To best answer this question, let's first look at the definitions of primary and secondary industries.
A primary industry involves the collection and production of raw materials. For example, a farmer who produces and collects his crop would be considered a primary industry. Also, a fisherman fishes to collect "raw" material to sell.
A secondary industry is one that takes raw materials and transforms them into goods to sell. For example, a car manufacturer would be considered a secondary industry. Also, those who produce clothing would be considered secondary as well.
Because construction takes raw materials such as wood, steel, iron, etc. and transform it into something else, it would be considered a secondary industry. Construction companies take materials from primary industries in order to build residential and commercial buildings.