from point 1). Fe was presented in chemical composition, and it was confirmed on the EDS spectrum.

Nevertheless, as mentioned before, there was no clear evidence of the presence of a C₃(A,Fe)H₆ solid solution based on XRD studies, but in the literature sources, it was reported [38].

Hydration of calcium aluminate cement ‘Gorkal 70’

24 h after starting the hydration process, the following products were identified on the basis of the strongest ‘100’ peaks (XRD, Fig. 7a): C₃AH₆, AH₃, C₄A${\bar{\text{C}}}$H₁₁, little quantity of CA₂ and a lack of CA. On the base of gas evolution curves (DSC–TG–EGA(MS)) (Fig. 10), the main products of hydration were C₃AH₆ (decomposition temperature: 307 °C), AH₃ (decomposition temperature: 283 °C) and C₄A${\bar{\text{C}}}$H₁₁ (decomposition temperature: 152 °C) [23,24,25,26,27,28]. SEM observations confirmed the presence of cubic C₃AH₆ with a grain size of 1.5–3 μm, hexagonal platelets of metastable hydrates and residue of AH₃ (Fig. 24).

72 h after starting the hydration process, XRD patterns (Fig. 7b) confirmed the presence of C₃AH₆, AH₃ and C₄A${\bar{\text{C}}}$H₁₁ and lack of CA₂. Results of gas evolution curves were not changed significantly (decomposition temperatures: of C₃AH₆: 303 °C, of AH₃: 283 °C, of C₄A${\bar{\text{C}}}$H₁₁: 150 °C) [23,24,25,26,27,28]. SEM observations confirmed the presence of cubic C₃AH₆ with grain size about 2 μm (Fig. 25), the presence of C₃AH₆ in shape of truncated octahedron with a grain size of 3.5–6 μm (Fig. 26 EDS spectrum obtained from point 1), hexagonal platelets of metastable hydrates (Fig. 27 EDS spectrum obtained from point 2) and residue of AH₃ (Fig. 28 EDS spectrum obtained from point 3). Moreover, the mass CaO/Al₂O₃ ratio of C₃AH₆ was higher than for hexagonal hydrates, and it was visible on the EDS spectra. (A higher peak of Ca was recorded for C₃AH₆ than for hexagonal platelets in point 2).

Discussion and conclusions

The C₃AH₆ was successfully synthesized from various precursors. The other phases of hydration products of all precursors were: aluminum hydroxide AH₃ (except sample C₇A₂FZ) and calcium monocarboaluminate C₄A${\bar{\text{C}}}$H₁₁ (except sample C₇A₂FZ). Depending on the type of precursor, two different types of C₃AH₆ were obtained. In the case of C₇A₃Z and C₇A₂FZ, X-ray diffraction patterns of C₃AH₆ were identified in agreement with reference data, i.e., JCPDS card No. 98-006-2704 (2θ = 39.208°, d = 2.29587 Å). For other precursors (C₁₂A₇, C₆SrA₃Z, CA and Gorkal 70), C₃AH₆ was identified in agreement with different reference data, i.e., JCPDS card No. 98-009-4630 (2θ = 17.262°, d = 5.13291 Å). In all of the samples (except C₇A₂FZ), AH₃ was identified in agreement with reference data: JCPDS card No. 98-024-5301 (2θ = 18.268°, d = 4.84770 Å). C₄A${\bar{\text{C}}}$H₁₁ was identified in large part of samples (except sample C₇A₂FZ) in agreement with reference data: JCPDS card No. 01-087-0493 (2θ = 11.706°, d = 7.55376 Å).

At a broad temperature range, probably in the temperatures of 25–200 °C, the phases presented as a hydrated amorphous alumina gel-phase AH₃ or as a calcium aluminum hydrate (C-A-H)-type gel were dehydrated and could partially overlap with dehydration of crystalline calcium aluminate hydrates. At higher temperatures, mass losses were associated with the loss of the structural water from the numerous calcium aluminate hydrates of the defined composition (Fig. 8). Moreover, the maxima of EGA peaks corresponded to the minima of DSC peaks (Figs. 9, 10).

The results of the investigation confirmed the influence of precursor on the shape and grain size of C₃AH₆. The smallest grain size of C₆AH₃ was obtained for C₁₂A₇ and Gorkal 70 (grain size ~ 1–4 μm), medium grain size for CA and C₆SrA₃Z (grain size ~ 10 μm) and the biggest grain size for C₇A₂FZ and C₇A₃Z (grain size ~ 20 μm). In the case of samples containing zirconium, C₃AH₆ was obtained in truncated octahedron shape, while in the other samples C₃AH₆ was obtained in cubic shape. Probably the presence of zirconium is conducive to obtaining C₃AH₆ in truncated octahedron shape and with a bigger size. Grains formed after 24 h did not increase during the hydration process up to 72 h.

Some general conclusion can be drawn as follows:

• 1. The key novelties of this work lie in the precursor-controlled synthesis of C₃AH₆ and its structure, microstructure and thermal stability.

• 2. From the presented XRD, DSC–TG–EGA(MS) and SEM–EDS results, it is clear that the type of precursor, i.e., pure single-cementitious phases CA, C₁₂A₇ and C₇A₃Z (C = CaO, A = Al₂O₃, Z = ZrO₂), calcium aluminate cement, Fe-doped C₇A₃Z and Sr-doped C₇A₃Z are used to study the hydration which is influenced by the properties of the final cement pastes.

• 3. The size of C₃AH₆ crystals can be formed depending upon the precursor used; the smallest grain size of C₃AH₆ was obtained for C₁₂A₇ and Gorkal 70 (grain size ~ 1–4 μm), medium grain size for CA and C₆SrA₃Z (grain size ~ 10 μm) and the biggest grain size for C₇A₂FZ and C₇A₃Z (grain size ~ 20 μm).

• 4. The mass CaO/Al₂O₃ ratio of precursors before the hydration process affects the size of C₃AH₆ crystals in cementitious pastes. The higher the CaO/Al₂O₃ value, the larger the size of the crystals of C₃AH₆.

• 5. Direct evidence for the formation of Sr-doped C₃AH₆ and Fe-doped C₃AH₆ crystals came from the chemical composition analysis in micro-areas, which was performed by means of the scanning electron microscope (SEM–EDS). The formation of (Ca,Sr)₃AH₆ solid solution was confirmed by XRD.

• 6. Under given hydration condition (temperature above 50 °C and w/c ratio 1), the most stable hydration structure was formed from Fe-doped C₇A₃Z, whereas the least stable hydration structure was formed from Sr-doped C₇A₃Z.