A Rock Solid Idea: Impermeable Cement


On the first day in the field, our group walked down the road from our hotel to Fira Quarry. From the top of the trail, I had a great view of the size of the rock outcrops that extended all the way down into the mined out area, but from the bottom of the quarry I was staring up at 20 meters of Minoan eruption ash and pumice. The wall of rock that stood dauntingly above me made me think about why it stopped where it did and what happened to all the other material that once filled in the area we were standing in, and why it would benefit anyone to mine the pumice at the quarry.

IMG_0099.jpg(figure 1) This picture shows the boundaries of the quarry and where pumice was mined 

I remembered hearing a quick comment in one of the lectures about how the ash and pumice was mined from Santorini in the late 1800s to build the Suez Canal because it can make water resistant cement. I was bewildered by the idea that the rocks in this quarry had been there, untouched and unmoved since the Minoan eruption in 1613 BC (+/- 13 years) and then one day people just started mining. Some of the material that was ejected from Santorini’s magma chamber in Santorini is now in the walls of the Suez Canal in Egypt.

This stuck with me and left me wondering about the composition of the cement, what about the ash and pumice made it water resistant? Why had all the pumice on the island not been mined if it made such great cement? What other constructional purposes could ash be used in to improve the structures? As I began researching the topic the more it started to make sense and I realized that the aftermath of the enormous caldera explosion has some long-lasting positive effects, such as creating economic resources for this small island.

IMG_0090(figure 2) Outcrop at the quarry showing pumice from the Minoan eruption

Pozzolanic cement is made with silicious volcanic material (the pumice in the quarry has ~73% silica), which when added to other cement mixtures, produces water-resistant cement. Silica reacts with lime, a chemical that is produced as the cement solidifies and helps stop it from cracking as it hardens, so it makes sense to use silica-rich pumice in the mixture[1].

Pumice works for this purpose because the reactions require high silica content and pumice is a felsic silica-rich rock (the pumice is ~73% silica). You could not take basalt (between 50-60% silica), crush it up and add it to the cement mixture expecting the same results as using pumice. The reaction would not be the same and the cement would not have the same properties as pozzolan because of the lack of silica present to interact.

Pozzolan from Santorini, called Santorini earth, was mined starting in 1867 for building the Suez Canal, up until around 1980 when they stopped in order to preserve the environment of the island. Similar types of cement are used around the world because it is lower cost while maintaining durability and quality of cement.

Ash can also be used for constructional purposes outside of cement because of it durable nature. The runway of Santorini airport is founded on volcanic ash and a large portion of the pavement is made of compacted ash. The capacity of compacted ash to sustain loads of overlying pavement is very high compared to other small particle substances like sand.

IMG_0093.jpg(figure 3) Pumice and ash at the Fira Quarry

Pumice is not necessarily a renewable resource so people cannot just mine it infinitely for the purpose of making cement. We would have to wait for another eruption just to get the pumice again, and large, highly explosive volcanic eruptions equivalent to that of Santorini’s almost 4,000 years ago are relatively rare.

As we walked out of Fira Quarry, just south of our hotel, I was left with the thought that someone before us had the idea to take the pumice deposits from the highly explosive eruption and make something new out of it. The make up and silica content of the pumice makes it the perfect candidate for incorporation into cement because it strengthens bonds and creates durable building material. An eruption that destroyed and incinerated an entire island produced material that is now being repurposed and used to build and strengthen modern cities.



[1] Stamatopoulas, A.C., Kotzias, P.C., 1990, Volcanic Ash in Ancient and Modern  Construction, in Hardy, D., and Doumas, J., Eds, Thera and the Agean World III (proceeding from the third International Congress, Santorini, Greece, 3-9 September 1989)


5 thoughts on “A Rock Solid Idea: Impermeable Cement

  1. Sam – thank you for incorporating all of my comments. The featured image is too small amd so it does not take up the entire space (can you chnage to a photo in landscape format?). My advice for your next post is to try to find better ways to expain ‘bewildered’ (a little dramatic?) and “perfect.” Also I’m hoping we can learn more about your experiences here in your next post (sounds, vistas, feelings, growth, struggles…?)

  2. Hi Sam!

    This is an interesting topic for me to read about as an outsider, and your post went into great detail. Your use of images is perfect for a blog post because it broke up your paragraphs in a way that I was just reading little chunks at a time, instead of being confronted by only text. This is great.

    You did a nice job at creating a narrative that brought me into your world and thought processes as you explored and researched your topic. While I liked reading about what questions prompted your research, I had a tricky time actually understanding the answers to those questions. Something you can do to address this discrepancy is to start with one question at a time, and then answer your own question by explaining the research you have done. By focusing on one topic at a time, you help guide your readers through your growing understanding of the topic. This approach also helps readers who may be unfamiliar with this material not get too overwhelmed.

    With this post, I got a bit lost after the third paragraph where you went into specific details about the pumice. This information was presented all at once and was a lot to digest. When you have this much information, and especially when you’re looking at information from the past to the present, you can organize your posts so that you present the information in a chronological order. For example, at the end of your third paragraph you mention that one of the effects of the caldera explosion was the creation of economic resources for the island. This would lead perfectly into your discussion about the cement that was mined as a resource for the Suez Canal. Within that discussion, you would then incorporate details about the composition of the pumice and why it was so perfect for this project. In general, you want to create transitions between ideas and organize your post so that your reader will easily follow along with your ideas.

    What I really liked about your second to last paragraph was that you tied your research about pumice into the larger idea that this is not a convenient resource. Several geological elements need to fall into place before we can use this rare resource again. This conclusion brings your research questions and subsequent research together to show the larger implications of pumice. As an outside reader, this is a strong conclusion because it relates everything together in an explicit manner that I can easily understand.

    Overall, this post is informative and gives insight into your research. Your personal narrative is intriguing and makes me want to know more about this topic. For future posts, I would recommend continuing to keep that “lay audience” in mind and think about how to organize your post in a way that allows for easy understanding from outside readers. Great first post!


    1. Thank you so much for taking the time to read and analyze my post and provide me with such great feedback! Your comments will help me a lot in writing my next blog and I really appreciate it.

  3. Hey Sam,

    I remember learning the pumice and ash were mined for the Suez Canal because it made great cement, but I never thought about that again. Kudos for delving deeper and reporting what you found; I enjoyed this post.
    When they mine the pumice, the deposits also have varying amounts of lithic fragments that widely range in size. Since these lithic fragments are basaltic and you said basaltic cement wouldn’t be beneficial, how do they pick out the smaller lithic fragments?


  4. Alex,
    Thanks for taking the time to read my blog! It was pretty difficult to find solid information on this topic so I’m not really sure how to answer your question. My guess would be that they filter or sift the lithic fragments out somehow, and I’m sure pieces still get mixed in. The pieces of basaltic rock would not necessarily make the pumice cement less beneficial, but making purely basaltic cement would. Thanks for the question, it made me think more about my topic!

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