Study on The Durability of Self - Luminous Magnesium Phosphate Cement in Low Temperature Climate Building Exterior Wall

. Self-luminous magnesium phosphate cement is a new material that combines long-afterglow luminescent material with magnesium phosphate cement. This paper studies the performance of self-luminous magnesium phosphate cement material through experiments, so as to expand the applicability of this material to exterior walls of buildings in a cold climate. The experiment indicates that the Self-luminous magnesium phosphate cement has excellent corrosion resistance, impermeability, and salt-freeze resistance in a cold climate. It can be used as a new kind of beautiful, energy-saving and environment-friendly exterior wall paving material.


Introduction
Self-luminous magnesium phosphate cement is a new road energy storage luminescent material that combines long-afterglow luminescent material with magnesium phosphate cement [1].This new functional layer material can absorb and store visible light, ultraviolet light, X-ray and other radiation energy, and continue to emit light within ten hours after stopping excitation.
Long-afterglow luminescent materials do not require electricity, relying only on visible light, ultraviolet light, and X-rays to store energy and release energy to emit light.With the wide application of long afterglow luminescent materials in various fields, its related industries have also developed rapidly.For example, 'SM Krogh [2] prepared luminescent aggregate from luminescent silica crystals and calcium silicate glass materials, and S Timilsina [3]' prepared luminescent coating from long afterglow luminescent materials of rare earth aluminate system.'L Hong et al. [4] prepared a kind of self-luminous glass by mixing long-afterglow luminescent materials with glass-based materials and calcination at high temperatures.' In terms of architectural landscape, 'Han B. [5] et al.M Qi et al. [6]' discussed the application of the surface self-luminous coating in architectural environment decoration.The luminescent coating made of long afterglow luminescent material has the performance of the ordinary coating and the unique luminescent decoration function at the same time.The material is widely used in outdoor pavement and landscape pavement, which has the characteristics of supplementary lighting and strong decoration, As shown in Figure 1.Therefore, this paper studies the performance of the self-luminous magnesium phosphate cement material through experiments to expand the applicability of this material to the exterior walls of buildings in a cold climate.

Study on the durability of self-luminous magnesium phosphate cement materials
The durability of cement-based materials includes two aspects: First, the destructive power of structural failure or damage when the building structure fails or deforms; Second, the nature of the material itself in the face of failure or damage resistance [7].During the use of cement-based materials in a cold climate, due to the influence of external factors such as humidity, temperature and acid-base balance, the resistance and force of the cement structure are unbalanced, which reduces the stability of the material to a certain extent, resulting in the failure or deformation of the structure.The material test block is shown in Figure 2.In the study of ordinary Portland cement structure, it is found that carbonate corrosion, freshwater erosion, freeze-thaw cycle and so on can lead to a change in the internal stress of the material, which is easy to cause structural damage.On this basis, the self-luminous magnesium phosphate cement material prepared in this paper can effectively play its functions in practical engineering appli-cations, namely light absorption, light storage and luminescence properties.It must be ensured that the self-luminous magnesium phosphate cement material can effectively absorb visible light and the emitted light must also have sufficient identification in the dark [8].

The durability of chemical erosion
The specimens used to test the chemical erosion resistance were cured at a temperature of 0±2℃ and relative humidity of 70% for 1 d, after which the molds were removed and weighed, and then soaked in the specified solution.The erosion solution should be supplemented in time to ensure the concentration of the solution and stirred regularly.At present, weight loss and strength loss are the indexes of the research on the corrosion resistance of cement-based materials in corrosive chemical media.The material weight loss is determined by cleaning the sample removed from the solution, removing as much as possible of the eroded portion, and drying and weighing the sample.
The calculation formula for weight loss is: Type: G --before and after corrosion weight loss (%); G0 --Weight of test block before corrosion (Kg); G1 --Weight of corroded test block (Kg) The calculation formula for strength loss is: Type: R -before and after corrosion strength loss (%); Rp --Strength of standard curing test block at the same age (MPa); R1 --Strength of corroded block (Kg) In order to study the corrosion resistance of self-photo phosphate-based magnesium cement-based materials, corrosion tests were carried out in a simulated corrosion environment.Using tap water, sulfuric acid, sodium hydroxide, sodium sulfate and magnesium sulfate as erosion substances, the weight loss and strength loss of the experimental block were measured and recorded under the erosion action of the test solution.The ratio of materials in different tests is shown in Table 1, and the loss rates of strength and weight are shown in Table 2  As the main hydration products of spontaneous light magnesium phosphate cement, MgNH4PO4 6H2O crystallization conditions can, generation and solution of stability of straight self-illumination magnesium phosphate important influence to the strength and durability of cement base material.MgNH4PO4 6H2O in long-term corrosion solution will happen under the erosion of the crystalline structure damage and strength loss.
Studies found that MgNH4PO4 6H2O is formed in the spontaneous light magnesium phosphate cement base material intensity is the main component of the MG 3 (PO 4 ) 2 4H 2 O phosphate is not favorable to the development of strength and other by-products, especially the Mg(OH) 2 crystals with large stack will produce porous structure, Seriously affect the mechanical properties and durability of materials.On spontaneous light corrosion resistance of magnesium phosphate cement base material study found that the material will be generated in the process of hydration reaction MgNH4PO4 6H2O, the material in the corrosion of strong acid and strong alkali solution and under the action of generation is reduced, still can make flocculent production material in the material struc-ture of Mg(OH) 2 , lead to material strength greatly reduced.Due to the infiltration of magnesium sulfate solution, spontaneous light magnesium phosphate cement base material internal structure magnesium Mg 2+ concentration increases, material internal MgNH4PO4 6H2O generation increases, its maintaining high strength in the long-term immersion environment.
To sum up, the chemical erosion resistance of self luminous magnesium phosphate cement-based materials is in the order of tap water >10% magnesium sulfate solution >10% sodium sulfate solution >2% sodium hydroxide solution.

Durability against leakage
The ability of the material to resist the penetration of the corrosive medium into the interior is called impermeability.The chemical erosion resistance and freezing resistance of the material are closely related to the impermeability.In the study of the impermeability of cement structure, it is found that the cement, sand and other materials inside the cement structure will be affected by hydration reaction in the process of setting, which makes the water gradually accumulate in the structure, resulting in the space and capillary inside the material [9].In addition, in the study and analysis of hydration product volume, it is found that the microstructure of cement-based materials is closely related to the change of its volume.In order to accurately measure the impermeability of the self-luminous magnesium phosphate cement-based material, the impermeability test was carried out on the round bench test block with the upper mouth diameter of 70 mm, the lower mouth diameter of 80 mm and the height of 30 mm.The mix ratio of the self-luminous magnesium phosphate cement-based material in the experiment was shown in Table 4.No water seepage occurred when the impermeable pressure of the test block was 4.0 MPa.The test was stopped after the pressure was maintained for 24h, and the test block was split immediately after taking out the test block to determine the height of the water seepage.It can be seen from the test results that the self-luminous magnesium phosphate cement material has good impermeability.The main reason for the good impermeability of the self-luminous magnesium phosphate cement magnesium cement-based material is that the water-binder ratio of the material is very low (0.1), and the phosphors and hydration products form a compact structure and fewer pores are formed.At the same time, the shrinkage of the material is very low, and the possibility of cracks caused by shrinkage is greatly reduced.

Resistance to salt freezing
Frost resistance is one of the important performance indexes of cement-based materials, which has a great influence on the stability and bearing capacity of cement structures [10].When the cement structure is affected by the external environment and temperature, the structure of the temperature change under the action of denudation and cracking.In order to study the salt-freeze resistance of the self-photo phosphate-based magnesium cement-based material, the material was immersed in 3% NaCl solution for 28 days after natural curing in the air environment for a freeze-thaw cycle test.The size of the test block was 40×40×160 mm, the NaCl solution was 5 mm, and the temperature was controlled at -20±2℃ for the freeze-thaw cycle test.The experimental data are shown in Table 5.As can be seen from Table 5, the self-luminous magnesium phosphate cement material showed good freezethawing denudation resistance during the freeze-thawing cycle test.There was no freeze-thawing denudation damage even after 40 freeze-thawing cycles, and the material spalling amount was only 0.74 kg/m3 when the freezethawing cycle was 100 times.The reason is that during the hydration reaction of the spontaneous photo phosphatebased magnesium cement-based material, a large amount of NH3 gas will be produced, and a good bubble structure will be generated inside the material, which plays a certain role in physical air entraining.At the same time, the waterbinder ratio of the self-luminous magnesium phosphate cement material is very low, which reduces the amount of freezable water in the system, so that the material has excellent freezing resistance.

Conclusion
(1) Spontaneous light magnesium phosphate cement base material resistance to sulfate attack ability is strong, its hydration products MgNH4PO4 6H2O in different erosion solution of stability is the key to influence the chemical erosion resistance material.The chemical erosion resistance of self luminous magnesium phosphate cement materials was in the order of tap water >10% magnesium sulfate solution >10% sodium sulfate solution >2% sodium hydroxide solution.
(2) The self luminous magnesium phosphate cement material has good impermeability and no water seepage occurs when the impermeability pressure of the test block is 4.0 MPa.(3)The self-light magnesium phosphate cement-based material has good salt-freezing resistance, and the erosion damage of 100 freeze-thaw cycles is only 0.74 kg/m 3 .
Magnesium phosphate cement-based materials are combined with fluorescent materials to make self-luminous magnesium phosphate cement-based materials, which have the advantages of high early strength, sulfuric acid corrosion resistance, salt and freeze resistance in low temperature environment, and can be used as a new kind of beautiful, energy saving and environmental protection building exterior wall paving materials.

Table 2 .
and Table 3.The age is 7 d, 28d and 60 d, respectively.Strength of self-luminous magnesium phosphate cement-based materials after solution erosion

Table 3 .
Mass loss rate of self-luminous magnesium phosphate cement-based materials

Table 4 .
Date of impermeability test

Table 5 .
Date of freezing and thawing test