I planning a cinder block bench with a limestone top, should I use portland cement or masonry cement ?

What product to attach the limestone top to the cinder block would be best to use ? I have covered the cinder block with stack stone. I find the limestone to be very powdery, how would it best to seal the limestone? Also, some of the limestone slabs have a slight color difference, how can I get them to be more similar in color......bleach?