The cement is the benchmark cement produced by Qufu Zhonglian Cement Co., Ltd., and the test of fly ash meets the technical requirements of F-class and II-class fly ash in GB/T 1596-2005 Fly Ash for Cement and Concrete. The content of fly ash is 30%. Sand adopts medium sand with fineness modulus of 2.7, and coarse aggregate is gravel, with continuous gradation of 5 ~ 20 mm ..

2. Influence of magnesium oxide on impermeability of concrete

Impermeability is an important physical property of concrete, which measures the ability of concrete to resist liquid and gas penetration. In order to study the influence of magnesium oxide content on impermeability of concrete, impermeability tests were carried out on concrete with magnesium oxide content of 0, 2%, 4% and 6% at 3.5MPa, and the average permeability heights were 27.8mm, 24mm, 22.5mm and 2 1.5mm respectively. We can see that adding magnesium oxide improves the impermeability of concrete. The average penetration height of concrete without magnesium oxide is 27.8 mm, while the impermeability of concrete with magnesium oxide is less than 27.8 mm. With the increase of magnesium oxide content, the penetration height gradually decreases, but the effect of the increase of magnesium oxide content on reducing the penetration height gradually weakens. This is because magnesium hydroxide crystals generated in the hydration process of magnesium oxide in concrete gather on the surface of magnesium oxide particles, which fills the pores inside the concrete and reduces the porosity of the concrete, thus enhancing the water resistance and improving the impermeability of the concrete.

3. Influence of MgO on frost resistance of concrete

The frost resistance of concrete refers to the ability of concrete to resist expansion and cracking under repeated freeze-thaw cycles, which can be measured by relative dynamic modulus and mass loss rate. At present, the research results of frost resistance of concrete mixed with magnesium oxide are not uniform. Li Fangxian used slow freezing method to carry out frost resistance test, in which magnesium oxide was added instead of cement. The results show that the frost resistance of concrete mixed with MgO expansive agent is much higher than that of ordinary concrete. Li Chengmu adopted the rapid freeze-thaw test. The results show that the relative dynamic modulus loss of magnesium oxide concrete is below 50% and the weight loss is about 65438 0.5% after 33 times of freezing and thawing, while the relative dynamic modulus loss of concrete without magnesium oxide is below 60% and the weight loss is about 2.4% after 23 times of freezing and thawing. The addition of magnesium oxide expansion agent makes the concrete structure more compact and the frost resistance of concrete is obviously improved. The test of magnesium oxide frost resistance of plum was carried out by quick freezing method. The test results show that the greater the content of magnesium oxide, the greater the decrease of relative dynamic modulus, indicating that the addition of magnesium oxide has an adverse effect on the frost resistance of this kind of concrete. Li observed the hydration process of magnesium oxide and cement on the microscopic scale, and pointed out that the particle disintegration accompanying the hydration process of magnesium oxide caused damage or even destruction of microstructure strength. In order to determine the influence of magnesium oxide on the frost resistance of concrete, the author carried out the frost resistance test of concrete according to the relevant provisions in SL 352-2006 "Test Specification for Hydraulic Concrete". When the relative dynamic modulus drops to 60% of the initial value or the mass loss rate reaches 5%, it is judged that the sample is damaged. The frost resistance test results of concrete with MgO content of 0, 2%, 4% and 6% are as follows: when the number of freeze-thaw cycles of concrete is 0, the relative dynamic modulus is 100% and the mass loss is 0; When the number of freeze-thaw cycles of concrete is 100, the relative dynamic modulus is 92.4%, 96. 1%, 97.4% and 97.0% respectively, and the mass loss is 1.3%, 0.7%, 0.5% and 0.5% respectively. When the number of freeze-thaw cycles of concrete is 200, the relative dynamic modulus is 765,438+0.5%, 865,438+0.7%, 82.2% and 84.8%, and the mass loss is 5.8%, 2.2%, 65,438+0.8% and 65,438+0, respectively. We can sum up the relationship between freeze-thaw cycles and relative dynamic modulus and mass loss of concrete with different magnesium oxide content: (1) With the increase of freeze-thaw cycles, the relative dynamic modulus decreases and the mass loss increases, while with the increase of freeze-thaw cycles, the relative dynamic modulus decreases and the mass loss increases. (2) The addition of magnesium oxide improves the frost resistance of concrete. After 200 freeze-thaw cycles, the relative dynamic modulus of concrete mixed with magnesia is obviously higher than that of concrete without magnesia, and the mass loss is obviously lower than that of concrete without magnesia. (3) With the increase of magnesium oxide content, the relative dynamic modulus gradually increases and the mass loss gradually decreases, that is, the frost resistance of concrete gradually increases.

4. Influence of MgO on impact resistance and wear resistance of concrete

The impact of water flow has a great influence on the durability of hydraulic concrete, which greatly shortens the service life of buildings. The abrasion damage of concrete is caused by the impact and friction of the medium in the water flow on the concrete surface [3]. According to the size, shape and density of particles and the velocity of water flow, the medium in water flow can be divided into suspended load and bed load. The erosion failure mechanism of suspended sediment is that the water flow drives the friction of suspended sediment with smaller particle size on the concrete surface, which leads to the abrasion and erosion of the concrete surface. The abrasion of concrete materials by high-speed water flow containing suspended sediment is realized by grinding the weak parts layer by layer. The erosion and wear damage of bed load to hydraulic concrete buildings includes sliding friction, rolling friction and jumping impact, among which jumping impact is the most destructive to concrete. This time, according to the corresponding test method (underwater steel ball method) in the Standard Specification for Hydraulic Concrete Test (DL/T 5 150-200 1), the impact and wear resistance of concrete were tested. Underwater steel ball method was put forward by 1980. When this method is tested, the wear and tear state of concrete surface is close to that of concrete in actual engineering. The wear index of concrete is expressed by wear strength or wear rate. The impact and wear resistance tests of concrete with 0 and 4% magnesium oxide content were carried out respectively. The test results show that the wear rate of concrete without magnesium oxide is 2.3 1%, and the wear strength is 14.05438+0h/kg? m-2； The wear rate of concrete with 4% MgO is 1.55%, and the wear strength is 18.22h/kg? M-2. From the test results, it can be seen that the wear rate of concrete decreased by 33% and the impact strength increased by 30% after adding 4% magnesium oxide. It can be seen that the addition of magnesium oxide can significantly improve the impact resistance and wear resistance of concrete.

5 conclusion

The volume micro-expansion caused by the hydration of magnesium oxide can effectively solve the problem of concrete cracking. In this paper, the impermeability test, frost resistance test and impact wear resistance test of concrete mixed with magnesium oxide are carried out respectively, and the influence law of magnesium oxide on each test is analyzed and discussed, and the following conclusions are drawn: (1) With the increase of magnesium oxide content, the permeability height of concrete gradually decreases and the impermeability of concrete increases. This is because the magnesium hydroxide crystals produced during the hydration of magnesium oxide gather on the surface of magnesium oxide particles, which will fill the pores in concrete and reduce the porosity of concrete, thus enhancing the water resistance of concrete and improving the impermeability of concrete. At the same time, it is concluded that the effect of increasing the content of magnesium oxide on improving the impermeability of concrete is gradually weakened. (2) The frost resistance test of concrete with different MgO content shows that the addition of MgO improves the frost resistance of concrete. After freeze-thaw cycle, the relative dynamic modulus of concrete with MgO is obviously higher than that without MgO, and the mass loss is obviously lower than that without MgO. With the increase of magnesium oxide content within 4%, the frost resistance of concrete is gradually improved. (3) After adding 4% magnesium oxide, the wear rate of concrete decreased by 33% and the impact strength increased by 30%. Adding magnesium oxide can significantly improve the impact resistance and wear resistance of concrete.

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