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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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    Whst do you mean by "waterproof"? Mortar isn't rapidly damaged by water, as witness many thousands of brick structures. It is porous, however. What's the real problem you're trying to solve? – keshlam Oct 28 '15 at 18:38
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    We have a new septic tank made of solid stone masonry. The masonry is held together by cement mortar and has been plastered with cement plaster. A professional working on the job has ensured us that the tank is waterproof (i.e. it should not egress water into the soil.) However, posts online are telling me that neither mortar nor plaster is waterproof. So is this guy totally wrong? Do we need to apply a waterproofing system to the septic tank? – AlfaZulu Oct 28 '15 at 18:46
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    Edit your question to make clear that this is what you really need to know. You'll get better answers. – keshlam Oct 28 '15 at 18:51
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    I'd prefer that septic tank bit as a new question. I like this one's second part about classifications, so I answered it ;) – Mazura Oct 29 '15 at 2:25
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Mortar is not waterproof. However, there are products that can be applied to mortar (and other concrete materials), that can make the mortar waterproof.

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    I'm still trying to make sense of the advice I was given. Can ordinary mortar be described as water resistant? Or was the guy I was talking to just totally wrong? – AlfaZulu Oct 28 '15 at 18:02
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    @AlfaZulu If mortar (or most any concrete material) is thick enough, it can prevent water from passing all the way through it, given that the pressure of the water is not too high. For example. If you spray a garden hose at a brick wall, you'll not likely have water pass through. However, ground water can be pushed up through the concrete slab of your basement, if the water table raises too high. – Tester101 Oct 28 '15 at 18:55
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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.

  • The permeability of concrete decreases as its strength (and age) increases:

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.

enter image description here

–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

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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.

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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.

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Mortar is definitely not waterproof as such but neither is it affected by water (unless freezing).

Water penetrates a brick wall so far under normal conditions and then dries out during intervening dry periods.

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