Optimization of hydrothermal autoclaving parameters for the synthesis of porous ceramics from porcelain tile polishing residue

Porous ceramics were synthesized using porcelain tile polishing residue (PTPR) and slaked lime (Ca(OH)2) as a reinforcing agent through a hydrothermal autoclaving method. The process parameters, including the quantity of slaked lime added, the hydrothermal autoclaving temperature, and the reaction duration, were optimized meticulously. The composition, structure, thermal and physical properties of the samples were thoroughly analyzed via Brunauer–Emmett–Teller (BET) measurements, powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM). The results indicated that the incorporation of slaked lime and hydrothermal autoclaving led to the formation of calcium silicate hydrate, which corresponded with an enhancement in the strength of the sample. Notably, when the quantity of slaked lime added was optimized at 30 wt%, the formation of tobermorite (5CaO·6SiO2·5H2O) was detected. At a hydrothermal autoclaving temperature of 150 °C, the formation of only sheet-like calcium silicate hydrate was observed. In contrast, at an elevated temperature of 180 °C and 210 °C, needle-like tobermorite was successfully synthesized. The porous ceramic with the most favorable structure was obtained through autoclaving at 180 °C for 10 h with 30 wt% slaked lime, exhibiting a total pore volume of 0.11 mL/g, a specific surface area of 26.35 m2/g, and a mesoporous volume fraction of 90.40%.

from such residue, particularly under non-sintering conditions, remains in a nascent stage of exploration.The hydrothermal autoclaving method is characterized by low energy consumption due to its mid-to-low temperature operations, broad material applicability, precise reaction condition control, and an environmentally benign enclosed system for managing toxic reactions 17 .These features indicate its substantial potential in non-sintering conditions.
In this research, porous ceramics were synthesized via hydrothermal autoclaving method, employing porcelain tile polishing residue as the primary raw material and slaked lime (Ca(OH) 2 ) as a reinforcing agent.The composition, pore structure, and microstructure of the fabricated ceramics were thoroughly characterized through powder X-ray diffraction (PXRD), Brunauer-Emmett-Teller (BET) analysis, and scanning electron microscopy (SEM).The effect of hydrothermal autoclaving parameters (the amount of slaked lime added, the autoclaving temperature, and the reaction duration) on thermal and physical properties was analyzed through comprehensive balance method of orthogonal experiment design, which is adept at addressing multi-index optimization challenges, facilitating in harmonizing various objectives to derive an optimal, holistically performant solution [18][19][20] .This research not only establishes a theoretical foundation for the synthesis of porous ceramics as building insulation materials without sintering but also proposes an energy-efficient method for the utilization of porcelain tile polishing residue.

Raw materials
The primary raw material employed in this research was porcelain tile polishing residue, sourced from a production line of 600 mm × 1200 mm polished tiles in Monalisa Group Co., Ltd, utilizing diamond as the abrasive.Table 1 elucidates its chemical composition.Figure 1 depicts the mineral composition, while Fig. 2 shows the particle size distribution, with D10, D50, and D90 values of 1.5 μm, 7.1 μm, and 29.3 μm, respectively.Analyticalgrade slaked lime, procured from Tianjin Hongyan Chemical Company, served as the reinforcing agent.

Sample fabrication
Figure 3 illustrates the methodology utilized in producing porous ceramics from porcelain tile polishing residue.To fully react and form calcium silicate hydrate and tobermorite, the expected calcium to silicon ratio (Ca/Si) should be approximately 1.5 and 0.8, respectively 21,22 .However, such a high Ca/Si ratio is not essential, considering that porcelain tile polishing residue already possesses a porous structure.Thus, only a modest amount of CaO is required to produce a certain quantity of calcium silicate hydrate and tobermorite, which will enhance the mechanical properties of the specimen and improve its pore structure, ultimately maximizing the utilization of ceramic polishing waste.Consequently, the samples were synthesized by altering the slaked lime quantity-0, 10, 20, and 30 wt%-added to the porcelain tile polishing residue, designated as PTPR0, PTPR10, PTPR20, and PTPR30, respectively.After homogenous stirring of the reaction mixture, 10wt% water was incorporated for granulation.This was followed by an aging process lasting 12 h to ensure moisture uniformity, then molding at 25 MPa pressure, and subsequent drying.The resultant green bodies were then placed in a reactor and subjected to hydrothermal autoclaving at diverse temperatures (150, 180, or 210 °C) for varying durations (7, 10, or 13 h).
Figure 1.PXRD pattern of the polishing residue used in this study.
A three-factor, three-level orthogonal test design, L 9 (3 3 ), was employed to analyze the influence of three variables-the quantity of slaked lime added, the hydrothermal autoclaving temperature, and the reaction duration-on the compressive strength and apparent porosity of the samples.Table 2 outlines the orthogonal test design parameters.

Performance characterization
The apparent porosity of the samples was determined using Archimedes' principle in water, as outlined in Eq. (1): where m 1 is the mass of the dry sample, m 2 is the apparent mass of the saturated sample in water, and m 3 is the mass of the saturated sample in air.
The mechanical load that the samples can withstand was measured using a testing machine (CSS44100), at a loading rate of 0. 5 kN/s.The compressive strength was calculated using Eq. ( 2): (  where F is the maximum load and S is the area of the force applied to the sample.The thermal conductivity of the samples was measured using a thermal conductivity tester (Xiangtan Xiangyi Instrument Co., Ltd., DRH-III) was used.In the test, the temperatures of the hot and cold surfaces were maintained at 35 °C and 25 °C, respectively, with the sample dimensions being 300 mm × 300 mm × 30 mm.
The mineral composition of the sample was analyzed using an X-ray diffractometer (MDI, WJP75-91WJQ9).The micromorphology of the samples was examined using a scanning electron microscope (FEI, Quanta 600FEG).The pore size distribution and characteristic parameters of the samples were determined using an automatic surface and porosity analyzer (McMurray Instrument Co., Ltd, ASAP2460).

Compressive strength, apparent density, and thermal conductivity of porcelain tile polishing residue-based porous ceramics
Systematic orthogonal tests facilitated the preparation of various samples under specific hydrothermal autoclaving conditions.The indicators (compressive strength, apparent density, and thermal conductivity) of the samples were subsequently quantified.The results are presented in Table 3, where ' A' indicates the amount of slaked lime added, 'B' the autoclaving temperature, and 'C' the reaction duration.
Table 3 illustrates that the compressive strength of the specimens exhibits considerable variation across different factors and levels, achieving a maximum of 41.6 MPa.This strength is adequate for application in certain load-bearing wall structures.Conversely, the thermal conductivity demonstrates minimal fluctuation, ranging from 0.049 to 0.069, which is significantly lower than the maximum thermal conductivity of ≤ 0.23 required for building insulation materials.Furthermore, these values comply with the Chongqing region's building energy conservation standard of 65%.
Table 4 is the result of calculations performed on Table 3 using a comprehensive balance method.The findings are exhibited in Table 4, where K i represents the sum of indicators measured at an identical level for each factor, and ∆R denotes the range of the sum of K i .Figure 4 facilitated the elucidation of the relationship among each factor, level, and K value, as depicted in.
For instance, in analyzing compressive strength values, the compressive strength value corresponding to (K A1 ) is 27.7 MPa, which is the sum of the compressive strength results measured for factor A (the quantity of slaked lime added) at the same level 1 (10wt%): 6.3 MPa + 7.9 MPa + 13.5 MPa = 27.7 MPa.The range (∆R A ) is the difference between the maximum and minimum values among (K A1 ) (27.7 MPa), (K A2 ) (53.8 MPa), and (K A3 ) (79.8 MPa): 79.8-27.7 = 52.1 MPa.A larger ∆R indicates a greater impact of this factor on compressive strength.Because of ∆R A (52.1 MPa) > ∆R B (33.7 MPa) > ∆R C (20.1 MPa), the order of influence on compressive strength is: quantity of slaked lime added (A) > hydrothermal autoclaving temperature (B) > reaction duration (C).
For apparent porosity and thermal conductivity, the order is: reaction duration (C) > amount of slaked lime added (A) > hydrothermal autoclaving temperature (B).Distinct optimal solutions emerged for the three www.nature.com/scientificreports/properties; for compressive strength, the optimal solution is A3B3C2, and for apparent porosity and thermal conductivity, it is A1B3C2.Considering that A3 yields the highest strength, moderate porosity, and thermal conductivity, and A1 the lowest strength, the comprehensive analysis of optimal parameters for compressive strength, apparent porosity, and thermal conductivity led to the selection of A3B3C2 as the optimal solution.This corresponds to adding 30 wt% slaked lime, a hydrothermal autoclaving temperature of 210 °C, and a reaction duration of 10 h.

Pore structure characteristics of porcelain tile polishing residue-based porous ceramics
The hydrothermal autoclaving temperature exhibited a minimal influence on the compressive strength, apparent porosity, and thermal conductivity.In order to further determine the optimal experimental parameters of hydrothermal autoclaving temperatures, nitrogen adsorption measurements were carried out to characterize the pore structures of the samples prepared at different hydrothermal autoclaving temperatures for 10 h with the addition of 30 wt% slaked lime.Nitrogen adsorption-desorption isotherms of the PTPR30 samples, synthesized under these conditions, are depicted in Fig. 5. Figure 5 demonstrates that the isotherms exhibit typical type II adsorption characteristics 23 .The nitrogen adsorption-desorption isotherms of the PTPR, with slaked lime added before and after hydrothermal autoclaving, display distinct profiles, indicating the presence of desorption hysteresis.A pronounced 'hysteresis loop' emerges when the relative pressure P/P 0 surpasses 0.45, with the hysteresis area of the PTPR30 sample being most extensive at a hydrothermal autoclaving temperature of 180 °C.Consequently, it is evident that, following hydrothermal autoclaving, the PTPR30 sample exhibits mesoporous characteristics.
Regarding the polishing residue amended with slaked lime, the nitrogen adsorption-desorption isotherms of the samples show divergence pre-and post-hydrothermal autoclaving, indicative of desorption hysteresis.A marked hysteresis loop becomes evident when the relative pressure P/P 0 surpasses 0.45, with the hysteresis area being most significant for the PTPR30 sample at a hydrothermal autoclaving temperature of 180 °C.Consequently, it is apparent that the PTPR30 sample acquires mesoporous properties following autoclaving.
Table 5 delineates the characteristic parameters of the pore structures of the PTPR30 samples, ascertained at different hydrothermal autoclaving temperatures via the BET method.www.nature.com/scientificreports/As demonstrated in Table 5, an increase in the hydrothermal autoclaving temperature initially leads to an augmentation of the total pore volume and specific surface area, followed by a subsequent decline.Notably, at 180 °C, the PTPR30 sample exhibits the highest specific surface area, measured at 26.35 m 2 /g, coupled with a pore volume of 0.11 mL/g, an average pore diameter of 12.67 nm, and a mesoporous volume percentage of 90.40%.Remarkably, the specific surface area of this sample is 3.3 times that of the sample that underwent no hydrothermal autoclaving, and its total pore volume has also increased 2.7 times, signifying that hydrothermal autoclaving induces a significant pore expansion effect.Table 5 demonstrates that the pore volume reaches its zenith at 180 °C, resulting in the maximal apparent porosity observed macroscopically.In contrast, elevating the temperature to 210 °C diminishes the pore volume, indicating that the apex of apparent porosity is not achieved at the highest autoclaving temperature, but rather within a specific thermal range, proximate to 180 °C.
Integrating the data from Table 5 with the outcomes of the orthogonal tests, the optimal experimental parameters to achieve a material with superior physical properties-specifically, maximal porosity and mesoporous volume-were identified.The ideal conditions include a hydrothermal autoclaving temperature of 180 °C, a reaction duration of 10 h, and the addition of 30 wt% slaked lime.

Mineral composition of porcelain tile polishing residue-based porous ceramics
The phase compositions of the samples prepared under various reaction conditions were analyzed using powder X-ray diffraction.The results of these analyses are presented in Fig. 6.
Figure 6a displays the PXRD patterns of hydrothermally autoclaved samples with varying slaked lime content.Post-hydrothermal autoclaving, the distinct peak of calcium silicate hydrate is prominently observed at 2θ = 29.4°,indicative of the reaction between SiO 2 from the porcelain tile polishing residue and Ca(OH) 2 under hydrothermal conditions 24 .The presence of tobermorite (5CaO•6SiO 2 •5H 2 O) is evident with the addition of 20 wt% slaked lime.Furthermore, an increase in slaked lime content correlates with a heightened intensity of the tobermorite peak in the samples.This observation, coupled with the compressive strength data, implies that slaked lime promotes phase development and strengthens the samples 25 .As depicted in Fig. 6b, at 150 °C, only calcium silicate hydrate is formed, whereas the tobermorite peak appears when the hydrothermal autoclaving temperature exceeds 180 °C. Figure 6c reveals that over a period of 7-13 h, the characteristic peak intensity of tobermorite reaches its maximum value at 10 h, indicating that it is the optimal reaction duration conducive to the formation of this phase.

Microstructure of porcelain tile polishing residue-based porous ceramics
The PTPR30 samples were observed by SEM at different hydrothermal autoclaving temperatures, the microstructures of which are shown in Fig. 7. Figure 7a depicts the sample surface exhibiting a porous morphology.The BET results corroborate that materials from porcelain tile polishing residue inherently exhibit a porous structure, even before autoclaving.Figure 7b reveals that at a hydrothermal autoclaving temperature of 150 ℃, the reaction between SiO 2 from the porcelain tile polishing residue and slaked lime leads to the formation of calcium silicate hydrate, characterized by sheet-like particles, which markedly improves the samples' strength.Figure 7c demonstrates that at a hydrothermal autoclaving temperature of 180 °C, tobermorite forms with a needle-like structure.This phase not only enhances the strength but also significantly increases the porosity of the samples, as confirmed by the pore volume data in Table 5.In contrast, Fig. 7d shows that at a temperature of 210 °C, the ongoing production of calcium silicate hydrate results in a denser structure with improved strength, yet with reduced porosity, as indicated by the lower pore volume in Table 5.

Conclusions
Upon reaching a slaked lime content of 30 wt% in the reactions, and setting the hydrothermal autoclaving temperature and duration to 180 °C and 10 h respectively, the resulting porcelain tile polishing residue-based porous ceramic exhibits optimal physical properties and pore structure.This is substantiated by a total pore volume of 0.11 mL/g, a specific surface area of 26.35 m 2 /g, and a mesoporous volume fraction of 90.40%.
After the addition of slaked lime and hydrothermal autoclaving, a novel calcium silicate hydrate phase materializes within the porous ceramic.When the amount of lime added increases from 20wt% to 30wt%, characteristic peaks of tobermorite appear in the sample, correlating with an enhancement in sample strength.At an autoclaving temperature of 150 °C, only calcium silicate hydrate with a sheet-like structure form.In contrast, temperatures of 180 °C or above induce the formation of a tobermorite phase with a needle-like structure.The peak intensity of the tobermorite is observed to culminate at a reaction duration of 10 h.

Table 2 .
Factor levels of the orthogonal tests.

Table 3 .
Results of orthogonal tests on the samples prepared in this work.

Table 4 .
Analysis of the experimental results for the samples prepared in this study.

Table 5 .
Characteristic parameters of the pore structures of the PTPR30 samples at different hydrothermal autoclaving temperatures measured using the BET method.