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I have just been told that tar alone makes a good enough mortar because of its adhesive capabilities.

This, however, does not change the fact that, since the Roman Empire, concrete has been the mortar of choice. Still, this brings up an interesting scenario.

True to the spirit of Life After People, this scenario focuses on what happens to a building when we vanish rather than how we actually vanished. Even though Roman concrete stood the changes of 2,000 years, most scientists doubt that in a Life After People, skyscrapers built on modern concrete would last longer than 150 years.

So in this alternate scenario, tar was the mortar of choice. For a building in a temperate climate built on bricks baked in an oven rather than by the sun (the former being more durable than the latter) and glued together by tar, how long would this building last in a Life After People? Does tar have what it takes to outdo concrete, or did the Romans make a wiser decision in inventing concrete?

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  • $\begingroup$ I don't think tar as mortar would support that much weight. If you're just building two or three story buildings you might be OK, but no skyscrapers (and likely not many bridges, either). $\endgroup$ – TMN Jul 21 '16 at 18:49
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    $\begingroup$ Mortar and concrete are two very dfferent materals. Mortar is actually the name of a class of materials, which are used to bind bricks together. $\endgroup$ – AlexP Jun 4 '17 at 16:34
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Tar is definitely not a good mortar. Tar is fluid and never forms a solid. It doesnt do well in heat, either. Even modern asphalt, made from tar, will melt in hot weather. Tar is is good for creating waterproof seals, especially when the seal needs to be flexible. That is why it is used on boats to waterproof and seal ropes, rigging, and decks.

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  • $\begingroup$ Oh yes. We had a tarmac school playground when I was a kid. On really hot days you could write your name it it by gouging the letters out of the tarmac with a wooden lollipop stick! $\endgroup$ – DrBob Jul 21 '16 at 13:07
  • $\begingroup$ Worth reading this. Besides reviewing whether that surface is properly called Tarmac®, it reviews such terms as “Asphalt concrete”, tar-grouting, and even mentions adding portland cement to tar! $\endgroup$ – JDługosz Jul 21 '16 at 18:12
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Concrete can be made using tar as the cement, as for roads. I think you are messing up your terminology, as with the previous concrete question. Concrete is not mortar. Mortar is not made from concrete.

So mortar would be tar with sand. Concrete has large chunks and chunks across size scales.

I recall reading that tar-based concrete was used in parts of California because of earthquakes. Perhaps mortar would be the same: use it because of the properties it has! While permanently set cement would crack with motion, a re-setable cement can heal itself and withstand slow “creep”, or be reset by heating the wall.

It is also waterproof, so is used in that capacity for ships. How about stone foundations of swamp areas? They might make mortar from tar because it works for ships.

And, maybe it's simply what is available.

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  • $\begingroup$ The terminology of "mortar" is "something that glues the outer shell together". $\endgroup$ – JohnWDailey Jul 21 '16 at 12:12
  • $\begingroup$ So concrete could only be used as a mortar if the bricks were more than 4 inches apart, as it contains large chunks. Thin-set is the same idea but with (only) pea-sized chunks. $\endgroup$ – JDługosz Jul 21 '16 at 18:03
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Petroleum derived tar would be an exceptionally poor material to bond bricks and blocks into large load-bearing walls. On the other hand, no one uses concrete as mortar, as far as I know (not too far, admittedly). Perhaps you meant "cement"? Note that all of these terms have a generally understood civil engineering meaning, but also have more general, more ambiguous meanings. They especially should be avoided when discussing novel applications. Portland cement mortar is the most common type used in modern construction, but lime mortar is also still used. Tar is used to seal and waterproof, it is a weak adhesive and as mentioned is thermoplastic and will flow under load and heat.

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  • $\begingroup$ I saw a documentary that explained the Great Wall of China used boiled rice as an ingredient to strengthen their cement. Not an answer, but an interesting aside. $\endgroup$ – CaM Jun 5 '17 at 20:29
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I learned something the other day as brick layers were laying bring I asked them which is stronger mortar or brick, and they replied you can't use concrete as mortar because of the rocks in it. So I'm not sure exactly what the difference is but there is a difference. Also, it was said that you can't use tar to build skyscrapers well Genesis 11:3 states that the building was to be made from brick and tar as mortar. Some recent archaeological studies have found that some middle eastern ziggurats were believed to be at least 500 feet tall based on the dimensions of the bases that remain. It is has been computed that brick can sustain a building of that size and weight. So maybe we should discount the ability of tar to be used a mortar. We may not have the exact chemical composition that was used in that era. They may have put additives that strengthened it. I personally still want my house to be built with good old fashioned mortar, but I still don't want to disqualify it. BTW we can't compare modern concrete to roman concrete. They still made a better concrete than modern society. Roman concrete gets harder with time, while modern concrete weakens in time. Roman concrete was more advanced than ours.

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    $\begingroup$ You are confused between concrete and cement. See en.wikipedia.org/wiki/Concrete If you had asked which is stronger, cement or brick, you might have gotten the answer you wanted. Cement-mortar has a yield strength of about 20 - 30 MPa, Building brick can go anywhere from about 15 to 60 MPa. So the answer to your question is "Yes". $\endgroup$ – WhatRoughBeast Jun 4 '17 at 15:46
  • $\begingroup$ «you can't use concrete as mortar because of the rocks in it.» yes, as older answers explain. I mention “thin-set” too. “concrete could only be used as a mortar if the bricks were more than 4 inches apart, as it contains large chunks” nearly a year ago. $\endgroup$ – JDługosz Jun 5 '17 at 9:06
  • $\begingroup$ «They still made a better concrete than modern society. Roman concrete gets harder with time, while modern concrete weakens in time. Roman concrete was more advanced than ours.» I would disagree with all of those statements. The cement used in modern concrete is engineered and assayed for consistent properties matching its use; the exact ampunt of magnesium and whatnot can be tweeked as desired. Why does modern concrete get weaker with age, as you claim? $\endgroup$ – JDługosz Jun 5 '17 at 9:11

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