Research on the Performance of Recycled-Straw Insulating Concrete and Optimization Design of Matching Ratio

Abstract

Construction solid waste and agricultural waste, as renewable resources, have gained increasing attention recently. This research aims to explore the mechanical and thermal properties of recycled-straw insulating concrete commonly made with construction waste and straw in northern Fujian, which can provide useful suggestions for the practical use of recycled-straw insulating concrete. The effects of recycled coarse aggregate, fly ash, and straw on the mechanical and thermal properties of recycled-straw insulating concrete were investigated by orthogonal tests. The results of the orthogonal tests were optimized by the total efficacy coefficient method to obtain the optimal mix ratio of recycled-straw insulating concrete. Combined with the finite element analysis software ANSYS Workbench, the heat transfer performance of the recycled-straw insulating concrete walls was analyzed to simulate the insulation performance of the walls. The compressive strength of the recycled-straw insulating concrete with the optimal ratio was found to be 30.93 MPa, and the thermal conductivity was 0.5051 W/(m·K). The steady-state thermal analysis of the recycled-straw insulating concrete wall and the plain concrete wall was carried out by finite element software, and the simulation results showed that the insulation performance of the recycled-straw insulating concrete walls was improved by 145% compared with the plain concrete wall. These results indicate that the recycled-straw insulating concrete wall has better thermal insulation performance and can be applied to building envelopes to save heating costs in winter and reduce carbon dioxide emissions, which has significant economic and environmental significance for areas with low outdoor temperatures in winter and long heating periods.

1. Introduction

As China’s national economy rapidly develops, living standards have continually improved alongside the continuous development of the construction industry. This growth has accompanied the widespread alteration, demolition, and new construction of buildings, which has produced a large amount of construction solid waste [1,2,3,4]. China currently produces about 1.55–2.4 billion tons of construction solid waste annually, accounting for 30–40% of urban waste. If not properly managed, this waste will become a serious burden on urban development and the natural environment [5,6,7]. There is an urgent demand for the effective resourcing of construction solid waste, which has become a fundamental goal for the construction industry. Additionally, a significant amount of energy is consumed by buildings during operation and maintenance, with heating and air conditioning accounting for the majority of this consumption. It has been predicted that heating and cooling will account for more than 50% of total building energy consumption in 2050 [8,9,10]. The building envelope is the most intense energy consumer among all the parts of a building, making it crucial to resolve technical issues related to building insulation and energy efficiency [11,12].

To date, many studies have been conducted on the mechanical and durability properties of recycled concrete, but relatively little attention has been given to its thermal properties [13,14,15,16,17]. The incorporation of recycled aggregates alters the internal microstructure of recycled concrete, resulting in differences in its mechanical and thermal properties compared to common concrete [18,19,20]. Da Silva [21] studied recycled concrete, using recycled aggregates as replacements for some natural aggregates, while fly ash replaced some cement. Although this adversely affected the mechanical and durability properties of the material, it helped to reduce the environmental CO₂ production, lower the consumption of natural resources, and decrease production costs. Xiao [22,23] analyzed the significance of factors affecting the thermal conductivity of concrete. The significance of the influencing factors was as follows: wet and dry conditions, the replacement ratio of recycled coarse aggregate, temperature, the aggregate volume fraction, aggregate type, the reinforcement volume fraction, the water–cement ratio, and the type of admixture.

The physical properties of recycled aggregates from different sources also differ, which can also lead to varying thermal conductivities [24,25]. Miguel et al. [26] investigated the thermal conductivity of concrete made from recycled aggregates from different locations. When the replacement ratio of recycled coarse aggregate is 100%, the thermal conductivity of concrete from two different sources of recycled coarse aggregate is 1.7 and 1.6 W/(m·K). Wang et al. [27] added glazed hollow beads to recycled concrete at a volume ratio of 1:1 to produce recycled insulating concrete with a minimum thermal conductivity of 0.322 W/(m·K), which is much lower than that of plain concrete, and demonstrated good mechanical and durability properties. Pavlu et al. [28] mixed EPS (expanded polystyrene) into recycled concrete and found that 30% EPS reduced the thermal conductivity of the recycled concrete by 68.5%.

Straw fiber is a renewable natural building material that can also be recycled. Compared with traditional synthetic fibers, plant fibers not only have the characteristics of low carbon, environmental protection, and sustainable development but also can improve the tensile strength and toughness of concrete [29] However, in China, the current solution to waste straw is dominated by burning, which increases haze and deteriorates air quality over time [30]. It also leads to a waste of renewable resources like straw fibers. Incorporating straw fibers into concrete causes a decline in its mechanical properties, but for some building envelopes with low mechanical requirements, it can lend a certain insulation effect and reduce costs while satisfying certain mechanical properties [31,32,33]. Li [34,35] conducted experimental research on concrete mixed with different types of rice straw at various proportions. They found that adding rice straw fiber reduced the compressive strength, splitting tensile strength, and flexural strength of concrete but enhanced its toughness. A material of this strength can be used in non-load-bearing and infill parts of building frame structures to reduce costs. Kammoun [36] prepared eco-lightweight concrete by adding straw to ultra-high performance concrete containing dune sand. They found that the addition of straw with a volume admixture of 20 kg/m³ reduced the density of concrete by 33% with a 60% reduction in thermal conductivity. Ashour [37] prepared eco-optimized straw concrete by mixing granulated blast furnace slag after treatment with a NaOH solution. The NaOH-treated straw fiber concrete had a higher density, lower porosity, higher compressive strength, higher splitting tensile strength, and higher flexural strength than untreated straw fiber. Zhang [38] investigated the addition of straw ash instead of cement for preparing straw ash concrete. After replacing the cement with the straw ash admixture, the compactness of concrete is improved. In addition, the rebar corrosion rate is lowered, and the corrosion resistance of rebar is improved effectively, thereby enhancing the durability of concrete structures.

Song [39] mixed fly ash and silica fume in straw concrete and found that increasing the amount of fly ash improved the insulation performance of the straw concrete but lowered its compressive strength. Straw concrete mixed with silica fume had higher compressive strength within a certain range as well. Chen et al. [40] studied the effects of different types and forms of straw and fly ash compounding on the workability, mechanical properties, and thermal insulation properties of concrete. They found that incorporating straw reduced the slump, apparent density, strength, and thermal conductivity of concrete. Among the three kinds of straw, the workability, strength, and insulation performance of concrete mixed with rape straw are better than those of wheat straw and corn straw. Moussi [41] investigated the effects of different proportions of rice straw and rice husk blended into concrete on thermal conductivity and mechanical properties. They found that increasing the content of rice straw reduced the thermal conductivity of concrete and that the thermal conductivity is inversely proportional to the porosity.

China is one of the main producers of crop straw fiber in the world. Therefore, the application of straw fiber in building materials is worth exploring in order to effectively utilize the abundant agricultural waste resources [42,43]. Compared with plain concrete, recycled-straw insulating concrete is the recycling of construction solid waste and agricultural waste resources, which can reduce the waste of resources and realize the recycling of resources, which is in line with the characteristics of sustainable development. The effect of the recycled coarse aggregate replacement ratio, fly ash replacement ratio, and straw blending amount on the mechanical and thermal properties of recycled-straw insulating concrete was investigated in this research based on orthogonal tests. The optimal mix ratio of recycled-straw insulating concrete was obtained based on the test results using the total efficiency coefficient method. The temperature field inside the recycled-straw insulating concrete wall was simulated using the finite element analysis software ANSYS Workbench(ANSYS 2022 R2). A temperature distribution cloud of the recycled-straw insulating concrete wall in the steady state was obtained, which can analyze the heat transfer performance of the recycled-straw insulating concrete wall. This research can promote the application of recyclable solid waste materials in engineering. At the same time, the research of new wall materials is of great significance to promote the rapid development of building structure industrialization, save energy, and reduce environmental pollution.

2. Materials and Testing Procedure

2.1. Materials

The cement used in this research is Wannianqing brand P·O 42.5R grade cement (Jiangxi Wannianqing Cement Co., Ltd., Nanchang, China) with an apparent density of 3100 kg/m³. Its chemical composition is shown in

Table 1. Chemical composition of cement.

Components	SiO2	CaO	Al2O3	Fe2O3	SO3	MgO	Loss
Content/%	20.30	60.39	5.10	4.30	2.24	2.57	2.1

.

Natural coarse aggregate was sourced locally from Wuyishan with apparent density of 2660 kg/m³, water absorption of 1.1%, and aggregate particle size of 5–16 mm, as shown in Figure 1a. The recycled coarse aggregates were taken from a road pavement renovation project, crushed in a crushing yard, cleaned, initially sieved, and then sieved again in a laboratory sieving machine to produce aggregate particles with a size range of 5–16 mm, apparent density of 2380 kg/m³, and water absorption of 3.4%, as shown in Figure 1b. The sieving curves of recycled aggregate and natural aggregate are shown in Figure 2.

2.2. Orthogonal Test Design

As there are currently no established standards for designing recycled-straw insulating concrete, the preliminary basic formulation of concrete was designed by considering the intended uses of the material and following the relevant specifications [44]. The purpose of this experiment was to investigate recycled-straw insulating concrete as a new wall material for building envelopes. Considering that the addition of straw and recycled coarse aggregate can significantly reduce concrete strength, a sufficient strength reserve was retained by setting the strength class of the foundation concrete to C30. The basic formulation of recycled-straw insulating concrete is shown in

Table 4. Basic formulation of recycled-straw insulating concrete.

Materials	Water (kg/m3)	Cement (kg/m3)	Fine Aggregate (kg/m3)	Natural Coarse Aggregate (kg/m3)
Content	220	440	643.8	1096.2

.

The orthogonal test method was used in this experiment [45]. The orthogonal test table is shown in

Table 5. Factors and levels of orthogonal test design.

	Factors	Recycled Coarse Aggregate Replacement Ratio (A)/%	Fly Ash Replacement Ratio (B)/%	Straw Content (C)/%
Level
1		0	0	0
2		30	10	1
3		70	15	2
4		100	20	3

, with compressive strength and thermal conductivity as test indicators. Three factors were selected: recycled coarse aggregate, fly ash, and straw. Four levels of each factor were selected using the L₁₆ (4³) orthogonal table, as shown in

Table 6. Mix proportion design of recycled-straw insulating concrete in orthogonal test.

Number	Orthogonal Test	Cement (kg/m3)	Fine Aggregate (kg/m3)	Natural Coarse Aggregate (kg/m3)	Recycled Coarse Aggregate (kg/m3)	Fly Ash (kg/m3)	Straw(kg/m3)	Net Water Consumption (kg/m3)	Additional Water Consumption (kg/m3)
1	A1B1C1	440	643.8	1096.2	0	0	0	220	0
2	A1B2C2	396	643.8	1096.2	0	44	4.4	220	13.2
3	A1B3C3	374	643.8	1096.2	0	66	8.8	220	26.4
4	A1B4C4	352	643.8	1096.2	0	88	13.2	220	39.6
5	A2B1C2	440	643.8	767.34	328.86	0	4.4	220	20.76
6	A2B2C1	396	643.8	767.34	328.86	44	0	220	7.56
7	A2B3C4	374	643.8	767.34	328.86	66	13.2	220	47.16
8	A2B4C3	352	643.8	767.34	328.86	88	8.8	220	33.96
9	A3B1C3	440	643.8	328.86	767.34	0	8.8	220	44.05
10	A3B2C4	396	643.8	328.86	767.34	44	13.2	220	57.25
11	A3B3C1	374	643.8	328.86	767.34	66	0	220	17.65
12	A3B4C2	352	643.8	328.86	767.34	88	4.4	220	30.85
13	A4B1C4	440	643.8	0	1096.2	0	13.2	220	64.81
14	A4B2C3	396	643.8	0	1096.2	44	8.8	220	51.61
15	A4B3C2	374	643.8	0	1096.2	66	4.4	220	38.41
16	A4B4C1	352	643.8	0	1096.2	88	0	220	25.21

. The content of recycled coarse aggregate is the mass percentage replacing natural aggregate; fly ash content is the mass percentage of substituted cement; and straw fiber content is the mass percentage of cement [40].

The water consumption for this test included net water consumption and additional water consumption, the latter of which is composed of the additional water consumption of recycled coarse aggregate and straw. The water absorption of recycled coarse aggregate is approximately three times that of natural aggregate, so additional water was supplied to the recycled coarse aggregate to minimize the impact of its high water absorption on the effective water–cement ratio [46,47]. To maintain the desired workability of concrete, around 10–13 kg of additional water per cubic meter of concrete was added for every 1% increase in the straw admixture during the experiment [35].

2.3. Test Preparation and Performance Testing

The process for preparing recycled-straw insulating concrete was as follows:

• Natural aggregate, recycled coarse aggregate, and sand were placed into the concrete blender for mixing, and half of the required water was added.
• Straw fibers were added while stirring, and they were stirred for 30 s after all the straw was added.
• Cement and fly ash were added and stirred for about 1 min. Then, the remaining half of the water was added and stirred for 2–3 min.
• The mixture of concrete was poured into a mold and demolded after 48 h. The concrete test specimens were kept in a standard maintenance room at 20 ± 2 °C and 95% humidity. The compressive strength test was conducted after 7 d and 28 d of standard maintenance. Then, the thermal conductivity test was conducted after 28 d of standard maintenance.

The compressive strength test was conducted according to the GB/T 50081-2002, “Standard for Mechanical Properties of Ordinary Concrete” [48]. HCT306A servo universal testing machine was used to test the compressive strength, and the test specimen was a 100 mm × 100 mm × 100 mm cube test block, as shown in Figure 5.

The orthogonal test was designed with 16 groups of six specimens each. Half of the specimens were used to test the 7 d compressive strength of concrete, and the remaining three were used to test the 28 d compressive strength of concrete.

The steps for testing compressive strength were as follows:

• The specimens were removed from the curing room. Then, we wiped the attached water off the surface of the specimen with a wet wrung-out washcloth.
• The specimens were placed in the center of the lower pressure plate surface of the testing machine. Then, we started the testing machine.
• For uniform and continuous loading in the test process, the loading speed was set at 0.4~0.6 MPa per second. Compressive strength was recorded until the specimen was damaged.

The thermal conductivity of the specimens was determined according to GB/T 10294-2008, “Determination of steady-state thermal resistance of insulation materials and related properties by the protective thermal plate method” [49]. The test equipment used for the measurement of thermal conductivity was a JTRG-III thermal conductivity tester, which was set up according to the national standard GB/T 10,294 for single-specimen, double-flowmeter type thermal conductivity instruments (Figure 6). According to the orthogonal test, 16 groups of tests were designed, with three specimens tested per group. The specimen size for recycled-straw insulating concrete was 300 mm $\times$ 300 mm $\times$ 30 mm (length $\times$ width $\times$ thickness) in accordance with the equipment requirements, as shown in Figure 7.

The steps for measuring thermal conductivity data were as follows:

• Specimens were kept in the curing room for 28 d and then placed in the dryer at 105 °C to dry to constant weight.
• After the dried specimens cooled to room temperature, the cold plate temperature of the thermal conductivity tester was set to 15 °C, and the hot plate temperature was set to 35 °C. The specimens were then tested for thermal conductivity.
• After the test data stabilized, records were taken and organized.

2.4. Finite Element Simulation of Wall Temperature Field

Heat energy transfer occurs within an object or between an object and another object. According to the different heat transfer mechanisms, there are three methods of heat energy transfer: heat conduction, heat convection, and heat radiation [50]. Wall heat energy transfer involves conduction, convection, and radiation occurring over a complex heat transfer process [51,52]. As shown in Figure 8, this process can be divided into three stages: (1) heat absorption from the interior surface of the wall, (2) heat conduction from the wall material layer, and (3) heat dissipation from the outer surface of the wall.

In this research, the steady-state thermal analysis method in the finite element software ANSYS Workbench was used to simulate the temperature field inside the recycled-straw insulating concrete wall. Sixteen wall models were established by steady-state thermal analysis [53,54,55]. The heat transfer from the wall in the steady state satisfies Fourier’s law, in which the heat moving through a given area per unit time is referred to as “heat flow rate” and denoted as $\Phi$ (W) (Equation (1)). The heat flow rate through a unit area is called heat flux and denoted as $q$ (W/m²). When the temperature of the wall changes only in the X-direction, the heat flux is expressed as in Equation (2), according to Fourier’s law:

$$\Phi =-\mathsf{\lambda }A\frac{dt}{dx}$$

(1)

$$q=\frac{\Phi }{A}=-\mathsf{\lambda }\frac{dt}{dx}$$

(2)

where $\Phi$ is the heat flow rate, $\lambda$ is the thermal conductivity, $A$ is the heat transfer area, $q$ is the heat flux, and $\frac{dt}{dx}$ is the temperature change rate. The negative sign indicates an opposite direction of heat transfer to the direction of temperature increase.

The structural model used for the finite element simulation is a wall. The steps of the software ANSYS Workbench for steady-state thermal analysis of recycled-straw insulating concrete walls were as follows:

• Establishing the model of recycled-straw insulating concrete wall: in the geometric modeling platform of Design Modeler in the software, the dimensions of the recycled-straw insulating concrete wall model were established as 2 m high, 1 m wide, and 0.3 m thick.
• Creating a steady-state thermal analysis project: the recycled-straw insulating concrete wall model established in the Design Modeler module was imported into the steady-state thermal analysis system.
• Assigning material properties: Concrete materials were searched for in the engineering data of the software, and their performance parameters was added. The main performance parameter was the setting of concrete thermal conductivity. For the steady-state thermal analysis simulation process of the straw concrete wall, 16 groups of materials were set.
• Meshing of recycled-straw insulating concrete wall models: the automatic meshing method was used, which can automatically identify the object and match the appropriate meshing form, and the model mesh was generated with 188,232 cells and 43,287 nodes after automatic meshing.
• Adding load: The steady-state temperature field simulation of the recycled-straw insulating concrete wall was preset on the outer side of the wall. For the outer surface of the wall, the outdoor ambient temperature was set to 0 °C with temperature transfer. A temperature load was applied to the interior surface of the wall, and the heat flux was set to 40 W/m². The remaining four surfaces of the wall were assumed to be insulated.
• Solving: the solving option was set to achieve the purpose of the data results we wanted to obtain from the simulation.

The steady-state thermal analysis was performed with the software to extract the temperature difference between the interior and outer wall surfaces at steady-state as a basis for further analyzing the advantages and disadvantages of these types of wall insulation.

3. Results, Analysis, and Discussion

3.1. Results

The orthogonal test results for recycled-straw insulating concrete are shown in

Table 7. Performance test results of recycled-straw insulating concrete.

Number	Recycled Aggregates (A)/%	Fly Ash (B)/%	Straw (C)/%	7 d Compressive Strength/(MPa)	28 d Compressive Strength/ (MPa)	Thermal Conductivity/ [W/(m·K)]
1	0	0	0	35.23	42.44	0.7846
2	0	10	1	30.77	34.55	0.6812
3	0	15	2	19.07	28.43	0.5760
4	0	20	3	10.63	15.06	0.4990
5	30	0	1	30.11	32.72	0.6351
6	30	10	0	29.10	35.12	0.5591
7	30	15	3	12.34	17.48	0.4329
8	30	20	2	17.17	22.04	0.4703
9	70	0	2	19.32	22.86	0.5215
10	70	10	3	16.18	15.78	0.4226
11	70	15	0	27.88	32.54	0.5418
12	70	20	1	24.65	30.93	0.5352
13	100	0	3	13.83	16.46	0.3893
14	100	10	2	16.76	21.87	0.4508
15	100	15	1	22.48	26.55	0.5289
16	100	20	0	24.61	31.56	0.5833

.

3.2. Analysis and Discussion of Mechanical Properties

3.2.1. Range Analysis

For the range analysis, $\overline{K}$_i is the average value of the same level for different factors, and the range value R refers to the difference between the maximum and minimum value of $\overline{K}$_i. The larger the value of R, the greater the influence of the level change of the factor on the performance of recycled-straw insulating concrete and the more important the factor [56].

The range analysis of 7 d and 28 d compressive strength of recycled-straw insulating concrete is shown in

Table 8. Range analysis of compressive strength.

Age	$\overline{\mathit{K}}$i	Recycled Aggregates (A)	Fly Ash (B)	Straw (C)
7 d	$\overline{K}$1	23.93	24.62	29.21
	$\overline{K}$2	22.18	23.20	27.00
	$\overline{K}$3	22.01	20.44	18.08
	$\overline{K}$4	19.42	19.27	13.25
	R	4.51	5.36	15.96
	Optimization	A1B1C1
28 d	$\overline{K}$1	30.12	28.62	35.47
	$\overline{K}$2	26.89	26.88	31.19
	$\overline{K}$3	25.53	26.25	23.80
	$\overline{K}$4	24.11	24.90	16.20
	R	6.01	3.72	19.72
	Optimization	A1B1C1

. The following conclusions can be drawn from the table: ① When the age is 7 d, the range value is R_C > R_B > R_A. That is, the order of the factors affecting the compressive strength of recycled-straw insulating concrete is straw > fly ash > recycled aggregate. According to the value of $\overline{K}$_i, the optimal solution for 7 d age is A₁B₁C₁. ② When the age is 28 d, the order of factors affecting the compressive strength of recycled-straw insulating concrete is straw > recycled aggregate > fly ash, and the optimal solution at 28 d is A₁B₁C₁. In summary, straw is the main factor affecting the compressive strength of recycled-straw insulating concrete, followed by the recycled aggregate replacement ratio.

Figure 9 depicts the effects of the recycled coarse aggregate replacement ratio on the compressive strength of different ages of recycled-straw insulating concrete. As shown in the graph, the compressive strength of the concrete at both 7 d and 28 d decreased as the replacement ratio of recycled coarse aggregate increased. This decrease in strength can be attributed to the presence of dust and old cement mortar on the surface of the recycled aggregate, which led to a high porosity in the interfacial transition zone between the old and new mortar. The resulting loose structure made it difficult for the new mortar and recycled aggregate interface transition zone to securely connect [6,13]. Therefore, as more recycled coarse aggregate was used, the strength of concrete tended to decrease.

Figure 10 shows the influence of the fly ash replacement ratio on compressive strength at different ages. The compressive strength appears to decrease by 13.5% as the fly ash replacement ratio increases from 10% to 15% at 7 d and by 2.4% when the replacement ratio increases from 10% to 15% at 28 d. The greater decrease in strength at 7 d is attributable to the fact that, in the preliminary process of concrete hardening, the cement first underwent a hydration reaction while fly ash, in the initial stage, acted as small particles to fill the role, and, thus, the reaction degree is lower. In the later stage of concrete hardening, SiO₂ and Al₂O₃ in the fly ash reacted with the hydration product Ca (OH)₂ in the cement via the pozzolanic reaction, leading to a slower decrease in compressive strength at 28 d [57,58]. However, fly ash is less active than cement, so the pozzolanic reaction cannot be effectively reacted, and the strength of the concrete decreased over time. When straw and fly ash were added simultaneously, the straw delaying the hydration reaction of cement will further prevent the pozzolanic reaction of fly ash, resulting in lower concrete strength [40].

Figure 11 shows the effect of percentage of straw content on compressive strength at various ages. The compressive strength of the recycled-straw insulating concrete appears to decrease at both 7 d and 28 d as the straw admixture increases. This is due to the fact that straw fibers mainly contain cellulose and hemicellulose. The sugars precipitated from organic cellulose will prevent the hydration reaction of cement, that is, hinder the growth of Ca(OH)₂ crystals, resulting in a “retarded coagulation” phenomenon. In addition, the compatibility between fiber and cement paste is poor, which will affect the internal molecular crystallization and reduce the amount of C-S-H (calcium–silicate–hydrate) generated. Hemicellulose is also known to delay the hydration of C₂S (dicalcium silicate) and hinder the generation of Aft (ettringite), which further reduces the strength of concrete [59]. Additionally, due to the high water absorption of straw, the additional water added to maintain the required compatibility increased the water–cement ratio of the concrete and directly impacted its strength [35]. The incorporation of straw also made a less dense linkage between the cement-based slurry and aggregate, causing an increase in porosity and a decrease in concrete strength.

3.2.2. Variance Analysis

The range analysis is a simple method for processing and calculating experimental data. The optimal mix ratio of concrete can be obtained using comprehensive comparison [60]. However, in regard to experimental errors, the range analysis method may yield inaccurate estimates. To determine whether there were errors or issues with different levels of multiple factors, the variance analysis method was applied.

The variance analysis results for the 7 d and 28 d compressive strength of recycled-straw insulating concrete are shown in

Table 9. Variance analysis of 7 d and 28 d compressive strength of recycled-straw insulating concrete.

Age	Factors	SS	df	MS	F	CV	Sig
7 d	Recycled Aggregate (A)	41.359	3	13.786	4.964	F0.01 (3, 6) = 9.78	*
	Fly ash (B)	72.7	3	24.233	8.726	F0.05 (3, 6) = 4.76	*
	Straw (C)	675.595	3	225.198	81.094	F0.1 (3, 6) = 3.29	**
	e	16.662	6	2.777		F0.2 (3, 6) = 2.1
	e△	806.316	15
28 d	Recycled aggregate (A)	79.238	3	26.413	4.932	F0.01 (3, 6) = 9.78	*
	Fly ash (B)	28.658	3	9.553	1.784	F0.05 (3, 6) = 4.76	△
	Straw (C)	862.888	3	287.629	53.708	F0.1 (3, 6) = 3.29	**
	e	32.133	6	5.355		F0.2 (3, 6) = 2.1
	e△	1002.917	15

Note: SS: Sum of squares of deviation from mean; df: degree of freedom; MS: mean square; F: statistic; CV: coefficient of variation; sig: significance. “△” indicates some influence; “*” indicates significance; “**” indicates high significance.

, which shows that the straw content has a significant effect on the compressive strength of recycled-straw insulating concrete at both 7 d and 28 d. The effect of the recycled aggregate replacement ratio appears relatively significant. Fly ash had a significant effect on the compressive strength at 7 d and some effect at 28 d. Additionally, the $F$ values of factors affecting 28 d compressive strength are $F_A=26.41$, $F_B=9.55$, and $F_c=286.63$. These values suggest that the order of strength of each factor on the 28 d compressive strength of recycled-straw insulating concrete is straw (C) > recycled aggregate (A) > fly ash (B), which is consistent with the range analysis results.

3.3. Analysis and Discussion of Thermal Conductivity

3.3.1. Range Analysis

The range analysis of the thermal conductivity of recycled-straw insulating concrete is shown in

Table 10. Range analysis of thermal conductivity.

Age	$\overline{\mathit{K}}$	Recycled Aggregate (A)	Fly Ash (B)	Straw (C)
28 d	$\overline{K}$1	0.8103	0.6954	0.7325
	$\overline{K}$2	0.5294	0.5609	0.6172
	$\overline{K}$3	0.4978	0.5493	0.5320
	$\overline{K}$4	0.4877	0.5194	0.4434
	R	0.3227	0.1760	0.2890
	Optimization	A4B4C4

, in which the range values fall into order as R_A >R_C >R_B. That is, the order of the factors affecting the thermal conductivity of concrete is recycled aggregate > straw > fly ash. The optimal mix ratio is A₄B₄C₄.

Figure 12 shows the impact of each factor on the thermal conductivity of recycled-straw insulating concrete. The recycled aggregate used was obtained by crushing construction waste, which resulted in a large amount of old cement mortar being adsorbed on its surface. This caused the recycled aggregate to have higher porosity, lower density, and higher water absorption than natural aggregate [6]. The crushing process of construction waste can also damage the recycled aggregate, further increasing its porosity. The internal porosity of concrete increases as the replacement ratio of recycled aggregate increases, so the thermal conductivity of concrete in this case decreased as the recycled coarse aggregate replacement ratio increased.

The straw fiber contains honeycomb pores and mainly comprises substances such as lignin and cellulose, which are poor conductors of heat [38]. This resulted in better insulation performance. Additionally, straw increases the internal porosity of concrete, leading to more closed voids and lower thermal conductivity, which improved its thermal performance. The thermal conductivity appears to decrease by 19.34% as the fly ash replacement ratio increases from 0% to 10% and by 7.4% when the replacement ratio increases from 10% to 20%.

3.3.2. Variance Analysis

The variance analysis results of the thermal conductivity of recycled-straw insulating concrete are shown in

Table 11. Variance analysis of thermal conductivity of recycled-straw insulating concrete.

Age	Factors	SS	df	MS	F	CV	Sig
28 d	Recycled aggregate (A)	0.284	3	0.095	7.45	F0.01 (3, 6) = 9.78	**
	Fly ash (B)	0.073	3	0.024	1.922	F0.05 (3, 6) = 4.76	△
Straw (C)		0.182	3	0.061	4.79	F0.1 (3, 6) = 3.29	*
	e	0.076	6	0.013		F0.2 (3, 6) = 2.1
	e△	0.615	15

Note: SS: Sum of squares of deviation from mean; df: degree of freedom; MS: mean square; F: statistic; CV: coefficient of variation; sig: significance. “△” indicates some influence; “*” indicates significance; “**” indicates high significance.

.

The thermal conductivity of recycled-straw insulating concrete was affected by three factors: the recycled aggregate replacement ratio, straw content, and fly ash replacement ratio. Among them, the recycled aggregate replacement ratio has the greatest impact, followed by straw content. The effect of the fly ash replacement ratio on thermal conductivity was not significant in our case. The $F$ values of each factor are $F_A=7.45$, $F_B=1.92$, and $F_c=4.79$. According to the $F$ values calculated by each factor, the order of strength of each factor’s influence on thermal conductivity is recycled aggregate (A) > straw (C) > fly ash (B), which is consistent with the range analysis results.

3.4. Optimal Design of Mix Ratio

The optimal mix ratio obtained from the range analysis and variance analysis of the orthogonal test results cannot simultaneously incorporate mechanical and thermal insulation properties. To find a balance between these two performance indicators and determine the optimal mix ratio of recycled-straw insulating concrete for proper mechanical properties with superior thermal insulation properties, the total efficiency coefficient method was used to find out [61]. Before calculating the single efficiency coefficient, it is necessary to normalize the test data. Accordingly, there are $n$ indicator types and $j$ groups in the test. The maximum test value corresponding to the $i$ indicator is $C_{max}$, the minimum value is $C_{min}$, and the normalized value of the $j$th group of test $i$ indicator data is $d_{ji}$.

In this test, compressive strength belonged to a “larger is better” type of efficiency indicator. Therefore, the variables with larger indicator values and higher single efficiency coefficients were considered significant. As the 28 d compressive strength characterizes the mechanical properties of concrete, it was normalized according to Equation (3). Thermal conductivity, conversely, belonged to a “smaller is better” type of cost indicator, in which variables with smaller indicator values and higher single efficiency coefficients are significant. Therefore, the thermal conductivity characterizing the thermal insulation performance of concrete was normalized according to Equation (4) [62].

The normalized calculation of efficiency-type indicators (where larger is better) is

$$d_{ji}=\frac{C_{ji}}{C_{max}}$$

(3)

The normalized calculation of cost-based indicators (where smaller is better) is

$$d_{ji}=\frac{C_{min}}{C_{ji}}$$

(4)

Based on the single efficiency coefficients of the evaluation indicators, the total efficiency coefficients for group j trials were calculated as follows:

$$D_j=\sqrt[n]{d_{j1}{d}_{j2}\dots d_{j,m-1}{d}_{jm}}$$

(5)

As shown in

Table 12. Normalized treatment and indicator test results.

Number	28 d Compressive Strength (MPa)	Thermal Conductivity [W/(m·K)]	Total Efficacy Coefficient
1	42.44 (1.000)	1.2357 (0.315)	0.561
2	34.55 (0.814)	0.7912 (0.492)	0.633
3	28.43 (0.670)	0.6854 (0.568)	0.617
4	15.06 (0.355)	0.5289 (0.736)	0.511
5	32.72 (0.771)	0.6351 (0.613)	0.687
6	35.32 (0.832)	0.5791 (0.672)	0.748
7	17.48 (0.412)	0.4329 (0.899)	0.609
8	22.04 (0.519)	0.4703 (0.828)	0.656
9	22.86 (0.539)	0.5215 (0.746)	0.634
10	15.78 (0.372)	0.4226 (0.921)	0.585
11	32.54 (0.767)	0.5418 (0.718)	0.742
12	30.93 (0.729)	0.5051 (0.771)	0.749
13	16.46 (0.388)	0.3893 (1.000)	0.623
14	21.87 (0.515)	0.4508 (0.863)	0.667
15	26.55 (0.626)	0.5372 (0.725)	0.673
16	31.56 (0.744)	0.5733 (0.679)	0.711

, Group 12 has the highest total efficiency coefficient. Thus, the optimal mix ratio of recycled-straw insulating concrete falls into Group 12, which has a recycled coarse aggregate replacement ratio of 70%, fly ash replacement ratio of 20%, and straw content of 1%. This optimal mix ratio results in a compressive strength of 30.93 MPa and a thermal conductivity of 0.5051 W/(m·K). The Group 12 mix ratio satisfies the required mechanical properties of the wall structure while providing satisfactory insulation. The application of recycled aggregates and straw can effectively utilize construction solid waste and crop waste, which contribute to environmental and social economic benefits.

3.5. Finite Element Simulation Analysis of Wall Temperature Field

The maximum temperature values of the interior surface of the wall were obtained by simulating recycled-straw insulating concrete walls in a steady state with a constant temperature on the outer surface of the wall and a thermal load applied on the other side in ANSYS Workbench, as shown in

Table 13. Maximum temperature values of internal temperature distribution of wall.

Number	Thermal Conductivity [W/(m·K)]	Maximum Temperature (°C)
1	1.2357	7.77
2	0.7912	12.13
3	0.6854	14.00
4	0.5289	18.15
5	0.6351	15.12
6	0.5791	16.58
7	0.4329	22.18
8	0.4703	20.41
9	0.5215	18.41
10	0.4226	22.72
11	0.5418	17.72
12	0.5051	19.01
13	0.3893	24.66
14	0.4508	21.29
15	0.5372	17.87
16	0.5733	16.75

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The influence of various factors on the maximum temperature inside wall is shown in Figure 13. As can be seen from Figure 13, the maximum temperature of the interior surface of the wall increases with the increase in the replacement ratio of recycled aggregate. As that ratio increased from 0% to 30%, the temperature on the interior side of the wall increased by 5.56 °C, indicating that the incorporation of recycled aggregate improved the thermal insulation performance of the concrete wall. Similarly, an increase in straw content caused the maximum temperature on the interior surface of the wall to increase, and thermal insulation performance increased by 49.08% when straw blending increase from 0% to 3%. This shows that straw can improve the thermal performance of the wall. The effect of the fly ash replacement ratio on the thermal insulation effect of the wall was not significant, which is consistent with the results of the orthogonal test.

Figure 14, Figure 15 and Figure 16 show the temperature clouds obtained from the finite element analysis of the recycled-straw insulating concrete wall model. Figure 12 shows the temperature distribution of the plain concrete, for which the maximum temperature on the interior surface of the wall was 7.77 °C. Figure 13 shows a wall temperature cloud map of the optimal mix ratio of recycled-straw insulating concrete. The maximum temperature on the interior surface of the wall increased to 19.01 °C, which is 1.45 times higher than that of plain concrete. This verifies the thermal insulation effect of recycled-straw insulating concrete. Figure 14 shows the temperature cloud of the wall with the lowest thermal conductivity and the optimal thermal insulation performance of recycled-straw insulating concrete, for which the maximum temperature on the interior surface of the wall is 24.66 °C, which is 2.17 times higher than that of plain concrete. It is further confirmed that the recycled-straw insulating concrete applied to the wall has a certain thermal insulation effect.

4. Conclusions

The influence of recycled aggregate, straw, and fly ash on the performance of recycled-straw insulating concrete was analyzed in this research based on orthogonal tests. The steady-state thermal analysis of the recycled-straw insulating concrete wall was carried out by using the finite element software ANSYS, and the thermal insulation effect of the recycled-straw insulating concrete applied to the wall was quantified. Based on the results and the analysis, the following conclusions and prospects can be drawn:

• For mechanical properties, the degree of influence falls into the order of straw > recycled aggregate > fly ash. For the thermal properties, the order of influence is recycled aggregate > straw > fly ash.
• The optimal mix ratio for recycled-straw insulating concrete falls into Group 12, which has a recycled coarse aggregate replacement ratio of 70%, fly ash replacement ration of 20%, and straw content of 1%. This mix ratio results in a compressive strength of 30.93 MPa and a thermal conductivity of 0.5051 W/(m·K).
• Finite element software was used to conduct steady-state thermal analysis. The simulation results showed that the insulation performance of the recycled-straw insulating concrete walls was improved by 145% compared with plain concrete, which verified that the recycled-straw insulating concrete applied to the wall has a better insulation effect.
• Recycled-straw insulating concrete can be considered for application in building envelopes with low mechanical properties. While satisfying certain mechanical properties, it can play a role in thermal insulation effect and reduce costs. For some areas with low outdoor temperature and long heating periods in winter, it can save winter heating costs and reduce carbon dioxide emissions, which has important economic and environmental significance.
• Only compressive strength was considered for the mechanical properties in this research. The tensile strength of recycled-straw insulating concrete and its durability performance can be considered in future research, which can enable recycled-straw insulating concrete to be used in different building structures and to improve the utilization of solid waste resources.
• In order to meet the needs of concrete engineering applications in different environments, it is necessary to further optimize and improve the compatibility between straw fibers and concrete. How to improve the mechanical properties and thermal insulation properties of recycled-straw insulating concrete without increasing the production cost is a problem that needs to be considered in future research. It is also an important guarantee for saving energy, reducing emissions, and promoting green ecology.