Pore structure evolution and fracture behavior of CAC bonded alumina-spinel castables: Role of curing time

Wenjing Liu , Ning Liao *, Mithun Nath, Zixu Ji, Yajie Dai, Liping Pan , Yawei Li *, Ilona Jastrzębska,

Jacek Szczerba—The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China —National-provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan 430081, China —Department of Ceramics and Refractories, AGH University of Science and Technology, Krakow, Poland—Email: liaoning@wust.edu.cn; liyawei@wust.edu.cn

IntroductionCalcium aluminate cement (CAC) bonded alumina-spinel castables are applied successfully in iron and steel, nonferrous, cement, and petrochemical industries due to their excellent corrosion

resistance and good workability. For example, the ladle lining castables has to endure repeatedly thermal stress damage, molten steel erosion, and slag corrosion. Especially the periodical steeltaping leads to thermal spalling and thermochemical corrosion, resulting in deteriorated performances at service in particular the adoption of waste steel. Therefore, strategies such asmicrostructure and composition design have been proposed to conquer the above challenges. Actually, the comprehensive properties of castables are depending on the hydration of cement, wherecuring time has been taken as one dominant factor. Extending the curing time would promote the hydration of cement and thus affect the pore structures and the final microstructures after heattreatment. To better understand the pore size (both in nanometer and micrometer range) distribution variation of castables cured for different times, mercury intrusion combined with micro-CTscanning have been adopted simultaneously. Besides, the fracture behaviors of the castables were evaluated with wedge splitting test together with acoustic emission. Finally, the influencesmechanisms of curing time on the pore structure, mechanical properties, thermal shock resistance and fracture behavior of CAC bonded alumina-spinel castables were discussed.

Results and discussions

Conclusion: The results show that the hydration degree is enhanced with extending the curing time, and more hydrates occupy the pores due to the migration of hydrated ions.Consequently, the extended curing time contributed to complex fractal dimension features at elevated temperatures based on the mercury intrusion combined with micro-CT scanningmeasurements. Furthermore, the 3d curing castables possess excellent HMOR and thermal shock resistance, attributing to induced tortuous crack propagation among matrix and along aggregate-matrix interfaces.

Fig.2 Pore diameter distribution of castables treated at different temperatures: 110 ℃ (a), 1600 ℃ (b)

Fig.3 Vertical force-displacement curves of alumina-spinel castables treated at 110 ℃ (a) and 1600 ℃ (b) after wedge splitting test.

Fig.1 SEM pictures of alumina-spinel castables cured for 1d (a) and 3d (b) treated at 1600 ℃

Wenjing Liu, Ning Liao *, Mithun Nath, Zixu J, Yajie Dai, Liping Pan , Yawei Li *, Ilona Jastrzębska, Jacek Szczerba, Pore structure evolution and fracture behavior of CAC bonded alumina-spinel castables: Role of curing time, Waiting for the submission.

Table 1 Calculated parameters of alumina-spinel castables after wedge splitting tests.

Index G'f (J/m2) lch (mm) σNT (MPa) G'f/σNT (um) (B+C)/A

1d-110℃ 141.78 258.47 7.90 17.95 2.89

3d-110℃ 169.50 229.29 9.42 19.99 2.33

1d-1600℃ 194.19 64.72 21.06 9.22 0.52

3d-1600℃ 207.89 86.66 18.91 11.00 0.87

Resource Crack’s category 1d-110℃ 3d-110℃ 1d-1600℃ 3d-1600℃

AE peak frequencies

Grain 5.63 2.34 14.41 11.10

Interface 18.61 13.62 19.77 19.03

Matrix 75.76 84.04 65.82 69.87

SEM counts

Grain 13.61 9.16 19.95 15.49

Interface 40.99 27.73 19.11 32.64

Matrix 45.40 63.11 60.94 51.87

Table 2 Crack propagation path ratio of alumina-spinel castables based on acoustic emission and SEM observation.

Fig.4 Peak frequency obtained from AE tests of alumina-spinel castables: 1d-1600 ℃ (a), and 3d-1600 ℃(b).

Microstructure of castables cured for 1d and 3d treated at 1600 ℃ are shown in Fig.1. The morphology demonstrates that pores found in sample 3d are smaller than those of sample 1d.Fig.2 shows the pore diameter distribution of castables from micro-CT scanning. Generally, samples 3d contain more finer pores than samples 1d regardless of the treating temperature,especially for the pore diameter between 30 and 200 μm. These results obtained from micro-CT scanning demonstrate that extending the curing time can refine the pore diameter in full range,which endows castables potential advantages against the thermal stress attack and molten slag penetration.

Wedge splitting test combined with acoustic emission-based method was conducted for investigating the fracture behavior. Fig.3 shows the vertical force-displacement curves obtainedfrom wedge splitting test. As seen from it, under 110 ℃, sample 3d shows higher load and slightly longer vertical displacement. But, under 1600 ℃, sample 3d possesses lower load butmuch longer vertical displacement. Table 1 gives the parameters obtained from vertical force-displacement. All the parameters demonstrates that sample 3d possesses better ability to resistcrack, including the crack initiation and propagation. During the crack propagation, there are three crack propagation ways. Based on acoustic emission-based method and SEM observation,the ratio were calculated in this work. The obtained results are listed in Table 2, regardless the treating temperature, extending the curing time can induce more crack propagation amongmatrix and along aggregate-matrix interfaces.