比较图4中样品外观特征，未开裂的膨胀区表面和开裂内部的产物颜色存在明显

Comparing the appearance characteristics of the samples in Figure 4, there is a clear difference in the color of the product on the surface of the uncracked expansion zone and inside the crack. The outer surface is still grout grey, while the cracked interior has a large amount of brown product produced. It can be asserted that the expansion and cracking of the mortar specimen is caused by the formation of brown corrosion products. The surface layer did not crack because no brown corrosion products were formed.

Comparing the appearance characteristics of the sample in Figure 4, there is a significant difference in the product color between the surface of the uncracked expansion zone and the cracked interior. The outer surface is still cement slurry gray, while a large number of brown products are generated in the cracked interior. It can be concluded that the expansion cracking of mortar specimen is caused by the formation of brown corrosion products. The non cracking of the surface layer is also due to the non formation of brown corrosion products.

Comparing the appearance characteristics of the sample in Figure 4, it is found that there are obvious differences in product colors between the surface of the uncracked expansion zone and the cracked interior. The exterior surface is still gray with cement slurry, while a large number of brown products are generated in the cracked interior. It can be concluded that the expansion cracking of mortar specimens is caused by the formation of brown corrosion products. The surface layer did not crack because no brown corrosion products were formed.