An interesting role of QCA is that it can disentangle a Hamiltonian while quantum circuits cannot. We will begin with an example of QCA on 3d lattice that disentangles a Walker--Wang lattice model. We will identify an algebraic object that determines the boundary algebra in the special case of Clifford QCA, and interpret the group of QCA modulo circuits and shifts as a Witt group. We will end with a characterization of boundary algebras of general QCA.