Experimental Study on Mix Ratio Design of High Coal Gangue Content Concrete

Abstract

In order to improve the utilization rate of coal gangue as aggregate in concrete, based on the orthogonal experimental design method, the L16 (4⁵) orthogonal table was used to initially prepare the mix ratio, and the concrete compressive specimens were poured, and their compressive strength was tested after standard curing for 7 days. The significance of the three factors of coal gangue ratio, coal gangue-sand ratio and water-binder ratio on strength was preliminarily understood. Then adjust the factor levels, use the L9 (3⁴) orthogonal table to arrange the factor levels, formulate the mix ratio, test its strength after curing under the same conditions, and finally analyze that the optimal mix ratio of high-volume coal gangue concrete is 90% of coal gangue. Coal gangue sand accounts for 60% of the total sand, the water-binder ratio is 0.4, the sand rate is 35%, and the amount of cement per cubic meter is 350 kg. This mix ratio can use a large amount of coal gangue as a concrete cushion or sub-base in road engineering.

1 Introduction

Concrete is the most widely used material in civil engineering construction. According to the data released by the National Development and Reform Commission, the output of commercial concrete in 2021 will be 3293.3 million cubic meters, an increase of 6.9%, and the growth rate will increase by 4 percentage points year-on-year. The output and increment of the above concrete show that my country still has a large demand for concrete in infrastructure construction, and it will not decrease in a short time. In the face of such a huge amount of concrete, coupled with the carbon neutrality goal proposed by the central government, all industries are required to steadily promote the “dual carbon” work. In the “14th Five-Year” development guideline for the concrete and cement products industry, it is proposed to develop low-carbon carbon-fixing concrete materials and use suitable industrial solid waste to manufacture concrete. Coal gangue is a solid waste generated in the process of coal mining. The existing coal gangue stockpile in my country is about 7 billion tons, and the annual output is increasing by more than 3 tons [1]. The existing coal gangue in northern Shaanxi alone accounts for one-seventh of the country’s total, more than 1 billion tons [2]. The storage of coal gangue not only takes up a lot of cultivated land, but also causes serious pollution to the environment. Due to its strong water absorption, it will bring serious haze under the catalysis of wind after drying and weathering, resulting in environmental pollution. The annual sales of sand and gravel in my country can reach 20 billion tons, and the production of machine-made sand has exceeded 18 billion tons [3]. Due to the limitation of natural sand and gravel mining, it is necessary to find as many sources of machine-made sand and gravel as possible. By comparing the composition of coal gangue sand with natural sand, researchers believe that coal gangue sand can be used as a raw material for preparing concrete [4]. However, due to the different sources of coal gangue and different dosages, it will affect the strength of concrete. Some scholars believe that the content of coal gangue instead of coarse aggregate below 30% has the least effect on the compressive strength of concrete [5], which seriously restricts the utilization rate of coal gangue in concrete. In response to the above situation, the team of this research group, based on the concept of green development, carried out experimental research on high-volume coal gangue concrete, improved the utilization rate of coal gangue in concrete, and provided solutions for consuming a large amount of coal gangue.

2 Materials and Methods

2.1 Experimental Materials

The materials tested in this paper are coal gangue, coal gangue sand, natural crushed stone, water-washed machine-made sand, cement, water reducing agent and tap water.

• 1. Coal gangue

The coal gangue selected in this experiment is all from northern Shaanxi. According to the literature research, the chemical composition of coal gangue in this area is mainly SiO2 and Al2O3, the mineral composition is mainly quartz and clay minerals, and the aluminum–silicon ratio is between 0.31 and 0.39. Intermediate aluminum–silicon ratio coal gangue. The calcium and magnesium content is between 2.11 and 5.03%, which belongs to the silicon-alumina gangue, and the water absorption rate is 5–8% [6]; it can be used as concrete aggregate.

The coal gangue particles sampled on site are large, and they need to be crushed first, and the experimental jaw crusher is used for crushing. The crushing value of the crushed coal gangue was tested, and the crushing value was higher than 19%.

The coal gangue used in this experiment was screened with a square-hole sieve, and its gradation curve was obtained (Fig. 1). According to the gradation curve, it can be seen that the coal gangue particle size is relatively continuous between 0 and 10 mm.

Fig. 1

Gradation curve of gangue after crushing

• 2. Coal gangue sand

The coal gangue sand in this paper is the sand obtained after crushing the above coal gangue with a hammer crusher, and then passing through a 0.16 mm sieve (Fig. 2).

Fig. 2

Preparation process of coal gangue sand

• 3. The natural crushed stone

The natural crushed stone (Fig. 3) comes from around Zhengzhou City, with a particle size of 10–20 mm and a crushing value of 2–10%.

Fig. 3

Natural gravel

• 4. Machine-made sand

Machine-made sand choose water-washed machine-made sand (Fig. 4), the fineness modulus is about 3.2, and the average particle size is above 0.5 mm.

Fig. 4

Machine-made sand

• 5. Cement

P·O 42.5 bagged ordinary Portland cement is used for cement, and its main component content is shown in Table 1.

Table 1 Contents of main components of ordinary Portland cement in bags

• 6. Water reducer

The water-reducing agent is a high-efficiency polycarboxylate water-reducing agent (Fig. 5). The water-reducing rate is about 20%. This time, the powder is directly added, and the dosage is 1.5% of the amount of the cementitious material.

Fig. 5

Polycarboxylate water reducer

2.2 Experimental Method

This experiment needs to study the influence of coal gangue content, coal gangue sand content and water-binder ratio on the strength of coal gangue concrete. Four levels are initially proposed for each factor. If a comprehensive test is to be done, 4³ = 64 groups of tests need to be completed. Each group of three specimens needs to be prepared and maintained for 192 specimens. The workload is very large and it is difficult to complete in a short time; Arrangement of factor water using orthogonal experimental design method flat. Orthogonal experimental design is to select some representative points from the comprehensive test according to the orthogonality. These representative points have the characteristics of “evenly dispersed, neat and comparable” orthogonal table, only 16 groups of experiments can be arranged to complete the preliminary experimental plan.

In order to use coal gangue in a large amount in concrete, the ratio of coal gangue and coal gangue sand is initially planned to be four levels of 70 ~ 100%, and the water-binder ratio is four levels of 0.35 ~ 0.50. The factor level table is shown in Table 3. The parameter combination of the test plan was designed according to the orthogonal experimental design method, and the amount of materials in each group was calculated according to the compressive test block of 150 × 150 × 150 mm, and each group of 3 test blocks. When calculating the amount of material, the sand rate of 35% is selected with reference to the relevant specifications [7], and the concrete density is designed according to 2300 kg/m³ (Table 2).

Table 2 Level table of preliminary factors for coal gangue concrete

Table 3 The parameter combination of the test scheme and the amount of materials in each group

According to the material dosage in Table 4, mix it in a 30L horizontal concrete mixer for 3 min, pour the concrete into a 150 × 150 × 150 mm compressive test mold, put it into a concrete curing box for standard curing for 7d, and test its compressive strength [8]. Because the water absorption of coal gangue was not considered in the preliminary test mix ratio, the fluidity of individual groups of concrete was poor, and the strength was not formed after curing for 7 days. Therefore, the test plan was designed again according to the preliminary test results. The orthogonal table design of L9 (3⁴) was used to design the experimental mix ratio. The proportion of coal gangue was 80 ~ 100%, the proportion of coal gangue sand was 60 ~ 80%, and the water-binder ratio was 0.40 ~ 0.60.

Table 4 L9 (3⁴) orthogonal experiment design factor levels

3 Results and Discussion

The compressive strength of coal gangue concrete (7d) under the mix ratio of two orthogonal tests was tested respectively.

According to Table 7, the range analysis of the compressive strength of coal gangue concrete with the mix ratio of the first orthogonal test shows that the coal gangue ratio has little effect on the strength, and the water-binder ratio has the greatest influence, followed by the coal gangue sand ratio. The compressive strength increases with the increase of the water-binder ratio and decreases with the increase of the proportion of coal gangue and coal gangue sand (Tables 5 and 6).

Table 5 L16 (4⁵) orthogonal test mix ratio compressive strength test results

Table 6 L9 (3⁴) orthogonal test mix ratio compressive strength test results

Table 7 L16 (4⁵) orthogonal test range analysis results

Because the strength of the 11th and 16th groups of the first mix ratio test was not formed, some errors were caused to the results of this test; for this reason, the research group summed up the experience, adjusted the factor level and carried out the L9 (3⁴) orthogonal test. The experimental design, the test results are shown in Table 7, and the range analysis is as follows (Table 8):

Table 8 L9 (3⁴) orthogonal test range analysis results

Table 7 shows that under the design level of this orthogonal test, the most significant effect on the compressive strength (7d) of coal gangue concrete is the ratio of coal gangue, followed by the ratio of coal gangue and sand, and the third is the water-binder ratio. In order to see the results of the range analysis more intuitively, draw the curve with the factor level as the abscissa and the average value of the compressive strength under each factor level as the ordinate (Fig. 6).

Fig. 6

Analysis of compressive strength (7d) of coal gangue concrete with the factors level

According to the L9 (3⁴) orthogonal test design range analysis results, the compressive strength (7d) of coal gangue concrete increases first and then decreases with the increase of the proportion of coal gangue, first decreases and then increases with the increase of the proportion of coal gangue sand, and increases with the increase of the proportion of water glue. The increase of the ratio also decreases first and then increases. Combined with the mix ratio results of the two orthogonal test designs, it is necessary to maximize the amount of coal gangue and not to reduce the strength. Comprehensive analysis shows that the optimal mix ratio is that 90% of the coarse aggregate uses coal gangue, and the coal gangue mechanism. The amount of sand can reach 60% of the proportion of sand, and the water-to-binder ratio is 0.4.

4 Conclusion

Under the dominance of the high content of coal gangue, the proportion of coal gangue, the proportion of coal gangue and sand and the water-binder ratio were preliminarily calculated, and the L16 (4⁵) orthogonal experimental design table was used to arrange the test mix ratio; compressive strength factor. Based on the test results of the first orthogonal test, considering the strong water absorption of coal gangue, adjust the level of the above three factors to carry out the second orthogonal test design mix ratio arranged by the L9 (3⁴) orthogonal table, after 7 days of standard curing, The compressive strength (7d) of coal gangue concrete was tested. The significance of the factor level was obtained by range analysis.

Combined with the results of two orthogonal test designs, it is concluded that the optimal mix ratio of high-volume coal gangue concrete is 90% coal gangue, 60% coal gangue sand, and 0.40 water-binder ratio. Large, natural crushed stone should not use crushed stone with a larger average particle size than coal gangue, and should use “rice stone” (0 ~ 5 mm crushed stone) that can hold the coarse aggregate of coal gangue. The sand rate is 35%, and the amount of cement per square is 350 kg. The compressive strength (7d) of gangue concrete under this mix ratio can reach more than 10 MPa. Innovation points and application prospects.