| The influence of B2O3 in the calcium aluminate cement (CAC) clinker and the particle size (and particle size distribution) of CAC on the setting behaviors (setting times, flowability and demolding strength) and application properties (strengths at medium and high temperatures) of CAC-bonded refractory castables is investigated in this work. This is performed by measuring the hydration heat and electrical conductivity of CAC-water mixture to monitor the dissolution-precipitation behavior of CAC during hydration, examining the phase composition and microstructure of the formed hydrates with X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively, of the frozen-evacuated CAC-pastes. The flowability, de-molding strength of refractory castables and the mechanical properties of refractory castables after heat-treatment were also analyzed according to Chinese National Standards.The XRD patterns of two kinds of CACs with different B2O3 contents (0 wt% and 2.8 wt%) show that, both CA and CA2 in the cement with B2O3 have smaller interplanar spacing but larger full width at half maximum than those in the B2O3-free sample. These observations indicate that B2O3 in the CAC clinker dissolves into the intrinsic structure of CA and CA2, increasing the defect concentration and thus the activity of these phases. The exothermic and electrical conductivity curves of the two CACs were measured at 10?-40?. Both the incubation period (-3 h) and nuclei-precipitation period (-4h) of hydration are shorter of the B2O3-containing cement than of B2O3-free sample (6h and 5h, respectively). These results imply that B2O3 in the CAC clinker accelerates the dissolution-nuclei-precipitation process, thereby increasing the setting process of CACs. Microstructure analyses of the frozen-evacuated CAC-pastes show that more hydrates are generated in the B2O3-containing cement, further confirming that B2O3 in the CAC clinker enhances the precipitation of hydration products.The cement mortar prepared with B2O3-containg cement has a shorter setting time (190 min) and smaller flow value (125 mm) than that with B2O3-free cement. This is because B2O3 in the cement enhances the precipitation of hydration products, and therefore promotes the formation of interlocking structures. After curing at 110? for 24 h, both the cold crush strength (CCS) and modulus of rupture (MOR) of the refractory castable prepared with B2O3-containg cement (86.10 and 12.03 MPa, respectively) are higher than those with B2O3-free cement (72.50 and 10.50 MPa, respectively). These results further confirm that B2O3 in the cement enhances the hydrates precipitation, and therefore promotes the formation of interlocking structures.Similarly, the CCS and permanent linear change (PLC) of castable bonded with B2O3-containg cement after firing at 1100? and 1450? are 75.3 and 139.59 MPa, and-0.02% and-0.42%, respectively, whereas these values are 60.07 and 130.61 MPa, and-0.01% and 0.46%, respectively, of the sample without B2O3. The phase composition analyses of the castable matrix after firing at same temperatures show that the formation of CA2 and CA6 bonding phases is not affected by B2O3. Thermodynamic calculations by FactSage suggest that B2O3 in the cement could promote the formation of liquid phase, facilitating the sintering of castable and thus lowering the PLC and increasing the strengths at room temperature. However, the hot MOR at 1400? of castable bonded with B2O3-containing cement (7.24 MPa) is lower than that with B2O3-free cement (11.05 MPa). This is because B2O3 in the castable increases the amount of liquid phases.After grinding for Oh, 1h and 2h, the two CACs, CA70 and Secar 71 show different particle size distribution (PSD). The former cement after grinding has a narrow PSD, whereas the later sample has a wide PSD after grinding. The exothermic and electrical conductivity curves of the two CACs were measured at 10 ?-40?. At 20?, after grinding for 2 h, the peak of the exothermic curve of CA 70 decreases from 3.5 h without grinding to 1.5 h after grinding for 2 h, whereas the peaks are 10h and 25 h of the Secar 71 after grinding for Oh and 2 h, respectively. As aforementioned, the ground CA70 has a narrow particle size distribution, and a loosen structure, and hydrates also have a loosen structure. As a result, the diffusion of water is facilitated, enhancing the dissolution of CACs and increasing the concentration of Ca2+ and [Al(OH)4]-, and nuclei and precipitation of hydrates, thereby shortening the setting times. In comparison, Secar 71 after grinding has a wider particle size distribution, and a lower porosity, and the formed hydrates have a compacted structure. These microstructures could slow the diffusion of water, retarding the dissolution of CACs, and nuclei and precipitation of hydration products, thereby prolonging the setting times. Correspondingly, at 20?, the setting times and demolding strengths of the cement mortars prepared with CA70 after grinding for 0h,1h and 2h increase. They are 215,310 and 400 min, and 36.6,45.3 and 48.6 MPa, respectively. These values decrease of Secar 71 after grinding. They are 200,185 and 168 min, and 49.4,48.7 and 48.1 MPa, respectively. This is because the dissolution-nuclei-precipitation process of CA70 after grinding is enhanced, facilitating the formation of hydration products and interlocking structure, thereby shortening the setting times and increasing the demolding strength. In contrast, the dissolution-nuclei-precipitation process of Secar 71 after grinding is retarded, hindering the formation of hydration products and interlocking structure, thereby prolonging the setting times and decreasing the demolding strength. However, at 10, 30 and 40?, the hydration behaviors of CA70 and Secar 71 after grinding are more related to the temperature than the particle size distribution. For example, at 10?, the kinetics of dissolution, nuclei and precipitation of ground CA70 and Secar 71 are hindered, leading to a longer setting time and lower demolding strengths, e.g.,0 h, 25 min and 32.8 MPa; 1 h,285 min and 32 MPa and 2 h,285 min and 24.7 MPa. In contrast, the kinetics of dissolution, nuclei and precipitation of ground CA70 and Secar71 decrease, resulting in longer setting times but reduced demolding strengths, for instance, Oh,295 min and 49.3 MPa; 1 h,425 min and 47.3 MPa,2 h,465 min and 24.7 MPa. However, at 30?, the kinetics of dissolution, nuclei and precipitation of ground CA70 and Secar 71 are favored, resulting in a shorter setting time and higher demolding strengths. For example, the setting times and demolding strengths of CA70 are,180 min and 38.6 MPa for 0 h,165 min and 45.7 MPa for 1 h, and 168 min and 52.6 MPa for 2 h, respectively. Likewise, these values of Secar 71 are,200 min and 42.5 MPa for 0h,165 min and 50.4 MPa for 1 h, and 140 min and 55.3 MPa for 2 h.After curing at 110? for 24 h, the CCS of castables bonded with CA70 and Secar 71 after grinding for 1 h and 2 h increase from 83.4 MPa and 57.6 MPa of unground cement, to 88.5 MPa and 92.6 MPa, and 83.8 MPa and 88.7 MPa, respectively. This is because the decrease of cement particle size accelerate the dissolution-nuclei-precipitation process, facilitating the formation of hydrates and interlocking structure of hydrates; and also favoring the homogeneous distribution of hydrates in the castable, thereby increasing the strength.The CCS of castable after firing at 1100? and 1450? and hot MOR of castable fired at 1450? and tested at 1400? also increase when the castable is bonded with ground CA70 and Secar 71 cements. The CCS and hot MOR values of the castable bonded with unground cements are,70 MPa (1100?),120MPa (1450?) and 11.05 MPa (1400?) for CA70, and 65.2 MPa (1100?),124MPa (1450?) and 12.10 MPa (1400?) for Secar 71. When the castable are prepared with 2h ground cements, these values are 90 MPa (1100?),130 MPa (1450?) and 16.85 MPa (1400?) for CA70, and 83.1 MPa (1100?),139.5 MPa (1450?) and 17.66 MPa (1400?) for Secar 71. Besides, the PLCs of castable after firing at 1450? decrease from -0.15% and -0.19% of unground CA70 and Secar 71, respectively, to -0.18% and -0.30% of 2h ground CA70 and Secar 71, respectively. XRD analyses of the castable matrix illustrate that the amount of CA2 and CA6 does not increase, whereas the grain size of CA2 and CA6 reduces with the decrease of the cement particle size, thereby lowering the volume expansion of CA2 and CA6, and thus decreasing the PLC. Additionally, the homogeneous distribution of these phases resulted from the even distribution of hydration products, could also decrease the volume expansion, hindering the formation of microcrack, thereby increasing the volume stability and strengths of castables. |
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