What are the light

The scientific name of MgO is magnesia, commonly known as magnesia, magnesia, magnesia. Density 2.94, light yellow or white powder. Melting point 2852 ℃, boiling point 3600 ℃. It is soluble in acids and ammonium salts, but not soluble in water and ethanol. The specific surface area is 180 m2 / g at 400 ℃. The specific surface area is only 3 m2 / g at 1300 ℃. It is easy to deliquescence in the air. It is a kind of gas hard cementitious material.


Light burned magnesia is also called caustic bitter soil, active magnesia. It is a kind of light burned magnesia obtained by calcining natural magnesite ore, brucite and magnesium hydroxide Mg (OH) 2 extracted from seawater or brine at 700 ~ 1000 ℃.


2 ore source


（1） Magnesite ore is one of the main sources of MgO. MgO content is about 47%. MgCO3 is a kind of solid powder (MgO) formed by light calcination (400-600 ℃) and grinding. The mineral resources are widely distributed in Liaoning, Xinjiang, Sichuan, Shandong, Tibet and other provinces and autonomous regions, with reserves of 2.8 billion tons, accounting for 30% of the world's mineral resources.


（2） Dolomite [CaMg (CO3) 2] is also the main source of MgO, and its reserves are larger and more widely distributed, with MgO content accounting for about 22%. The content of Cao is about 30%, and the rest is 48%. The theoretical structure of MgCO3 and Cao is one to one. Dolomite should be defined according to the content of MgCO3 more than 25%, otherwise it can not be used as the ore for producing magnesium oxide and can not be called dolomite.


（3） Serpentine [MG6 (si4o10) (OH) 2] is also a raw material for the production of magnesium oxide, mainly magnesium silicate (3mgo · 2SiO2 · 2H2O)


（4） The slag from smelting light magnesium alloy is also the raw material for producing magnesium oxide.


3 admixtures


After the reaction of pure water solution and magnesium oxide, Mg (OH) 2 is in loose state and has no strength, so there is no practical value. In 1867, the Swedish scholar Sorrell invented magnesia cement. When magnesium chloride is used as curing agent, the compressive strength and flexural strength of Portland cement are almost the same in dry environment. Compressive strength 110MPa / 28d, flexural strength 20MPa / 28d.


（1） Magnesium chloride hexahydrate (MgCl2 · 6H2O) curing agent makes MgO cement have the same compressive and flexural strength as ordinary silica cement, but it can only be used in dry environment. Magnesium chloride hexahydrate, also known as bischofite, is soluble in cold water at 0 ℃, with a water-soluble ratio of 281g / 100g, relative density of 1.56 and melting point of 712 ℃. When heated to 118 ℃, it began to hydrolyze and decompose into MgO and HCl at high temperature.


MgCl2·6H2O→Mg(OH)Cl+HCl+5H2O


Mg(OH)Cl→MgO+HCl


To Sishui, Bishui, Yishui and anhydrous magnesium chloride [2], please refer to "Introduction to magnesium chloride of Anmei Industrial Development Co., Ltd.". The content of MgO is 15-20%


（2） Magnesium sulfate heptahydrate (MgSO4 · 7H2O) is both a curing agent and a waterproof agent. Its molecular weight is 246.48, colorless crystal or powder. No bromine and bitter taste. MgSO4 is dissolved in water at 250 ℃ and reduced to MgO above 400 ℃. It is a reaction product of MgO and H2SO4. The content of MgO is 15-20%. The cost is higher than that of MgCl2 · 6H2O.


（3） Ferrous sulfate heptahydrate (FeSO4 · 7H2O) is a waterproof agent, light blue monocrystal, also known as alum. The relative density of FeSO4 · 7H2O is 1.898 (at 18 ℃), which is easily melted in water and has natural water producing chlorite. 3% of the total.


（4） Styrene butadiene latex (SBR solid content ≥ 45%) is waterproof latex, and its content accounts for 6-12% of the total content.


（5） Chloroprene latex, also known as chloroprene latex, is the most stable water-proof latex in magnesium oxide materials, accounting for 6-12%, and adding 1% of (ohch2ch2) 3N.


（6） Silica sol, styrene acrylic latex and butadiene acrylonitrile latex can be used as water-proof latex, and the content accounts for 6-12% of the total content.


（7） SiO2 type additives account for 10-15% of the total amount.


（8） Urea formaldehyde resin and melamine are also waterproof agents, accounting for 2-3% of the total content.


（9） Phosphoric acid (H3PO4), also known as orthophosphoric acid, is a good curing waterproof agent, accounting for 4% of the total content.


（10） The modified sodium silicate is a good curing waterproof agent, and its content accounts for 1-3% of the total amount


4. Four stages of hydration heat release cycle of magnesia cement


In the early stage of hydration induction, the first exothermic peak was formed in 10 minutes after mixing and molding.


The hydration induction period is the slow hydration reaction period within 60 min.


The second exothermic peak appeared in the hydration accelerating period within 240 min.


The stable period of hydration deceleration is from the second exothermic peak to stable, and the hydration reaction stops at 14 h. Through the four stages, it can be seen that the initial setting is 1.5h, the final setting is 4-5h, and the exothermic curing is completed for 14 days. After passing the test, it can be installed out of the warehouse for 20d (wall panel or various prefabricated parts), and the cast-in-place magnesia concrete can be removed after 6h.


Calcination temperature and time are the key to determine the activity and dispersion of MgO.


Whether it is magnesite or dolomite, the decomposition reaction in calcination is endothermic reaction. Due to the decomposition of CO2 and H2O, MgO forms a porous crystal structure, which is not dense. The product has good activity and high dispersion (180m2 / g). This means that when the product is ground at 400 ℃ to 600 ℃, the product will be hydrated in a few minutes at room temperature after being ground. Therefore, a good calcination process should control the temperature between 400 ℃ and 600 ℃, that is to say, energy saving, and the product quality is good. When the temperature is higher than 900 ℃, the densification dispersion of the crystal is less than 3 m2 / g at 1300 ℃. It takes 75 days for 95% MgO to be hydrated at room temperature. According to the lattice constant of Gleason, MgO reaches 0.425 nm and Mg (OH) reaches 0.424 nm at 400 ℃. That is to say, the light firing temperature of 400 ℃ ~ 600 ℃ is the best temperature, which is the source of the theory of light firing powder. The longer the calcination time is, the more compact the crystal structure is, and the hydration is difficult.


Several elements of optimum mix proportion:


（1） The specific gravity (relative density) of the water solution of the admixture is 1.35-1.50. Water in the preparation of lightweight wallboard concrete