Why are some ductile iron pipes cement-mortar lined?

Cement mortar lining provides a high pH in the inner surface of ductile iron pipes and it serves as a physical barrier to water to guard against corrosion of iron from acidic water. Moreover, owing to smooth nature of cement lining, it results in the provision of high flow.

For cement-mortar lined ductile iron pipes to convey water, water infiltrates the pores of lining and releases some calcium hydrate. The freed calcium hydrate form calcium carbonate with calcium bicarbonate in water so that it serves to clog the pores of linings and avoid further permeation of water. Moreover, iron also reacts with lime to precipitate iron hydroxide which also seals the pores of linings. As such, the lining provides both chemical and physical barrier to aggressive water.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.