Actually.. I'd put cement bags high on the non-durable scale.. They're essentially a double-ply grocery bag.. but the paper is thinner.
Their purpose is weighted more towards getting the cement out than keeping it in.

I need my laptop bag to be waterproof... Somehow one made of paper doesn't seem like the right choice.

I don't know why, but I really like the look of this thing. If I weren't hopelessly unemployed, I'd probably shell out the $80 even though I never use messenger bags.

"My grandmother was tougher than a South African Cement Sack"

Hello,

You are correct about the cement bag being three layers of thin brown paper. What makes the laptop bag strong is that bonded to each layer of paper is a normal weight fabric. This fabric makes the paper untearable and very durable. The bag is also double thick which means that on each side there are two layers of fabric (inside) with two layers of paper bonded on the outside.
I hope I am making sense.

Kind regards,
Wendren