  Let $k = \mathbb{Q}(\sqrt{\alpha})$ be a real quadratic number field, where
$\alpha$ is a positive square-free integer. Let $\mathcal{O}_k$ be the ring of
integers of $k$. In this paper, we prove that a certain set of $2 \times 2$
singular matrices with entries in $\mathcal{O}_k$ can be written as a product
of a bounded number of idempotent matrices. Our main theorem can be viewed as a
generalization of the recent result by Cossu and Zanardo, which studies finite
generation of certain singular matrices by idempotent matrices over
$\mathcal{O}_k$ instead of bounded generation of certain singular matrices by
idempotent matrices over $\mathcal{O}_k$ as considered in this paper.
