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

  • Ewa LitwinekEmail author
  • Dominika Madej
Open Access
Article
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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.

Keywords

Tricalcium aluminate hexahydrate (C3AH6Microstructure Hydration Calcium aluminate cement 

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 C1−xSrxAl2O4 (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. 27. 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. 811, 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, 2628, 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

Reference

Year

Method

Decomposition temperature/°C

C2AH8

C3AH6

C4AH19

C4A \({\bar{\text{C}}}\) H11

AH3 (cryst.)

[32]

1983

170

    

[32, 33]

1983, 2001

DTA

 

300–330

   

[31]

2015

DTA/TG

  

200–280

  

[35]

1985

DTA

   

160–200

 

[32, 34]

1983, 1984

DTA

    

275–280 ± 10–20

[30]

2007

DTA/TGA

110 175 295

    
Table 2

Crystallographic data of hydrates detected by XRD

Crystallographic parameters

C3AH6 (reference data: JCPDS Card No. 98-009-4630

C4A \({\bar{\text{C}}}\) H11 (reference data: JCPDS Card No. 01-087-0493

C2AH8 (reference data: JCPDS Card No. 00-011-0205

C4AH19 (reference data: JCPDS Card No. 00-042-0487)

Al(OH)3 (reference data: JCPDS Card No. 98-024-5301)

Crystal system

Cubic

Anorthic

*Not provided by the reference card

Hexagonal

Monoclinic

Space group

I a–3 d

P1

*

P 1 21/c 1

Space group number

230

1

*

14

a

12.5730

5.7747

5.7470

8.6750

b

12.5730

8.4689

5.7470

5.0690

c

12.5730

9.9230

63.8500

12.5086

Alpha/°

90.0000

64.7700

90.0000

90.0000

Beta/°

90.0000

82.7500

90.0000

129.1860

Gamma/°

90.0000

81.4300

120.0000

90.0000

Calculated density/g cm−3

2.53

2.18

*

2.43

Volume of cell (106 pm3)

1987.54

433.02

1826.31

426.34

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

Sample

Hydration time/h

% mass loss below 200 °C

% mass loss in the temperature range 200–350 °C

C7A3Z

24

5.77

16.30

72

4.67

18.70

C12A7

24

4.90

20.26

72

4.47

21.39

C6SrA3Z

24

10.29

10.45

72

5.15

16.09

C7A2FZ

24

5.11

14.66

72

3.60

15.15

CA

24

3.02

23.34

72

2.69

23.94

‘Gorkal 70’

24

4.36

21.19

72

3.95

22.33

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. 1.

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

     
  2. 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. 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. 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. 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. 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.

     

Notes

Acknowledgements

This project was financed by the National Science Centre, Poland, Project Number 2017/26/D/ST8/00012 (Recipient: Dominika Madej).

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Authors and Affiliations

  1. 1.Department of Ceramics and Refractories, Faculty of Materials Science and CeramicsAGH University of Science and TechnologyKrakówPoland

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