1) The mixture has good workability, large fluidity, cohesiveness, water retention, self-tightness and other properties. Under the premise of ensuring the quality, the slump value is designed to a maximum value of 230 mm to prevent the temperature of the mixture from being high and the slump loss is too large and the concrete pseudo-condensation phenomenon occurs under high temperature and long-distance pumping conditions. It can also reduce unfavorable factors such as excessive mixing error caused by uneven water content of the material.

2) Considering the unfavorable factors such as mixing error and the effect of low temperature in winter to reduce the strength of concrete, the natural strength of the prepared concrete specimens must be greater than 90% for 3 days to meet the construction needs of prestressed concrete, and reduce the concrete creep variables to ensure the tensile quality. The 28-day compressive strength of the formulated C30 concrete is not less than 30 MPa.

3) The retardation time of the prepared mixture meets the construction requirements, and the concrete pouring is 16 - 18 hours according to the pouring amount and the production capacity of the mixing station. The retarding time should not be too long, which will affect the early strength growth of concrete; it should not be too short, because it will affect the quality of concrete.

4) Select retarding high-efficiency water reducing agent and admixture to improve various properties of concrete, reduce hydration heat, and delay the release rate of hydration heat. The concrete mix ratio design meets the requirements of the design specifications of the anti-seepage concrete mix ratio and improves the crack resistance of the concrete.

5) Under the premise of ensuring the good working degree of the concrete mixture, the water-to-binder ratio can be reduced as much as possible to improve the strength of the concrete, reduce the shrinkage of the concrete, and reinforce the durability of the concrete.

6) The concrete elastic modulus Eh is not less than 3.0 × 10⁴ MPa.

According to the above objectives, combined with the C30 grade concrete mix ratio test in the construction site test room, the Best Way low alkali cement with a strength grade of 42.5 is used, and all indicators meet the requirements of Pakistan standards; high quality class II fly ash produced by Tacote Power Plant, Pakistan; S95 grade mineral powder produced in Pakistan, each indicator meets the requirements; The machine-made sand is the coarse sand of Zone I from the Tarcott Stone Field, with a fineness modulus of 3.4 and a stone powder content of 13%; The coarse aggregate is a 5 - 31.5 mm continuous graded gravel in the Tarcott stone field; The admixture is selected from polycarboxylate superplasticizer produced in Pakistan; Use clean groundwater for mixing water. Some of the material pictures are as follows: (Figures 1-4).

According to the C30 grade concrete reference mix ratio by the laboratory early stage test and machine-made sand replaced in proportion, five ideal C30 grade concretes was initially determined, as shown in Table 1.

The compressive strength of each age of each mix concrete was compared by the following test: (Table 2).

The test results show that the optimum replacement rate of the concrete performance of each mix ratio is selected to determine the most economical mix ratio.

It can be seen from the test results that when the substitution rate of the machine-made sand is between 30% and 50%, the working performance of the concrete is basically the same, but the substitution rate is relatively good at 30%. The main reason is that the machine-made sand particles have more angular edges and the surface is rougher than natural sand. With the increase of the amount of machine-made sand, the frequency of the machine-made sand surface contact with each other increases, which affects the fluidity of the concrete.

The following is an attempt to replace the natural sand with 50% high stone powder sand to prepare C20 to C50 pumped concrete to see if it can meet the working performance, pouring performance and strength.

Then try to apply to the C20 to C50 grades of concrete, and compare the corresponding work performance and compressive strength (Table 3 and Table 4).

The results showed that:

When the high stone powder content machine-made sand replaces 50% of the natural sand, in order to make the working performance of the concrete meet the requirements, it is necessary to increase the dosage of the admixture, especially the high-grade concrete. Thus, the cost advantage of using machine-made sand for concrete is compromised and can also cause other problems.

Therefore, the next step is to pass the high-silver content of the machine-made sand through a 0.630 mm sieve, sieve the stone powder and fine

particles and then test it to see if the amount of admixture can be reduced after the stone powder is reduced, meanwhile the replacement rate is also 50% natural sand (Table 5 and Table 6).

The results showed that:

The high stone powder content factor has a certain influence on the factors affecting the working performance of the concrete prepared by replacing the natural sand with the high sand powder content machine-made sand. However, the mixed concrete work performance issues can be better solved by improving or increasing the amount of the water reducing agent.

Next, the natural sand was replaced by 30% and 50% substitution rate for medium and low-grade pumping concrete, and the corresponding work performance and compressive strength were compared (Table 7 and Table 8).

The results showed that:

For medium and low-grade pumping concrete, the mixed concrete workability is better when we replace 30% natural sand with machine-made sand, and the concrete with 50% substitution rate is relatively poor. Even if the dosage of admixture is increased, it is still impossible to solve the problem of poor compatibility.

The following experiment was conducted with the use of machine-made sand at 50% and 100% replacement rate for ordinary pavement concrete and low slump concrete (Table 9 and Table 10).

1) The machine-made sand produced in the process of crushed stone production has high content of sandstone powder, poor particle size distribution, and high stone powder content can only be used to prepare concrete less than C30 grade. If the concrete is mixed with machine-made sand natural sand in a ratio of 3:7, the pumped concrete of less than C40 grade can be well prepared.

2) Replace the natural sand with 30% high stone powder content machine-made sand to prepare pumped concrete. The mechanical properties of the powder are slightly better than pure natural sand due to the filling effect of the stone powder. It is effective to replace natural sand with 50% machine-made sand to prepare ordinary pavement concrete and low slump concrete, and it is not necessary to significantly increase the dosage of admixture, and the economic benefit is obvious.

3) Using high stone powder content machine-made sand to replace part of natural sand to prepare concrete can not only save natural resources, but also save costs and have significant economic benefits.

4) In combination with the actual situation in the region, the high stone powder content machine-made sand needs to be properly treated before use to better ensure the quality stability of the machine-made sand.

5) From the experimental research and experience accumulation, determine the optimal cooperation of mixed sand concrete and pure natural sand concrete, such as the following Table 11.