If $K$ is a class of semiassociative relation algebras and $K$ contains the relation algebra of all binary relations on a denumerable set, then the word problem for the free algebra over $K$ on one generator is unsolvable. This result implies that the set of sentences which are provable in the formalism $\mathscr{L}w^x$ is an undecidable theory. A stronger algebraic result shows that the set of logically valid sentences in $\mathscr{L}w^x$ forms a hereditarily undecidable theory in $\mathscr{L}w^x$. These results generalize similar theorems, due to Tarski, concerning relation algebras and the formalism $\mathscr{L}^x$.