Identification of Calcium Sulphoaluminate Formation between Alunite and Limestone

Abstract

This study was carried out to identify the conditions of formation of calcium sulphoaluminate (3CaO·3Al₂O₃·CaSO₄) by the sintering of a limestone (CaCO₃) and alunite [K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃] mixture with the following reagents: K₂SO₄, CaCO₃, Al(OH)₃, CaSO₄·2H₂O, and SiO₂. When K₂SO₄, CaCO₃, Al(OH)₃, CaSO₄·2H₂O were mixed in molar ratios of 1:3:6:3 and sintered at 1,200∼1,300 °C, only 3CaO·3Al₂O₃·CaSO₄ and calcium langbeinite (2CaSO₄·K₂SO₄) were generated. With an amount of CaO that is less than the stoichiometric molar ratio, 3CaO·3Al₂O₃·CaSO₄ was formed and anhydrite (CaSO₄) did not react and remained behind. With the amount of CaSO₄ that is less than the stoichiometric molar ratio, the amounts of 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ decreased, and that of CaO·Al₂O₃ increased. In the K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system, to stabilize the formation of 3CaO·3Al₂O₃·CaSO₄, 2CaSO₄·K₂SO₄, and β-2CaO·SiO₂, the molar ratios of CaO: Al₂O₃: CaSO₄ must be kept at 3:3:1 and that of CaO/SiO₂, over 2.0; otherwise, the generated amount of 3CaO·3Al₂O₃·CaSO₄ decreased and that of gehlenite (2CaO·Al₂O₃·SiO₂) with no hydration increased quantitatively. Therefore, if all SO₃(g) generated by the thermal decomposition of alunite reacts with CaCO₃ (or CaO, the thermal decomposition product of limestone) to form CaSO₄ in an alunite- limestone system, 1 mol of pure alunite reacts with 6 mol of limestone to form 1 mol of 3CaO·3Al₂O₃·CaSO₄ and 1 mol of 2CaSO₄·K₂SO₄.

1. Introduction

Cement has been prepared using alunite [1], and a type of special cement containing calcium aluminate, anhydrite, and potassium sulfate has been prepared using anhydrite formed by the reaction between SO₃(g) that evolves from alunite and limestone via the following reaction:

$${\mathrm{K}}_2{\text{SO}}_4\cdot {\text{Al}}_2{({\text{SO}}_4)}_3\cdot 4\text{Al}{(\text{OH})}_3+{\text{mCaCO}}_3\to {\mathrm{K}}_2{\text{SO}}_4+3(\text{nCaO}\cdot {\text{Al}}_2{\mathrm{O}}_3)+3{\text{CaSO}}_4+{\text{mCO}}_2(\mathrm{g})+6{\mathrm{H}}_2\mathrm{O} (\mathrm{g})$$

Choi et al. [2] synthesized a calcium sulphoaluminate clinker consisting of 3CaO·3Al₂O₃·CaSO₄, CaO and CaSO₄ through the sintering for 2 hrs at a temperature of 1,200 °C of alunite, limestone and an anhydrite mixture at a weight ratio of 1:13:5. Their research results showed that alunite could be used in the preparation of a 3CaO·3Al₂O₃·CaSO₄ clinker provided that adequate mixing conditions are provided, despite that the expansion is relatively small compared to that of a clinker synthesized from reagents. However, the exact formation conditions of 3CaO·3Al₂O₃·CaSO₄ in the sintering state were not discussed. In addition, Han et al. [3,4] synthesized a clinker containing calcium fluoroaluminate (C₁₁A₇·CaF₂) from domestic alunite and investigated its characteristics in an effort to develop a type of fast-hardening cement.

The authors [5] carried out an investigation of the conditions under which 3CaO·3Al₂O₃·CaSO₄ is formed when mixtures of alunite and limestone are sintered. It was concluded that calcium langbeinite (2CaSO₄·K₂SO₄) forms from 700 °C and that calcium sulphoaluminate forms from 800 °C. Both are stable up to 1,300 °C, as shown in the equation below:

$${\mathrm{K}}_2{\text{SO}}_4\cdot {\text{Al}}_2{({\text{SO}}_4)}_3\cdot 4\text{Al}{(\text{OH})}_3+6{\text{CaCO}}_3\to 4\text{CaO}\cdot 3{\text{Al}}_2{\mathrm{O}}_3\cdot {\text{SO}}_3+2{\text{CaSO}}_4\cdot {\mathrm{K}}_2{\text{SO}}_4+{6\mathrm{H}}_2\mathrm{O}(\mathrm{g})+{6\text{CO}}_2(\mathrm{g})$$

However, when alunite or limestone is incorporated in SiO₂ so as to enable the formation of calcium sulphoaluminate, the molar ratios of CaO/alunite and CaO/SiO₂ must be kept over 6.0 and 2.0, respectively. A clinker composed of calcium sulphoaluminate and calcium langbeinite was transformed into ettringite(3CaO·Al₂O₃·3CaSO₄·32H₂O) in water as calcium langbeinite is transformed into CaSO₄·2H₂O(s) and K₂SO₄ (aq) in water.

In the present study, the formation of 3CaO·3Al₂O₃·CaSO₄ is identified in a K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system using reagents of various types in a detailed investigation of the formation conditions of this species through the sintering of a mixture of alunite and limestone.

2. Experimental

The reagents listed in

Table 1. List of reagents used in this study.

Reagents	Chemical formula	Purity	Manufacturer
Potassium Sulfate	K2SO4	First grade	Ducksan Pharmaceutical Co., Ltd
Aluminum Hydroxide	Al(OH)3	First grade	Shinyo Pure Chemical Co., Ltd
Calcium Carbonate	CaCO3	min 98.0%	Kanto Chemical Co., Inc.
Gypsum	CaSO4·2H2O	Extra pure	Junsei Chemical Co., Ltd.
Silicate Dioxide	SiO2	Extra pure	Junsei Chemical Co., Ltd.

were used to investigate the reaction products arising from the K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system. K₂SO₄ and Al(OH)₃ were used in substitution for the components of alunite; calcium carbonate (CaCO₃) substituted for limestone, and CaSO₄·2H₂O, substituted for anhydrite (CaSO₄), which is formed by the sintering reaction between alunite and limestone.

The mixture of reagents were sintered in a programmable electric furnace (Barnstead /Thermolyne F46120 CM High-Temperature Furnace : 240 V, 40 A, 2,500 Watt, 50/60 Hz, 1 Phase) below 1,300 °C in an air atmosphere. The mineral phases of the manufactured clinker were then analyzed by XRD(PW-1700, Philips, 30kV, 25mA, Cu target, Ni filter, at a scanning rate of 2^°/min). After the prepared clinker was finely milled with a laboratory ball mill, the mineral phases of the crushed clinker were analyzed by XRD.

3. Results and Discussion

Alunite (K₂SO₄·Al₂(SO₄)₃·4Al(OH)) is transformed into KAl(SO₄)₂ and Al₂O₃ by dehydration at 500∼580 °C and KAl(SO₄)₂, K₂SO₄ and Al₂O₃ by desulphurization at 700∼780 °C via the reaction of K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃ → K₂SO₄ + 2Al₂O₃ + 3SO₃(g) + 6H₂O(g), regardless of the partial pressure of CO₂(g). However, limestone decomposes from 650 °C in air and from 900 °C in a CO₂(g) saturated atmosphere [6].

When the mixture of alunite and limestone is sintered in air and in a CO₂(g) saturated atmosphere, the rate of formation of anhydrite is relatively low, at 76.0% and 67.0%, respectively, at a CaCO₃/alunite stoichiometric molar ratio of 3. However, the rate of formation increases as the molar ratio of CaCO₃/alunite (particle size, 37∼44 μm) exceeds 6, showing rates of more than 99.0% and 95.0% in air and in a CO₂(g) saturated atmosphere, respectively [7].

As shown in the results of aforementioned experiment, if alunite and limestone are mixed and sintered in air, most of the generated SO₃ reacts with limestone to form anhydrite (CaSO₄). Additionally, ignoring impurities such as Fe₂O₃, TiO₂, and P₂O₅ included in the alunite, because alunite ore is composed of alunite, quartz(SiO₂), and the aluminum silicate minerals of kaolinite, dickite, and pyrophyllite, the alunite and limestone mixture can be said to have five components, as does K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂.

If 1 mol if pure alunite is heated to temperatures that exceed 800 °C, it is pyrolyzed as K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃ → K₂SO₄ + 3Al₂O₃ + 3SO₃ + 6H₂O. Therefore, to formulate SO₃(g) of 3 mol into anhydrite, 3 mol of CaCO₃ is required, and to change 3 mol of Al₂O₃ into 3CaO·3Al₂O₃·CaSO₄, 3 additional mol of CaCO₃ and 1 mol of CaSO₄ will be needed. Therefore, to change all of the Al₂O₃ in alunite into 3CaO·3Al₂O₃·CaSO₄, and to generate SO₃(g) by the thermal decomposition of alunite into anhydrite, theoretically, pure alunite and limestone should be mixed at a molar rate of 1:6. This mixture will be composed of five component systems as in K₂SO₄-CaO-Al₂O₃-CaSO₄.

The present study aims to determine the mineral phases of a clinker generated in the K₂SO₄-CaO-Al₂O₃-CaSO₄ and the K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ systems, using several types of reagents, including K₂SO₄, Al(OH)₃, SiO₂, CaSO₄·2H₂O, and CaCO₃.

Figure 1 shows X-ray diffraction patterns of the materials generated, when a compound consisting of K₂SO₄-3CaO-3Al₂O₃-3CaSO₄ was sintered at 1,100 °C, 1,200 °C, and 1,300 °C, respectively, for 2 hours. As shown in this figure, 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ is formed, but CaSO₄ does not react and remains at 1,100 °C. Meanwhile, at temperatures in excess of 1,200 °C 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ were mainly generated. Accordingly, 1 mol of pure alunite reacts with 6 mol of limestone to form 1 mol of 3CaO·3Al₂O₃·CaSO₄ and 1 mol of 2CaSO₄·K₂SO₄ as follows; K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃ + 6CaCO₃ → 2CaSO₄·K₂SO₄ + 3CaO·3Al₂O₃·CaSO₄ + 6H₂O(g) + 6CO₂(g).

Figure 2 displays the materials generated from K₂SO₄-nCaO-3Al₂O₃-3CaSO₄ with various amounts of CaO sintered at 1,200 °C for two hours, as determined via an XRD analysis.

When it CaO is included in at an amount of 3 mol, 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ are mainly generated. However, it was found that as the amount of CaO is increased from 3 mol to 5 mol, CaO does not participate in the formation reaction of 3CaO·3Al₂O₃·CaSO₄. Thus, it can be said that 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ are generated stably when CaO is 3 mol in the K₂SO₄-nCaO-3Al₂O₃-3CaSO₄ system.

According to Fukuda [8] 3CaO·3Al₂O₃·CaSO₄ is the only compound in the CaO-Al₂O₃-SO₃ system. This line of research was started in 1962 by Halstead, et al. [9] with Ca²⁺ and SO₄²⁻ ions in a three-dimensional crystal structure sharing the angular point of a AlO₄ tetrahedron; an Al³⁺ ion is coordinated with four O²⁻ ions, a Ca²⁺ ion is surrounded asymmetrically by O²⁻ ions, and an isolated SO₄²⁻ ion is characterized by its ability to readily react with water. Kondo [10] discovered that these ions become hardened in water. In general, they are produced by re-sintering after producing 3CaO·Al₂O₃ and regulating the mixture ratio of 3CaO·Al₂O₃, CaSO₄·2H₂O and Ca(OH)₂ and by sintering a mixture of CaCO₃, Al₂O₃, and CaSO₄·2H₂O.

2CaSO₄·K₂SO₄ is easily generated in the CaO-Al₂O₃-SiO₂-Fe₂O₃-MgO-CaSO₄-K₂SO₄ system and has a considerable influence on the condensation time and the hardening characteristics of cements. Known as a water-soluble alkali, as 2CaSO₄·K₂SO₄ can be easily separated from the liquid state of a clinker oxide and is said to become K₂SO₄·CaSO₄·H₂O(syngenite) and CaSO₄·2H₂O upon exposure to water [11].

Figure 3 shows the materials generated from the K₂SO₄-3CaO-3Al₂O₃-nCaSO₄ system sintered at 1,200 °C for two hours with varying amounts of anhydrite, as determined via an XRD analysis.

As shown in this figure, when anhydrite is included in an amount of 3 mol, 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ are generated. However, as the amount of anhydrite is decreased from 3 mol to 1 mol, it was observed that the K₂SO₄ that did not react with CaO·Al₂O₃ remained. As the amount of anhydrite is increased from 3 mol to 5 mol, only the diffraction strength of the CaSO₄ that does not react increases. Thus, it can be said that an amount of CaSO₄ in excess of the stoichiometric molar ratio is necessary to generate 3CaO·3Al₂O₃·CaSO₄ stably.

As noted above, 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ were mainly generated in the K₂SO₄-3CaO-3Al₂O₃-3CaSO₄ system. However, because alunite from nature has impurities of SiO₂ and aluminum silicate minerals such as kaolinite, dickite, and pyrophyllite, the alunite and limestone mixture is believed to be comprised of a K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system to which SiO₂ is added.

Figures 4 and 5 show, by XRD analysis, the materials generated from the K₂SO₄-3CaO-3Al₂O₃-3CaSO₄-nSiO₂ system and the K₂SO₄-(3+m)CaO-3Al₂O₃-3CaSO₄-nSiO₂ system, respectively, sintered at 1,200 °C for 2 hours, with various amounts of CaO (3 + m : mol number) and SiO₂(n: mol number). In Figure 4, when there is no SiO₂ in the mixture, the result is identical to that obtained when 3 mol of CaO is used (Figure 2). It is expected that 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ will be generated. However, as the amount of SiO₂ is increased from 1 mol to 5 mol, the amount of 3CaO·3Al₂O₃·CaSO₄ generated is reduced, because gehlenite (2CaO·Al₂O₃·SiO₂) and wollastonite (α-CaO·SiO₂), which do not react with water, are generated. In addition, as shown in Figure 5, when the mol rate of CaO/alunite is less than 6 (m less than 3), and that of CaO/SiO₂ is less than 2, the synthetic clinker contained 2CaO·Al₂O₃·SiO₂ and CaSO₄; these compounds do not participate in the formation reaction of 3CaO·3Al₂O₃·CaSO₄ because 3CaO·3Al₂O₃·CaSO₄ is the only compound in the CaO-Al₂O₃-SO₃ system as noted above[8] and CaO and Al₂O₃ participate preferentially in the formation reaction of 2CaO·Al₂O₃·SiO₂. Accordingly, if all SO₃(g) generated by thermal decomposition of alunite reacts with CaCO₃ (or CaO, the thermal decomposition product of limestone) to form CaSO₄ in the alunite-limestone system, when the molar ratios of CaO/alunite exceed 6 and that of CaO/SiO₂ exceeds 2, 3CaO·3Al₂O₃·CaSO₄, 2CaSO₄·K₂SO₄ and β-2CaO·SiO₂ are generated stably.

4. Conclusions

This study investigated the formation characteristics of calcium sulphoaluminate (4CaO·3Al₂O₃·SO₃) in a K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system using reagents of various types in an effort to identify the formation conditions of 3CaO·3Al₂O₃·CaSO₄ by the sintering of a mixture of alunite and limestone. In experiments with reagents for the mineral phases generated from the compound system (the K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system), if all SO₃(g) from alunite reacts with limestone to form anhydrite, it was found that 1 mol of pure alunite reacts with 6mol of limestone to form 1 mol of 3CaO·3Al₂O₃·CaSO₄ and 1 mol of 2CaSO₄·K₂SO₄, as of 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ are generated stably at a component ratio of K₂SO₄-3CaO-3Al₂O₃-3CaSO₄. Over-mixing of CaO and CaSO₄ has a slight effect on the generation of 3CaO·3Al₂O₃·CaSO₄. However, it is thought that under-mixing will have a substantial effect on the mineral phase of the clinker and on the amount of 3CaO·3Al₂O₃·CaSO₄ that is generated. Moreover, if the impurities of SiO₂ are incorporated in the alunite and limestone, the molar ratio of CaO/alunite must exceed 6 and that of CaO/SiO₂ must exceed 2 in order to ensure the stable formation of calcium sulphoaluminate, calcium langbeinite, and the β-2CaO·SiO₂ phases. Otherwise, the amount of gehlenite(2CaO·Al₂O₃·SiO₂), which does not react with water, may be increased.