Anorthite ceramics: grog and clay selection for lowering the sintering temperature

Figures & data

Figure 1. Equilibrium phase diagram CaO – Al₂O₃ – SiO₂ [23]: the theoretical melting temperatures of the test samples; ISQC (1), KAQC (2), ISAC (3) and KAAC (4).

Table 1. Composition and identification of test samples (wt.%).

sample	kaolin	ISC	quartz	alumina	calcite
KAAC	60	–	–	25	15
KAQC	60	–	25	–	15
ISAC	–	60	–	25	15
ISQC	–	60	25	–	15

Table 2. Chemical composition of the plastic raw materials in wt.% (LOI – loss on ignition).

	SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	K2O	Na2O	LOI
kaolin	46.8	36.6	0.9	0.2	0.7	0.5	1.2		13.2
ISC	50.2	30.3	0.4	0.0	0.9	2.3	7.2	0.1	8.7

Figure 2. Dilatometric curves of the KAAC, KAQC, ISAC and ISQC test samples.

Figure 3. Sintering curves – the effect of firing temperature on water absorption and the determination of sintering temperature Ts.

Figure 4. XRD patterns of the test samples fired at 950°C (A – anorthite, Q – quartz, W-wollastonite, Al – corundum, mu – muscovite).

Figure 5. XRD patterns of the test samples fired at 1050°C (A – anorthite, Q – quartz, Al – corundum).

Figure 6. XRD patterns of the test samples fired at 1150°C (A – anorthite, Q – quartz, M – mullite, C – cristobalite, Al – corundum), ISQC is already sintered (Figure 7).

Figure 7. XRD analysis of sintered bodies at the sintering temperature Ts, according to Figure 3 (A – anorthite, M – mullite, C – cristobalite, Al – corundum).

Table 3. Summary of mineral composition of sintered bodies – quantitative XRD analysis.

sample	mullite M [wt.%]	Cristobalite C [wt.%]	Anorthite A [wt.%]	Corundum Al [wt.%]	Amorphous [wt.%]
KAAC	8.4	0.0	34.2	29.3	28.1
KAQC	7.5	12.4	41.3	0.0	38.8
ISAC	0.0	0.0	39.8	27.8	32.4
ISQC	0.0	0.4	33.8	1.8	64.0

Figure 8. Change in the length of sintered bodies for the determination of the coefficient of linear thermal expansion, CLTE.

Table 4. Coefficient of linear thermal expansion (CLTE) of sintered bodies in the temperature range of 30–700°C. Modulus of rupture (MOR) at sintering temperature.

sample	CLTE*10^−7 (°C^−1)	MOR (MPa)
KAAC	54.44	75.4 ± 3.1
KAQC	150.40	71.0 ± 1.6
ISAC	47.81	73.5 ± 2.9
ISQC	53.12	81.6 ± 3.2

Figure 9. SEM – examples of typical fractured surface of ISQC (a) and KAQC (b) sintered bodies without HF application.

Figure 10. SEM – microstructure of fractured and HF etched sintered body KAAC (al-corundum, A-anorthite).

Figure 11. SEM – microstructure of fractured and HF etched sintered body KAQC (M-mullite, A-anorthite).

Figure 12. SEM – microstructure of fractured and HF etched sintered body ISQC (A-anorthite).

Table 5. Results of the EDS analysis at the points highlighted in the SEM microphotographs.

Identified minerals	Identified elements (K-series)
	Ca [wt.%]	Al [wt.%]	Si [wt.%]	O [wt.%]	Fe [wt.%]	Na [wt.%]	K [wt.%]
Al - corundum	0.83	51.86	0.97	46.34	-	-	-
A – anorthite (KAAC)	12.94	17.92	20.26	46.47	1.18	0.57	0.66
A – anorthite (KAQC)	13.37	19.06	20.53	45.83	0.57	0.64	-
A – anorthite (ISQC)	13.92	18.27	21.05	45.52	0.48	-	0.76
M - mullite	1.38	34.63	12.07	49.33	0.96	0.71	0.92

Levin EM, Robbins CR, McMurdie HF. Phase diagrams for ceramists: fig. 630. 2nd ed. Columbus (OH): American Ceramic Society; 1975. ISBN: 0916094049.

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Data availability statement

The data presented in this paper are available upon request from the corresponding author.