Can Keras be used to build clustering models
Yes, you can use Keras to build clustering algorithms. Recalling that Keras is a high-level api for Tensorflow or Theano, and that you can build a K-means algorithm (very well known clustering technique) in Tensorflow, you should be able to build it in Keras. Example (in tensorflow):https://learningtensorflow.com/lesson6/.

Nevertheless, sklearn has in-built clustering algorithms like K-means or Gaussian Mixture Models, so at first, I think a better approach is using these sklearn funcionalities.