Optimization Research of Sodium Hydroxide Pretreatment to Enhance the Thermal Properties of Straw–Mortar Composite Materials

: Although straw is being utilized as an additive in construction materials, the thermal properties of straw and building materials in combination are insufficient. The thermal properties of straw–mortar composite materials can be improved by the pretreatment of straw. The alkali treatment enhances the mechanical attachment between the fibers and the matrix material, assuring that the straw–mortar composite materials have solid thermal insulation characteristics. Pretreatment with sodium hydroxide was utilized in this work to enhance the thermal properties of straw–mortar composite materials. This study mainly investigated the thermal properties of straw–mortar composite material after sodium hydroxide pretreatment and its change rules under the condition of the freeze–thaw cycle. A three-factor, three-level Box–Behnken experimental design, with the straw content (%), pretreatment time (min), and reagent concentration (%) as process parameters, was used. The response variables were the thermal conductivity, thermal diffusivity, and thermal resistance. The findings revealed that all of the variables had a substantial impact on the replies. Optimization parameters of 17.95% for the straw content, 19.50 min for the pretreatment time, and 4.99% for the reagent concentration for the straw–mortar composite materials were achieved. A thermal conductivity of 0.211 W · (m · K) − 1 , a thermal diffusivity of 0.277 mm 2 · s − 1 , and a thermal resistance of 57.211 K · W − 1 were the optimal thermal property indices. Furthermore, during the freeze–thaw cycle, the thermal conductivity coefficient and thermal diffusion coefficient of the combined pretreatment composite were 26% and 9% lower than the materials without the treatment. The thermal performance of the mortar composites prepared by alkali-treated straw was better than that prepared by untreated straw.


Introduction
It is critical to develop alternative construction materials with minimal environmental impact and cost.Biological materials provide significant benefits for modern structures as part of integrating sustainable development strategies.Every year, China generates a considerable amount of straw.However, straw-use technology is not matching the demand, resulting in a significant impact on environmental and human health [1,2].
Straw is a lightweight, porous natural material with good thermal insulating characteristics.It provides a means to store carbon for traditional building materials such as concrete and to reduce the reliance on other energy-intensive materials.Adding straw fiber to standard building materials can improve the thermal insulation performance of composite materials by 85-90% and reduce the thermal conductivity of composite materials [3][4][5][6][7][8].Many factors affect the thermal properties of composite materials, including the straw content, pretreatment time, sodium hydroxide concentration, straw variety, etc. Kammoun and Trabelsi studied the incorporation of straw into concrete, and concluded Sustainability 2024, 16, 5239 2 of 18 that the thermal conductivity and thermal diffusion coefficient decreased with the increase in the straw content [8,9].Pachla researched the sound absorption performance and heat insulation performance of porous concrete when the straw content was 15% [10].Sun investigated straw pretreatment times of 3, 6, and 9 h, respectively, and concluded that the longer the straw treatment time, the greater the mass loss rate of the composite material [11].Liang discussed the feasibility of preparing straw as a composite insulation material under the condition of a pretreatment time of 30 min [12].Mittal found that 3% lye-treated straw can improve the thermal stability of composites [13].Ashour used NaOH to pretreat rice straw to improve composite concrete.The results showed that treating rice straw with 1% NaOH had a better bonding effect with the surrounding matrix, and the straw incorporation was 1%, 2%, and 3% [14].Meanwhile, the tensile strength and flexural strength of the composite material decreased with the increasing concentration of alkaline pretreatment [15].Examining the influence of straw varieties on the properties of composite materials, the authors showed that the influence of straw varieties was not obvious [16].In summary, the three factors of the straw dosage, pretreatment time, and concentration have the greatest impact on the composites.Furthermore, adding plant fiber to composite materials might increase their freezing resistance [17][18][19].Using straw fiber and other construction materials to create new green and ecologically friendly composite building materials has considerable economic and environmental advantages [20,21].
On the other hand, the quality of the straw, but also the thermal diffusivity, could be improved by pretreatment [22,23].After heat treatment and modified binder treatment, the thermal conductivity of straw could be reduced by 8-11% [24].The thermal properties of pretreated straw composites were good.The existing pretreatments include physical pretreatment, biological pretreatment, and chemical pretreatment.Straw contains large amounts of cellulose, hemicellulose, and lignin, and 15 g of straw is hydrolyzed to extract 46.88% by weight of cellulose sugar.The literature suggested that the NaOH solution was the most effective method to remove cellulosic sugars.The alkaline solution acted as a binder, alkali activator, and dispersant.Additionally, the alkaline pretreatment could remove the wax and other impurities on the fiber surface [25][26][27][28].Alkalinized straw fiber was introduced as a modifying agent into a 3D-printed cement matrix composite material, and the fluidity, availability, and mechanical properties of the alkalized straw fiber composite material were improved compared with the natural-straw-fiber group [29].However, the high crystallinity of cellulose and the complicated aggregate structure of straw-mortar composite materials decreased their characteristics [30,31].As a result, the problem of weak interfacial adhesion when straw fiber and cement-based materials were joined to generate straw-mortar composite materials appeared [21].The pretreatment of straw could increase the porosity of straw-mortar composite materials and improve the interfacial compatibility between straw and construction materials.The thermal insulation properties of composite materials made with pretreated rice straw fiber were superior than composite materials made without the pretreatment [32,33].For the high-property modification of plant fibers, there are now physical, chemical, biological, and combination techniques [34].Sodium hydroxide treatment provided the best pretreatment impact on rice straw among these approaches [35].Therefore, we used alkali treatment, and the conventional pretreatment was sodium hydroxide.Rice straw treated with NaOH could improve the durability of bricks [36].The alkali treatment eliminated the majority of non-cellulosic compounds from the surface of the straw, resulting in a substantially rougher surface on the corn stalks [37][38][39].When straw and building materials were composited to form straw-mortar composite materials, the surface of the straw fiber treated with sodium hydroxide was rougher, which overcame the problem of the poor interface adhesion.Furthermore, following the alteration, the internal porosity of the straw-mortar composite materials was enhanced, and their thermal insulation properties were improved.However, at present, it is rare to comprehensively study the thermal conductivity, thermal diffusivity, and thermal resistance coefficient of straw-mortar composites after considering the comprehensive action of factors such as the straw content, pretreatment time, and concentration.Therefore, we studied these and hoped to obtain the optimal thermal performance parameters under the values of each factor.
The primary goal of this research was to evaluate the thermal characteristics of strawmortar composite materials following pretreatment with sodium hydroxide and to determine the best mortar formulation.Previous research on the surface changes in straw fibers yielded excellent findings regarding the fiber surface activity following alkali treatment [40].The application of the sodium hydroxide pretreatment of rice straw fiber in cement mortar will be explained in this study based on the findings.Thermal conductivity, thermal diffusivity, and thermal resistance in normal-and negative-temperature settings were studied in relation to the straw content, pretreatment time, and reagent concentration.This study provides a theoretical basis for the pretreatment of composite mortar material as a non-load-bearing energy-saving wall material.

Experimental Design
Table 1 shows the actual values and coded factor levels of the independent variables in this study.The analysis of variance (ANOVA) and response surface technique were performed using Design-Expert 12 software (RSM).According to the three-factor Box-Behnken experimental design, we performed 17 groups of tests, with each group of tests having three specimens, and each specimen was measured for its thermal conductivity, thermal diffusivity, and thermal resistance coefficient.Then, they were measured separately under freeze-thaw cycle conditions.Finally, the results were analyzed, and the optimal treatment conditions and thermal parameters were obtained.The influence of the straw content, pretreatment time, and reagent concentration on the thermal conductivity, thermal diffusivity, and thermal resistance of straw-mortar composite materials are shown in the regression equations.RSM's key benefit is that it can examine several factors and their interactions with only a few tests.As a result, RSM was used to optimize the process parameters so that the response variables (the thermal conductivity and thermal diffusivity) were minimized while the thermal resistance was maximized.The straw content, pretreatment time, and reagent concentration were all investigated.Here, x 1 is the straw content (% mass fraction), x 2 is the pretreatment time (min), and x 3 is the reagent concentration (% mass fraction).

Rice Straw
In October 2020, mature rice straw was harvested from Shenyang Agricultural University's paddy field (123.34 • E, 41.50 • N).Rice straw that was not moldy was chosen for the drying process, and the moisture level was kept below 10%.Rice straw roots and ears that were no longer needed were removed.

Sand
Natural river sand from Shenyang's Hun River was used in the experiment.

Water
The testing water was typical tap water from Shenyang Agricultural University's laboratory, with a pH of around 7.

NaOH Reagent
This investigation employed a white solid-particle Macklin sodium hydroxide reagent (S835850-500g, Shanghai Chemical Industrial Park, Shanghai, China) with a component concentration of at least 99.5 percent.It is water-soluble, dissolves exothermically, and very corrosive.

Pretreatment
Rice straw was crushed with a straw grinder (FS-100, Xingyang Chuangyou Machinery Equipment Co., Ltd., Zhengzhou, Henan, China) and soaked in sodium hydroxide solutions of varying durations and mass fractions with a solid-liquid ratio of 1:15.The straw was removed after being soaked and washed until it was neutral, then dried and placed in a moisture-proof container for subsequent use, as illustrated in Figure 1.In October 2020, mature rice straw was harvested from Shenyang Agricultural University's paddy field (123.34°E, 41.50° N).Rice straw that was not moldy was chosen for the drying process, and the moisture level was kept below 10%.Rice straw roots and ears that were no longer needed were removed.

Sand
Natural river sand from Shenyang's Hun River was used in the experiment.

Water
The testing water was typical tap water from Shenyang Agricultural University's laboratory, with a pH of around 7.

NaOH Reagent
This investigation employed a white solid-particle Macklin sodium hydroxide reagent (S835850-500g, Shanghai Chemical Industrial Park, Shanghai, China) with a component concentration of at least 99.5 percent.It is water-soluble, dissolves exothermically, and very corrosive.

Pretreatment
Rice straw was crushed with a straw grinder (FS-100, Xingyang Chuangyou Machinery Equipment Co. Ltd., Zhengzhou, Henan, China) and soaked in sodium hydroxide solutions of varying durations and mass fractions with a solid-liquid ratio of 1:15.The straw was removed after being soaked and washed until it was neutral, then dried and placed in a moisture-proof container for subsequent use, as illustrated in Figure 1.(2) Thermal resistance The specimen's thermal resistance was determined using Formula (1).According to the formula, the greater the thermal resistance, the less heat travels through the wall at the same temperature difference circumstances.
Where: R -Thermal resistance of the straw composite material, K•W −1 ; δ -Thickness of the straw composite material, m; λ -Thermal conductivity of the straw composite material, W•(m•K) −1 ; A -Vertical temperature gradient area, m 2 .

The Freeze-Thaw Cycle
A freeze-thaw cycle of 12 h of freezing and 12 h of thawing was used in this experiment.Figure 3 depicts a freeze-thaw cycle device.A star refrigerator (Zhejiang Xingxing Cold Chain Integration Co., Ltd., Taizhou, Zhejiang, China) and a SM5-LCD (Shanghai Minrong Electric Group Co., Ltd., Shanghai, China) high-precision intelligent temperature controller were among the equipment used in the freeze-thaw cycle.(2) Thermal resistance

Results and Discussion
The specimen's thermal resistance was determined using Formula (1).According to the formula, the greater the thermal resistance, the less heat travels through the wall at the same temperature difference circumstances.
where: R-Thermal resistance of the straw composite material, K•W −1 ; δ-Thickness of the straw composite material, m; λ-Thermal conductivity of the straw composite material, W•(m•K) −1 ; A-Vertical temperature gradient area, m 2 .

The Freeze-Thaw Cycle
A freeze-thaw cycle of 12 h of freezing and 12 h of thawing was used in this experiment.Figure 3 depicts a freeze-thaw cycle device.A star refrigerator (Zhejiang Xingxing Cold Chain Integration Co., Ltd., Taizhou, Zhejiang, China) and a SM5-LCD (Shanghai Minrong Electric Group Co., Ltd., Shanghai, China) high-precision intelligent temperature controller were among the equipment used in the freeze-thaw cycle.(2) Thermal resistance The specimen's thermal resistance was determined using Formula (1).According to the formula, the greater the thermal resistance, the less heat travels through the wall at the same temperature difference circumstances.
Where: R -Thermal resistance of the straw composite material, K•W −1 ; δ -Thickness of the straw composite material, m; λ -Thermal conductivity of the straw composite material, W•(m•K) −1 ; A -Vertical temperature gradient area, m 2 .

The Freeze-Thaw Cycle
A freeze-thaw cycle of 12 h of freezing and 12 h of thawing was used in this experiment.Figure 3

Results and Discussion
Table 3 shows the outcomes of the experimental runs.The rice straw processed with sodium hydroxide had a mean thermal conductivity ranging from 0.252 W•(m•K) −1 to 0.612 W•(m•K) −1 .Meanwhile, the thermal diffusivity ranged from 0.289 mm 2 •s −1 to 0.623 mm 2 •s −1 , and the thermal resistance varied from 23.112 K•W −1 to 56.128 K•W −1 , respectively.
Table 3. Experimental design scheme and results.
The thermal conductivity and thermal diffusivity were inversely related to the increase in the straw content, pretreatment time, and reagent concentration, as shown in Table 3.The increase in the straw content, pretreatment time, and reagent concentration all increased the heat resistance.

Analysis of Variance for Experimental Results (ANOVA)
The summary of the analysis of variance (ANOVA) of the response variables (the thermal conductivity, thermal diffusivity, and thermal resistance, respectively) as affected by the straw content, pretreatment time, and reagent concentration after the stepwise regression are presented in Table 4.The F-value is an essential indication in the analysis of variance, and the p-value represents the confidence level.The p-value determines the model's significance, and the p-value number determines the significance of the regression equation.The experimental analysis' conclusions are more dependable, as seen by the greater F-value and smaller p-value [41].
The probability values indicated that the models were significant.The lack-of-fit was statistically insignificant, as the p-values were larger than 0.05.An insignificant lack-offit suggests that the model fits the data well.Statistical analysis revealed that the straw content, pretreatment time, and reagent concentration all had a significant impact on all of the responses.All of the responses were significantly impacted by the interaction between the straw content and the pretreatment time, straw content, and reagent concentration.Except for the thermal diffusivity, the interaction between the pretreatment time and reagent concentration had a substantial impact on all of the responses.The pretreatment period had a substantial impact on all of the responses in the quadratic term.The reagent concentration significantly affected all the responses, except for the thermal resistance.The straw content significantly affected the thermal diffusivity.Here, x 1 is the straw content (% mass fraction), x 2 is the pretreatment time (min), and x 3 is the reagent concentration (% mass fraction); "*" means generally significant difference at the p < 0.05 level; "**" means extremely significant difference at the p < 0.01 level.
Based on Table 4, it was indicated that the models sufficiently described the response surface of the thermal conductivity, thermal diffusivity, and thermal resistance.The response surface model for optimization and prediction was considered reasonable.The final regression model for the response variables and the corresponding coefficient of multiple determination (R 2 ) are shown in Table 5.The determination coefficient R 2 reflected the fitness of the model.The R 2 value was closer to one, and had the better correlation between the experimental value and the predicted value.

Effect of Interaction of Various Factors on Thermal Properties 3.2.1. Effect of Interaction of Various Factors on the Thermal Conductivity
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal conductivity generated by the Design-Expert 12 software are shown in Figure 4.
Thermal diffusivity

Effect of Interaction of Various Factors on the Thermal Conductivity
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal conductivity generated by the Design-Expert 12 software are shown in Figure 4.   Based on Figure 4, with the increase in the straw content, the higher reagent concentration and the longer pretreatment time led to the thermal conductivity of the straw-mortar composite materials decreasing faster.With the increase in the pretreatment time, the higher straw content and the higher reagent concentration led to the thermal conductivity of the straw-mortar composite materials gradually decreasing slowly.With the increase in the reagent concentration, the higher straw content and the longer pretreatment time led to the thermal conductivity of the straw-mortar composite materials decreasing faster.
The addition of straw fibers to cement mortar enhanced the porosity of the strawmortar composite materials, lowering their thermal conductivity.The overall characteristics of the straw-mortar composite materials were influenced by the contact between the straw fiber and the matrix [42].The surface roughness of the straw fiber was improved after the alkali treatment, and the surface of the straw fiber was uneven, which increased the interface compatibility.The surface pore structure of the straw improved the porosity of the straw-mortar composite materials [43,44].The thermal stability of the composite materials was shown to be inextricably linked to the interface compatibility.The thermal stability improved as the interface compatibility improved [45].

Effect of Interaction of Various Factors on the Thermal Diffusivity
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal diffusivity generated by the Design−Expert 12 software are shown in Figure 5.
stability improved as the interface compatibility improved [45].

Effect of Interaction of Various Factors on the Thermal Diffusivity
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal diffusivity generated by the Design−Expert 12 software are shown in Figure 5. Based on Figure 5, with the increase in the straw content, the longer pretreatment time, and the higher reagent concentration, the thermal diffusivity of the straw-mortar composite materials gradually decreased rapidly.With the increase in the pretreatment time and the higher straw content, the thermal diffusivity of the straw-mortar composite materials gradually decreased slowly.With the increase in the reagent concentration and the higher straw content, the thermal diffusivity of the straw-mortar composite materials gradually decreased slowly.
The straw fiber was a hydrophilic porous material, as was the straw-mortar composite material.The water absorption of the straw-mortar composite materials was decreased when the straw fiber was prepared with sodium hydroxide.Water absorption was favorably associated with the thermal insulation properties of the straw-mortar composite materials.The thermal insulating properties of the straw-mortar composite materials Based on Figure 5, with the increase in the straw content, the longer pretreatment time, and the higher reagent concentration, the thermal diffusivity of the straw-mortar composite materials gradually decreased rapidly.With the increase in the pretreatment time and the higher straw content, the thermal diffusivity of the straw-mortar composite materials gradually decreased slowly.With the increase in the reagent concentration and the higher straw content, the thermal diffusivity of the straw-mortar composite materials gradually decreased slowly.
The straw fiber was a hydrophilic porous material, as was the straw-mortar composite material.The water absorption of the straw-mortar composite materials was decreased when the straw fiber was prepared with sodium hydroxide.Water absorption was favorably associated with the thermal insulation properties of the straw-mortar composite materials.The thermal insulating properties of the straw-mortar composite materials will improve as the water absorption decreases.Plant-fiber concrete contains greater voids than normal concrete.The presence of convective heat transfer in the gas inside the pores and between the pore walls increased the thermal insulation properties of the straw-mortar composite materials [46].The pretreatment altered the contact interface of the straw surface, allowing the straw fiber to better interact with the mortar [47,48].

Effect of Interaction of Various Factors on Thermal Resistance
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal resistance generated by the Design-Expert 12 software are shown in Figure 6.
Based on Figure 6, with the increase in the straw content, the longer pretreatment time, and the higher reagent concentration, the thermal resistance of the straw-mortar composite materials increased significantly.With the increase in the pretreatment time, the thermal resistance of the straw-mortar composite materials increased first and then decreased when the straw content was low.The thermal resistance of the straw-mortar composite materials gradually increased slowly when the straw content was high.With the increase in the reagent concentration, the thermal resistance of the straw-mortar composite materials gradually increased slowly.With the increase in the reagent concentration, the more straw content, and the longer pretreatment time, the thermal resistance of the straw-mortar composite materials increased rapidly.
The thermal resistance of the straw-mortar composite materials was linked to their structure, density, and moisture content.The thermal resistance of the straw-mortar composite materials was inversely proportional to the density.The shape of the straw fiber was enhanced by the pretreatment.To lower the density of the straw-mortar composite materials, prepared straw fibers were added to the cement matrix [49].The porosity of the straw fiber composite materials was greater than that of the regular concrete, whereas air had a higher thermal conductivity than the straw fiber composite materials.As a result, the thermal insulation properties of the straw-mortar composite materials improved as the straw content increased [50].The straw pretreatment successfully reduced the heat transfer and increased the thermal insulation of the straw-mortar composite materials.
will improve as the water absorption decreases.Plant-fiber concrete contains greater voids than normal concrete.The presence of convective heat transfer in the gas inside the pores and between the pore walls increased the thermal insulation properties of the straw-mortar composite materials [46].The pretreatment altered the contact interface of the straw surface, allowing the straw fiber to better interact with the mortar [47,48].

Effect of Interaction of Various Factors on Thermal Resistance
Based on the results of Tables 4 and 5, the interaction effects of the independent variables on the response of the thermal resistance generated by the Design-Expert 12 software are shown in Figure 6.Based on Figure 6, with the increase in the straw content, the longer pretreatment time, and the higher reagent concentration, the thermal resistance of the straw-mortar composite materials increased significantly.With the increase in the pretreatment time, the thermal resistance of the straw-mortar composite materials increased first and then decreased when the straw content was low.The thermal resistance of the straw-mortar composite materials gradually increased slowly when the straw content was high.With the increase in the reagent concentration, the thermal resistance of the straw-mortar composite materials gradually increased slowly.With the increase in the reagent concentration, the more straw content, and the longer pretreatment time, the thermal resistance of the straw-mortar composite materials increased rapidly.

Optimization and Model Validation
Table 6 shows the optimization objectives.Thermal conductivity and thermal diffusivity are to be minimized and thermal resistance is to be maximized.The most significant response variable was the thermal conductivity, which is a useful predictor of heat transmission during the heat transfer of the straw-mortar composite materials.Thermal diffusivity was then determined, since a low value indicates high thermal insulating properties.For variable objectives, the straw content, pretreatment time, and reagent concentration were set to "in range".Table 7 displays the optimization option findings provided by the Design Expert software.The optimal factor levels of the straw-mortar composite materials were as follows: a straw content of 17.95% (mass fraction), a pretreatment time of 19.50 min, and a reagent concentration of 4.99% (mass fraction).The straw-mortar composite materials had a theoretical thermal conductivity, thermal diffusivity, and thermal resistance of 0.211 W (m•K) −1 , 0.277 mm 2 •s −1 , and 57.211 K•W −1 , respectively.The best procedure for the straw-mortar composite materials was established to be a straw content of 18% (mass fraction), a pretreatment time of 20 min, and a reagent concentration of 5% (mass fraction).To verify the optimized optimal conditions, the thermal conductivity was 0.221 W•(m•K) −1 , the thermal diffusion coefficient was 0.297 mm 2 •s −1 , and the thermal resistance was 53.458 K•W −1 .Table 8 shows the experimental verification of the optimal conditions after the optimization.The thermal conductivity, thermal diffusivity, and thermal resistance were verified to be close to the predicted values, and the relative errors were 1.8%, 2.4%, and 3.7%, respectively.The regression model established by the Design Expert 8.0.6 software was optimized and designed accurately.The experimental results showed that the pretreatment significantly improved the thermal properties of the straw-mortar composite materials.The thermal insulation properties of the straw fiber concrete were better than that of the ordinary concrete and the filament straw fiber was better than that of the rod.When the diameter of the straw fiber was smaller, the thermal insulation properties of the material were stronger [51,52].These conclusions are consistent with the results of this study.

Thermal Properties of Composite Materials after Freeze-Thaw Cycles
The thermal conductivity represents the capacity of a material to transfer heat [53].Thermal diffusivity is a measure of the rate at which a disturbance in the temperature at one point in an object is transmitted to another [54].Thermal conductivity and thermal diffusivity can well reflect the thermal insulation properties of the straw-mortar composite materials.Table 9 shows the thermal conductivity and thermal diffusivity of the strawmortar composite materials after 10, 20, and 30 freeze-thaw cycles with the pretreated straw.The thermal conductivity of the straw-mortar composite materials after different freeze-thaw cycles varied from 0.245 to 0.599 W•(m•K) −1 , 0.228 to 0.584 W•(m•K) −1 , and 0.197 to 0.558 W•(m•K) −1 , respectively.The thermal diffusivity varied from 0.281 to 0.613 mm 2 •s −1 , 0.264 to 0.593 mm 2 •s −1 , and 0.235 to 0.564 mm 2 •s −1 , respectively.When the straw content was 10%, 14%, and 18%, the thermal conductivity decreased by 12.04%, 8.24%, and 13.63%, and the thermal diffusion coefficient decreased by 9.60%, 10.05%, and 12.87%, respectively.Table 9. Experimental results of composite materials with the pretreatment after freeze-thaw cycles.

Number
x 1 (%) x 2 (min) Table 10 shows the thermal conductivity and thermal diffusivity of the straw-mortar composite materials without the pretreatment after 10, 20, and 30 freeze-thaw cycles.The thermal conductivity of the straw-mortar composite materials after different freezethaw cycles varied from 0.716 to 0.852 W•(m•K) −1 , 0.693 to 0.832 W•(m•K) −1 , and 0.647 to 0.796 W•(m•K) −1 , respectively.The thermal diffusivity varied from 0.750 to 0.906 mm 2 •s −1 , 0.724 to 0.819 mm 2 •s −1 , and 0.673 to 0.850 mm 2 •s −1 , respectively.When the straw content was 10%, 14%, and 18%, the thermal conductivity decreased by 7.66%, 8.55%, and 11.44%, and the thermal diffusion coefficient decreased by 6.94%, 8.82%, and 12.48%, respectively.It can be seen from Tables 9 and 10 that, after 10, 20, and 30 freeze-thaw cycles, the thermal conductivity and thermal diffusivity of the rice straw mortar composite material both decreased somewhat, and the reduction range of the thermal conductivity and thermal diffusivity of the composite material combined with the pretreatment was greater than 26% and 9% of the materials without the treatment, respectively.The results showed that the freeze-thaw cycle could further improve the thermal conductivity and thermal diffusivity of the composites, and the combined pretreatment was significantly better than the materials without the treatment, and these values increased with the increase in the number of freeze-thaw cycles.

Analysis of Thermal Conductivity of Composite Materials after Freeze-Thaw Cycles
Figure 7 shows the variation amplitude of the thermal conductivity of the pretreated straw-mortar composite materials after freeze-thaw cycles.After 10, 20, and 30 freeze-thaw cycles, the decreasing amplitude of the thermal conductivity of the pretreated straw-mortar composite materials varied from 0.007 to 0.012 W•(m•K) −1 , 0.020 to 0.034 W•(m•K) −1 , and 0.054 to 0.069 W•(m•K) −1 , respectively.Based on Figures 7 and 8, the thermal conductivity of the straw-mortar composite materials decreased after 10, 20, and 30 freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the pretreated straw-mortar composite materials was greater than that of the untreated straw-mortar composite materials under different freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the straw-mortar composite materials increased with the increase in freeze-thaw cycles.Moreover, the thermal conductivity of the pretreated straw-mortar composite materials decreased more than the    Based on Figures 7 and 8, the thermal conductivity of the straw-mortar composite materials decreased after 10, 20, and 30 freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the pretreated straw-mortar composite materials was greater than that of the untreated straw-mortar composite materials under different freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the straw-mortar composite materials increased with the increase in freeze-thaw cycles.Moreover, the thermal conductivity of the pretreated straw-mortar composite materials decreased more than the Based on Figures 7 and 8, the thermal conductivity of the straw-mortar composite materials decreased after 10, 20, and 30 freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the pretreated straw-mortar composite materials was greater than that of the untreated straw-mortar composite materials under different freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the straw-mortar composite materials increased with the increase in freeze-thaw cycles.Moreover, the thermal conductivity of the pretreated straw-mortar composite materials decreased more than the untreated ones.
The results showed that the thermal insulation properties of the straw-mortar composite materials improved with the increase in the freeze-thaw cycles.Moreover, the thermal insulation properties of the pretreated straw-mortar composite materials were better than the untreated ones.
During the freeze-thaw cycles, the water inside the composite materials transitioned between liquid and solid.As well as the moisture migration caused by the temperature gradient inside and outside of the samples during the freeze-thaw process, the internal voids of the composite materials were increased.The cyclic pressure of the migration of moisture in the internal void structure of the composite materials was the main reason for the creation of more and larger pores inside the composite materials.The main role of the fiber was to change the quality of the pore structure inside the concrete, resulting in more pores with a poor connectivity inside the fiber concrete [55].The pores present at the junction of the straw fibers and composite materials were especially weak areas.As the number of freeze-thaw cycles increased, subtle cracks formed, which led to the better thermal insulation properties of the straw-mortar composite materials [56].

Analysis of Thermal Diffusivity of Composite Materials after Freeze-Thaw Cycles
Figure 9 shows the variation amplitude of the thermal diffusivity of the pretreated straw-mortar composite materials after freeze-thaw cycles.After 10, 20, and 30 freeze-thaw cycles, the decreasing amplitude of the thermal diffusivity of the pretreated straw-mortar composite materials varied from 0.007 to 0.015 mm 2 •s −1 , 0.023 to 0.038 mm 2 •s −1 , and 0.050 to 0.065 mm 2 •s −1 , respectively.untreated ones.The results showed that the thermal insulation properties of the strawmortar composite materials improved with the increase in the freeze-thaw cycles.Moreover, the thermal insulation properties of the pretreated straw-mortar composite materials were better than the untreated ones.
During the freeze-thaw cycles, the water inside the composite materials transitioned between liquid and solid.As well as the moisture migration caused by the temperature gradient inside and outside of the samples during the freeze-thaw process, the internal voids of the composite materials were increased.The cyclic pressure of the migration of moisture in the internal void structure of the composite materials was the main reason for the creation of more and larger pores inside the composite materials.The main role of the fiber was to change the quality of the pore structure inside the concrete, resulting in more pores with a poor connectivity inside the fiber concrete [55].The pores present at the junction of the straw fibers and composite materials were especially weak areas.As the number of freeze-thaw cycles increased, subtle cracks formed, which led to the better thermal insulation properties of the straw-mortar composite materials [56].

Analysis of Thermal Diffusivity of Composite Materials after Freeze-Thaw Cycles
Figure 9 shows the variation amplitude of the thermal diffusivity of the pretreated straw-mortar composite materials after freeze-thaw cycles.After 10, 20, and 30 freezethaw cycles, the decreasing amplitude of the thermal diffusivity of the pretreated strawmortar composite materials varied from 0.007 to 0.015 mm 2 •s −1 , 0.023 to 0.038 mm 2 •s −1 , and 0.050 to 0.065 mm 2 •s −1 , respectively.Figure 10 shows the variation amplitude of the thermal diffusivity of the untreated straw-mortar composite materials after freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the untreated straw-mortar composite materials varied from 0.003 to 0.007 mm 2 •s −1 , 0.015 to 0.026 mm 2 •s −1 , and 0.041 to 0.051 mm 2 •s −1 , respectively, after 10, 20, and 30 times freeze-thaw cycles.Figure 10 shows the variation amplitude of the thermal diffusivity of the untreated straw-mortar composite materials after freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the untreated straw-mortar composite materials varied from 0.003 to 0.007 mm 2 •s −1 , 0.015 to 0.026 mm 2 •s −1 , and 0.041 to 0.051 mm 2 •s −1 , respectively, after 10, 20, and 30 times freeze-thaw cycles.
Based on Figures 9 and 10, the thermal diffusivity of the straw-mortar composite materials decreased after 10, 20, and 30 freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the pretreated straw-mortar composite materials was greater than that of the untreated straw-mortar composite materials under different freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the straw-mortar composite materials increased with the increase in the freeze-thaw cycles.Moreover, the thermal diffusivity of the pretreated straw-mortar composite materials decreased more than the untreated ones.Based on Figures 9 and 10, the thermal diffusivity of the straw-mortar composite materials decreased after 10, 20, and 30 freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the pretreated straw-mortar composite materials was greater than that of the untreated straw-mortar composite materials under different freeze-thaw cycles.The decreasing amplitude of the thermal diffusivity of the straw-mortar composite materials increased with the increase in the freeze-thaw cycles.Moreover, the thermal diffusivity of the pretreated straw-mortar composite materials decreased more than the untreated ones.
After freeze-thaw cycles, the pores of the cement-based composite materials were subjected to frost-heaving pressure and water pressure, resulting in pore enlargement [57].At low temperatures, the freezing of the pore water in the straw fiber concrete caused volume expansion, creating swelling stress on the pore walls in the composite materials.When the expansion stress exceeded the cementation stress of the composite materials, irreversible fine cracks occurred.In the melting state, the ice melted into water, and water from the outside entered the cracks.After refreezing, the cracks will develop even more [58].A large number of pores and fine cracks reduced the heat transfer rate of the strawmortar composite materials [59].Moreover, the surface of the pretreated straw was rougher, leaving more and larger pores in the straw-mortar composite materials after the freeze-thaw cycles.Therefore, the thermal properties of pretreated straw-mortar composite materials were better than those of untreated straw-mortar composite materials after freeze-thaw cycles.

Conclusions
The three-factor Box-Behnken experimental design technique, with the straw content (% mass fraction), pretreatment time (min), and reagent concentration (% mass fraction) as process parameters, was used to predict the thermal conductivity, thermal diffusivity, and thermal resistance of straw-mortar composite materials produced with straw pretreated by sodium hydroxide.The models were developed and subjected to diagnostic analysis.The following conclusions were obtained: (1) Each process parameter had a significant effect on the responses, and the primary and secondary relationships of each parameter on the responses included a straw content that was greater than the reagent concentration and pretreatment time.The response surface methodology was used to optimize all of the pretreatment parameters using selected response variables.The optimal combination for making the rice straw-mortar composite materials produced with the pretreated rice straw showed that a high straw content After freeze-thaw cycles, the pores of the cement-based composite materials were subjected to frost-heaving pressure and water pressure, resulting in pore enlargement [57].At low temperatures, the freezing of the pore water in the straw fiber concrete caused volume expansion, creating swelling stress on the pore walls in the composite materials.When the expansion stress exceeded the cementation stress of the composite materials, irreversible fine cracks occurred.In the melting state, the ice melted into water, and water from the outside entered the cracks.After refreezing, the cracks will develop even more [58].A large number of pores and fine cracks reduced the heat transfer rate of the straw-mortar composite materials [59].Moreover, the surface of the pretreated straw was rougher, leaving more and larger pores in the straw-mortar composite materials after the freeze-thaw cycles.Therefore, the thermal properties of pretreated straw-mortar composite materials were better than those of untreated straw-mortar composite materials after freeze-thaw cycles.

Conclusions
The three-factor Box-Behnken experimental design technique, with the straw content (% mass fraction), pretreatment time (min), and reagent concentration (% mass fraction) as process parameters, was used to predict the thermal conductivity, thermal diffusivity, and thermal resistance of straw-mortar composite materials produced with straw pretreated by sodium hydroxide.The models were developed and subjected to diagnostic analysis.The following conclusions were obtained: (1) Each process parameter had a significant effect on the responses, and the primary and secondary relationships of each parameter on the responses included a straw content that was greater than the reagent concentration and pretreatment time.The response surface methodology was used to optimize all of the pretreatment parameters using selected response variables.The optimal combination for making the rice straw-mortar composite materials produced with the pretreated rice straw showed that a high straw content (17.95% mass fraction), long pretreatment time (19.50 min), and high reagent concentration (4.99% mass fraction) were advantageous to the thermal insulation properties of the straw-mortar composite materials.The straw-mortar composite materials had a thermal conductivity, thermal diffusivity, and thermal resistance of 0.211 W•(m•K) −1 , 0.277 mm 2 •s −1 , and 57.211 K•W −1 .
(2) After different numbers of freeze-thaw cycles, the thermal conductivity and thermal diffusivity of the composite showed a general downward trend, and the thermal conductivity and thermal diffusivity of the combined materials after the pretreatment decreased by 50% and 30%, respectively, while the thermal conductivity and thermal diffusivity of materials without the treatment decreased by 24% and 21%, respectively.The reduction in the thermal conductivity and thermal diffusivity of the combined pretreatment composites was 26% and 9% larger than that of the materials without the treatment, respectively.The analysis showed that the thermal properties of the pretreated straw-mortar composite materials were better than those of the untreated ones.

Figure 1 .
Figure 1.Combined pretreatment.2.3.2.Determination of Thermal Property Indicators (1) Thermal conductivity and thermal diffusivity The HS-DR-5 transient plane heat source technique thermal conductivity instrument (model no.HS-DR-5, Hesheng Instrument Co., Ltd., Shanghai, China) was used to evaluate the thermal conductivity and thermal diffusivity of the straw composite materials, and the experimental block was 70.7 mm× 70.7 mm× 70.7 mm in size (shown as Figure 2).

Figure 1 .
Figure 1.Combined pretreatment.2.3.2.Determination of Thermal Property Indicators (1) Thermal conductivity and thermal diffusivity The HS-DR-5 transient plane heat source technique thermal conductivity instrument (model no.HS-DR-5, Hesheng Instrument Co., Ltd., Shanghai, China) was used to evaluate the thermal conductivity and thermal diffusivity of the straw composite materials, and the experimental block was 70.7 mm × 70.7 mm × 70.7 mm in size (shown as Figure 2).

Figure 2 .
Figure 2. Determination of thermal conductivity and thermal diffusivity.

Figure 2 .
Figure 2. Determination of thermal conductivity and thermal diffusivity.
(a) Response surface diagram of the interaction of pretreatment time and straw content to thermal conductivity (b) Response surface diagram of the interaction of reagent concentration and straw content to thermal conductivity (c) Response surface diagram of the interaction of reagent concentration and pretreatment time to thermal conductivity

Figure 4 .
Figure 4. Response surface diagram of the interaction of various factors to thermal conductivity.

Figure 4 .
Figure 4. Response surface diagram of the interaction of various factors to thermal conductivity.

Figure 5 .
Figure 5. Response surface diagram of the interaction of various factors to thermal diffusivity.

Figure 5 .
Figure 5. Response surface diagram of the interaction of various factors to thermal diffusivity.

Figure 6 .
Figure 6.Response surface diagram of the interaction of various factors to thermal resistance.

Figure 6 .
Figure 6.Response surface diagram of the interaction of various factors to thermal resistance.

Sustainability 2024 , 19 Figure 7 .
Figure 7. Reduction in the thermal conductivity of the pretreated composite materials after freezethaw cycles.

Figure 8
Figure 8 shows the variation amplitude of the thermal conductivity of the untreated straw-mortar composite materials after freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the untreated straw-mortar composite materials varied from 0.004 to 0.005 W•(m•K) −1 , 0.018 to 0.023 W•(m•K) −1 , and 0.036 to 0.046 W•(m•K) −1 , respectively after 10, 20, and 30 freeze-thaw cycles.

Figure 8 .
Figure 8. Reduction in the thermal conductivity of the untreated composite materials after freezethaw cycles.

Figure 7 .
Figure 7. Reduction in the thermal conductivity of the pretreated composite materials after freezethaw cycles.

Figure 8 19 Figure 7 .
Figure 8 shows the variation amplitude of the thermal conductivity of the untreated straw-mortar composite materials after freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the untreated straw-mortar composite materials varied from 0.004 to 0.005 W•(m•K) −1 , 0.018 to 0.023 W•(m•K) −1 , and 0.036 to 0.046 W•(m•K) −1 , respectively after 10, 20, and 30 freeze-thaw cycles.

Figure 8
Figure 8 shows the variation amplitude of the thermal conductivity of the untreated straw-mortar composite materials after freeze-thaw cycles.The decreasing amplitude of the thermal conductivity of the untreated straw-mortar composite materials varied from 0.004 to 0.005 W•(m•K) −1 , 0.018 to 0.023 W•(m•K) −1 , and 0.036 to 0.046 W•(m•K) −1 , respectively after 10, 20, and 30 freeze-thaw cycles.

Figure 8 .
Figure 8. Reduction in the thermal conductivity of the untreated composite materials after freezethaw cycles.

Figure 8 .
Figure 8. Reduction in the thermal conductivity of the untreated composite materials after freezethaw cycles.

Figure 9 .
Figure 9. Reduction in the thermal diffusivity of the pretreated composite materials after freezethaw cycles.

Figure 9 .
Figure 9. Reduction in the thermal diffusivity of the pretreated composite materials after freezethaw cycles.

Sustainability 2024 , 19 Figure 10 .
Figure 10.Reduction in the thermal diffusivity of the untreated composite materials after freezethaw cycles.

Figure 10 .
Figure 10.Reduction in the thermal diffusivity of the untreated composite materials after freezethaw cycles.

Table 1 .
Coded levels for independent variables used in the experiment.

Table 2
lists the fundamental performance indicators for P.O.42.5 Portland cement, which was used in the experiment.

Table 2 .
Physical and mechanical properties of the cement.

Table 4 .
Coefficient values of the fitted model for different responses of composite materials pretreated with sodium hydroxide.

Table 5 .
Regression equation for thermal properties of straw-mortar composite materials.

Table 6 .
Optimization objectives of various variables of the straw-mortar composite materials.

Table 7 .
Optimum conditions for producing straw-mortar composite materials pretreated by sodium hydroxide.

Table 8 .
Verification of experimental results.

Table 10 .
Experimental results of composite materials without the pretreatment after freeze-thaw cycles.