I was advised by a professional that mortar is waterproof, however sources online are telling me that it's not.
So is mortar waterproof or not? Does waterproofness vary by mortar classification (e.g. M1, M2, M3 etc.)?
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Yes, mortar is waterproof. It is "relatively unaffected" by water "under specified conditions".
Water-proof or water-resistant describes objects relatively unaffected by water or resisting the ingress of water under specified conditions. –Waterproofing, Wiki
However, anything claiming waterproof is likely a long way away from being watertight or impervious to water.
M4 mortar is actually just concrete, with one part portland and four parts sand, according to Boral. Where as, M1 is just one part hydrated lime and three parts sand (and it's the only entirely lime-based mortar, which should be used for repairs on old buildings that have it, precisely because otherwise you'll have moisture problems).
I speculate that the waterproofness of M1 ~ M4 mortar can be loosely determined, as they will each have a given cure strength according to ISO 13007.
From the presented results it was possible to present correlation curves with compressive strength, age and characteristic strength compression versus the passing water flow trough it.
–International Symposium (NDT-CE 2003) Non-Destructive Testing in Civil Engineering 2003, Permeability of Concrete: A Study Intended for the "in situ" Valuation Using Portable Instruments and Traditional Techniques
Morter is not waterproof. Bricks are not waterproof.
Ceramics, like kitchenware, are waterproof as long as they're not cracked. The difference is that ceramics are fired with a glaze on the outside.
To get the same outer layer of protection, you're looking for a silicon spray that can be brushed or pressure-sprayed on the outside. This layer will wash off over time and needs to be reapplied every decade, just after a clean.
In my research regarding ferrocement water tanks especially in less industrialized countries, a structures waterproofness depends on the preparation of the mortar mix, having much to do with hydrating the mix with the least amount of water possible that still gives you a fully hydrated workable mixture. As i understand it, less initial moisture prevents cracking in the finished surface. Also, it must be trowled to a proper finish, and the portland cement to sand ratio is important. Reasearch "ferrocement" which is a mortar like mix that utilizes metal meshes, such as chicken wire, expanded metal lath etc, as reinforcement rather than rebar.