Hi Thomas, thank you for your answer.
With your advice I succeeded in creating multi-column masonry on the mobile layout of my website.
Unfortunately, as you can see in this attached picture:
https://www.dropbox.com/s/ivapqo8ft1ivs4e/bug%20masonry.png?dl=0
I found a bug on the positioning of the posts inside the masonry.
Could you please help me to solve the problem?
Best regards,
Giacomo