This verse refers to building "houses of cement" but it is generally agreed that this means they build house of stones which were cemented together.
Now we come to several verses in which Mormon, the historian, indicates a fantastic growth in population. However, we are reading the words of a historian who is dealing with a long time span. Verse 10 indicates that not only was there a remarkable expansion in the population but there was extensive trade between the land southward and the land northward.