Experimental Study on the Mix Proportion Optimization of Full Tailings Concrete

Full iron tailings concrete can effectively use iron tailings as the raw materials of concrete. This paper used iron tailings powder, iron tailings sand and iron tailing gravel to replace fly ash, river sand, and ordinary gravel as concrete admixtures, fine and coarse aggregates, respectively. The orthogonal experimental method was used to optimize the mix proportion of C40 strength grade full iron tailings concrete. Research results showed that the optimal mix proportions of C40 strength grade full iron tailings concrete was water cement ratio 0.34, sand ratio 39%, and water reducing agent 0.4%.


Introduction
In the past 30 years, China's economy has been in a stage of rapid development, and the large demand for steel production capacity promotes the development of the iron ore industry, resulting in a large amount of iron tailings stockpiling.According to the "China Environmental Statistics Annual Inspection -2020" statistics, the production of iron tailings in China was close to 535.6 million tons, with a utilization rate of 21.6%.
Huang X and Mingjing yang et al. [1][2][3] showed that mechanical activation of iron tailings powder could stimulate its potential activity and iron tailings powder could be used as admixture in concrete .Jiwei Cai and Heping Cheng et al. [4,5] studied the workability and mechanical properties of iron tailings sand as fine aggregate for concrete.Xinxin Ma et al. [6] investigated the bending performance of full iron tailings concrete beam.
This paper used Qian'an iron tailings powder, iron tailings sand and iron tailing gravel as raw materials to replace fly ash, river sand, and ordinary gravel as concrete admixtures, fine and coarse aggregates, respectively.The orthogonal experimental method was used to optimize the mix proportion of C40 strength grade full iron tailings concrete.The research results provided a theoretical basis for fully utilization of iron tailings.

Test materials
The cement used was ordinary Portland cement P • O 42.5R.The iron tailings powder was from Qian'an City, Hebei Province, and was ground mechanically by ball mill to a fineness of 466 m 2 /kg of specific surface area [2] .The physical properties of cement and iron tailings powder are shown in Table 1.The iron tailings gravel and iron tailings sand from Qian'an City, Hebei Province were used as the coarse and fine aggregate respectively.The physical properties of coarse and fine aggregates are shown in Table 2.The water reducing agent used was a polycarboxylic acid high-efficiency water reducing agent with a water reduction of 25%.The water used was tap water of Baoding City.Considering that the main factors affecting the mechanical properties and workability of concrete were water binder ratio, sand ratio and water reducer dosage, these three factors were selected in the orthogonal test design to optimize the mix ratio of full tailings concrete of C40 strength grade, and were represented by label A,B and C respectively.In the test process, three levels of each factor were selected and name 1,2 and 3 respectively.The factors and levels of orthogonal test design are shown in Table 3.Using FTC1~FTC9 to represent different mix proportions of full tailings concrete, the orthogonal test combinations are shown in Table 4. Based on the above analysis and the specification [7] ,the orthogonal test mix proportions of C40 strength grade full tailings concrete are shown in Table 5.Table 5 shows that the utilization rate of iron tailings is over 83.8%.

Analysis methods of orthogonal test
To analyse the degree of influence of different factors on the working performance and mechanical properties of concrete, the limit difference analysis method of orthogonal experiments was used to discussing the experimental results in the following section.The meaning of the indicators used in limit difference analysis is as follows: jm K -the sum of the corresponding experimental indicators when the factor in column j is taken as m level.
jm K -the average value of K jm , reflecting the magnitude of the impact of different levels of the same factor on the experimental results, and determines the optimal level that should be taken for this factor.R j -the limit difference of factor j, referring to the difference between the maximum and minimum average indicator values at each level of the factors in column j.R j reflect the magnitude of the change in the experimental indicators when the level of factors in column j changes.The larger the R j , the greater the impact of this factor on the experimental indicators and is more important.The smaller the R j , the smaller the impact of this factor on the experimental indicators and is less important.

Test methods of mechanical properties for full iron tailings concrete
The working performance and mechanical property tests of full iron tailings concrete were carried out in accordance with the "Standard Test Methods for Physical and Mechanical Properties of Concrete" [8] .

Working performance of full iron tailings concrete
Table 6 shows the working performance of each group of full iron tailings concrete measured in the tests.
Table 6 shows that the 9 mix proportion tests using orthogonal method have good working performance with slump ranging from 170mm to 180mm, and meet the requirements for working performance of pumped concrete.From the R value in Table 6, it can be concluded that the water cement ratio, sand ratio, and water reducing agent have similar influence on slump of full iron tailings concrete, with sand ratio having the greatest influence and water reducing agent having the smallest influence.The values of sand ratio K1, K2, and K3 in the Tale 6 show that an increase in sand ratio will increase the slump of the full iron tailings concrete.When the sand ratio is 43%, the average slump is 177mm.When the water cement ratio is 0.36, the corresponding slump is the highest.When the water cement ratio is reduced, the corresponding slump will be reduced.The reason is that the decrease in the water cement ratio leads to an increase in the amount of cementitious materials used.When the sand ratio and water reducing agent remain unchanged, the cohesiveness of the full iron tailings concrete mixture increases, and leads to a decrease of the slump.The change in the dosage of water reducing agent is between 0.35% and 0.45%, which has little influence on the slump.Increasing the dosage of water reducing agent appropriately from 0.35% to 0.4% will increase the slump.

Strength of full iron tailings concrete
The strength test results of full iron tailings concrete with different mix proportions in each group are shown in Table 7.
The limit difference analysis results based on the test results of orthogonal experiment in From the limit difference analysis results of Table 8, it can be found that the optimal mix proportions for cube compressive strength, splitting tensile strength, and axial compressive strength are A3B3C3, A1B3C1, and A2B1C2, respectively.For factor B, cube compressive strength and splitting tensile strength B3 are the best levels.In B3 level, the R value of the axial compressive strength index is the smallest, and this means the influence of B3 to the splitting tensile strength is the smallest.Therefore B3 is selected as the optimal level.For the factor A and C, combined with the comprehensive comparison of the K value and R value, it can be seen that the two factors have different optimal results in different experimental indicators, and neither has the worst value.Therefore, A2C2 is selected.Based on the above comprehensive analysis, the optimal combination is selected as A2B3C2.The optimal mix proportions of C40 strength grade full iron tailings concrete is "water cement ratio 0.34, sand ratio 39%, and water reducing agent 0.4%.

Conclusions
Based on the above research, The conclusions are as follows: (1) The optimal mix proportions of C40 strength grade full iron tailings concrete is water cement ratio 0.34, sand ratio 39%, and water reducing agent 0.4%.
(2) Full tailings concrete can effectively utilize iron tailings.The utilization rate of iron tailings is over 83.8% for C40 strength grade full iron tailings concrete.
(3)The amount of sand content is the main factor affecting the slump of full iron tailings concrete mixture, and the water cement ratio is a secondary fact.
(4) The water cement ratio is the primary factor affecting the strength of full iron tailings concrete.The sand ratio and the amount of water reducing agent are secondary factors, and the appropriate addition of water reducing agent can improve the strength of full iron tailings concrete.

Table 1 .
The physical properties of cement and iron tailings powder.

Table 2 .
The physical properties of aggregate.

Table 3 .
Orthogonal test design of factor and level.

Table 5 .
Mix proportions of orthogonal tests for full iron tailings concreteMixing proportions of full iron tailings concrete C40.

Table 6 .
Working performance of concrete C40.

Table 7 .
Table 7 are shown in Table 8.Orthogonal test results of strength

Table 8 .
Limit difference analysis table