Ventilation ducts are mostly thin-walled and large painted products, which often experience warping and deformation issues during production and use. The main reasons are as follows: four points:

(1)镁水泥的体积是在自然固化条件下的连续膨胀过程。线膨胀率为0.1-0.2%，体积膨胀率约为1%。

(1) The volume of magnesium cement is a continuous expansion process under natural curing conditions. The linear expansion rate is 0.1-0.2%, and the volume expansion rate is about 1%.

(2)在服务燃烧过程中不可避免地发生菱镁矿的局部过烧，导致部分MgO处于死燃状态。过度燃烧的MgO的水合过程非常缓慢。当具有强度的水泥石中的MgO再次被水水合时，形成Mg(OH)2，并且体积膨胀为120%。

(2) In the process of service combustion, local overburning of magnesite inevitably occurs, resulting in some MgO being in a dead burning state. The hydration process of excessively burned MgO is very slow. When MgO in high-strength cement paste is hydrated again by water, it forms Mg (OH) 2 and has a volume expansion of 120%.

(3)轻度燃烧氧化镁中游离CaO的体积在水化过程中增加了90%，而SO42-存在于盐水片中。当Ca(OH)2转化为石膏时，固体体积将增加两倍以上，导致产品变形。这也是限制轻微燃烧的氧化镁和盐水薄片中的SO42-中游离CaO含量的原因。

(3) The volume of free CaO in lightly burned magnesium oxide increased by 90% during the hydration process, while SO42- was present in saline solution. When Ca (OH) 2 is converted into gypsum, the solid volume will increase by more than twice, leading to product deformation. This is also the reason for limiting the free CaO content in SO42- in slightly burned magnesium oxide and salt water flakes.

(4)MgO-Mg-C12-H2O体系的水化过程是一个强烈的放热反应过程，生产管道的体积很大。如果环境温度高，则反应温度升高。片材的整体温度太高并且反应不充分，导致更大的结晶应力和热膨胀应力，以及产品的体积膨胀。

(4) The hydration process of the MgO-Mg-C12-H2O system is a strong exothermic reaction process, and the volume of the production pipeline is large. If the ambient temperature is high, the reaction temperature increases. The overall temperature of the sheet material is too high and the reaction is not sufficient, resulting in greater crystallization stress and thermal expansion stress, as well as volume expansion of the product.

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