Cement mortar coating should generally be applied after the spiral steel pipe has been laid, pressure tested, and backfilled and compacted according to design requirements. This is because spiral steel pipes have low rigidity and thin walls, and the pipeline must be in a stable state during coating application. If the coating is applied before the pipe is laid, significant deformation during lifting, transportation, and backfilling can damage the anti-corrosion layer.

Before coating application, the inner wall of the spiral steel pipe must be cleaned to remove loose rust, dirt, scale, grease, welding slag, and other adhering substances. The protrusion of the inner wall of the spiral steel pipe should not exceed 1/3 of the designed thickness of the anti-corrosion layer; the vertical deformation of the pipeline should not exceed the design specifications and should not exceed 2% of the inner diameter of the spiral steel pipe.

The cement mortar internal anti-corrosion layer can be applied using mechanical spraying, manual troweling, drag-tube application, or centrifugal prefabrication. If prefabrication is necessary for the internal anti-corrosion layer, protective measures should be taken during transportation, installation, and backfilling. During construction, the first step is to mix cement mortar. The mass ratio of cement to sand should be 1:(1-2), the slump of the cement mortar should be 60-80 mm, and the compressive strength of the cement mortar should not be less than 30 MPa.

Mechanical spraying produces a smooth, dense, and uniformly thick cement mortar internal anti-corrosion layer with good results. Mechanical spraying began to be used abroad in the 1930s, and in China, it was adopted in cities such as Shanghai, Qingdao, and Dalian starting in the 1960s. When using mechanical spraying, elbows, tees, special steel pipe fittings, and pipe sections near gate valves can be manually coated, with a smooth transition section connecting to the mechanically sprayed material.

Other methods, such as drag-tube or centrifugal prefabrication, are also widely used, but will not be discussed in detail here. Regardless of the construction method used, the maintenance process is a crucial step in ensuring that the cement mortar’s internal anti-corrosion layer does not crack or become hollow. Therefore, after the cement mortar anti-corrosion layer is formed, it is necessary to immediately seal the steel pipe to prevent air convection; after final setting, it should be moistened for curing. Generally, the curing time for silicate cement should not be less than 7 days, and for slag silicate cement, it should not be less than 14 days; before water is supplied, it should be continuously sealed and kept moist.