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So I just got back from watching an episode of the Science Channel program Unearthed, a show focusing on archaeological discoveries. In the episode in question, the subject was on the Lighthouse of Alexandria, and one of the questions asked in the episode is how the lighthouse stood true in an earthquake hotspot for 1500 years. One theory was that the granite bricks were connected by a kind of mortar made from molten lead. A demonstration had been tested, and it turns out that a wall of granite bricks and molten lead mortar can withstand seismic shock without problem.

Now, obviously, lead is not a metal worth recommending, considering its toxic nature, but are there other metals or alloys that, when still molten, can be used to make mortar? And would it be practical to use them given today's technology?

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    $\begingroup$ I'm not sure I see a worldbuilding question here. $\endgroup$ – user535733 Aug 6 '19 at 2:57
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    $\begingroup$ I suspect that lead was used because it has a low melting point and because its malleability would allow for some energy absorption between the blocks rather than simply cracking like Iron would. So, I suspect that your answer is gold, but I'm not convinced that's commercially viable. $\endgroup$ – Tim B II Aug 6 '19 at 3:41
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    $\begingroup$ I'd seriously doubt that molten lead could be used as mortar, since it would almost certainly solidify before you could put another brick on top of it. (And you'd have an awful lot of burned masons...) What might be possible is to use lead strips between the blocks, similar to the way lead is/was used to join pieces of stained glass. $\endgroup$ – jamesqf Aug 6 '19 at 4:25
  • $\begingroup$ Isn't "molten metal mortar" just welding? $\endgroup$ – Psylent Aug 6 '19 at 4:59
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    $\begingroup$ @Psylent, it's called brazing $\endgroup$ – L.Dutch - Reinstate Monica Aug 6 '19 at 5:23
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Bi58, with a little help.

An alloy of bismuth and tin in the proportion 58:42 known as Bi58. by itself has a melting point of 138 °C (280.4 °F), but doesn't by itself have suitable properties to act as mortar, as it needs to be able to stay where it's troweled to and although denser needs the approximate properties of a well-whipped egg-white.

The builder's mate would need to either use a spot-board and shovel or a powered rotating mixer and add-in a goodly proportion of copper powder and a little flux (to aid wetting), this will give the desired properties. A rotating mixer might just be easier to maintain at the right temperature and small batches would be be ferried to the (brick) layer as needed.

On cooling, the mixture expands by a small percentage (1 or 2%) which gives a good grip on the surfaces of rough materials. It has the virtue of being fine with the day/night cooling cycle, so will stay intact over time.

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Using metal as binder between non metallic part is a knowledge we humans are using for quite some time. We still use it today in Kintsugi.

In the case of Kintsugi, however, gold particles are added to the binder, instead of melting the gold itself.

What would be the requirements for such a metal to be used as binder?

  • non brittle, to yield when (dynamic) loads are applied and possibly damp them
  • low melting point, to prevent that the high temperatures alter the other substance
  • resistant to oxidation, to prevent that in few years the binder is turned to dust

The above leave not many alternatives to lead, I am afraid.

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The Inca were reported to have used molten lead silver gold and a bitumen pitch concoction that is very thin and boils but hardens well after being poured into tight cracks. With today's alloys I am sure you could do better. However with silver being as cheap as it is, might be the way to go.

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  • $\begingroup$ The boiling point of pitch is well below the melting point of the three metals that you mention, perhaps you could add some detail as to how it could work? (From review). $\endgroup$ – Bitter dreggs. Dec 27 '19 at 23:05
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Metal matrix composites are used in a wide variety of high-end technical applications.

A composite material is a material made of two significantly different components that are combined together into one material without chemical mixing, and which typically consist of fibers of a reinforcement material "glued together" with a matrix material that fills up the spaces between them. An example of it is fiberglass, which uses glass fibers as the reinforcement material, and a polymer resin as the matrix material.

Metal matrix composites are composites that use metal as their matrix material; they can be manufactured using a variety of methods, but simply pouring molten metal over a fiber mesh is a simple (if relatively imprecise) way of producing one. Examples of it include tungsten carbide cutting tools that use cobalt or bronze as their matrix, aluminum-matrix boron carbide materials used for certain high-end automotive applications, and the monofilament silicon carbide fibers in a titanium matrix used in F-16 landing gear.

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