This is because the gaps among the original grains are often not
totally plugged with cementing chemicals ...
Grammatically, it's tempting to call "cementing" an adjective, but in fact it is a verb phrase modifying "chemicals", i.e. it's an attributive modifier. Semantically, the phrase "cementing chemicals" presumably means something like 'chemicals that are capable of cementing something'.
Note that "cementing" can't be an adjective for the following reasons:
 Unlike most adjectives, it can't be modified by "very": we can't say *"very cementing"
 It can't occur as complement to complex-intransitive verbs like "become": we can't say *"It became quite cementing".
 It can't occur as complement to complex-transitive verbs like "find": we can't say *"I found it quite cementing".
The range of expressions that can occur as pre-head modifier to a noun is very large and varied: we don't want to call them all adjectives. "Cementing" has none of the properties of indisputable adjectives and hence does not belong in that class. Which is why it is best analysed as a VP here.