Let S be an integrally closed torsion-free grading monoid with quotient group G, and R an integral domain with quotient field K. We show that if each non-zero element of G is of type (0, 0,... ), then every t-ideal J of K[S] is of the form J = Hk[I] for some h ∈ K[G] and t-ideal I of S. Using this, we also show that if the semigroup ring R[S] is a PVMD and each non-zero element of G is of type (0,0,...), then Clt(R[S]) ≌ Clt(R) Clt(S).