Concrete as used in the Pantheon is self healing. Using siiica fume, or metakaolin with lime based cementitious materials as used by the Romans could be of great interest. If only we could add domething to speed ip strength gain.
Dear Gordon,
roman cement isn’t one only recipe as recent articles published on LinkedIn said and isn’t necessarily self healing at all.
Romans knew several different technologies and for example Pantheon that you use as example isn’t properly made on Roman cement but it was made of several types of materials. For the bricks and for the structural parts was used proto (ancient) geopolymer like material done for the Coliseum too and other hi-tech buildings.
Check at the book of professor Davidovits from www.geopolymer.org there is a complete chapter dedicated and full of geo-chemical explanations.
Roman cement was realized using natural or artificial pozzolanic materials found locally in center of Italy (from volcanic areas: pozzolana, lapillo and volcanic ashes like tuffs or other local zeolites) in precise ratio on the hydrate lime. Then several types of aggregates were added (sands, cocciopesto, gravel and crushed rocks) depending on the application‘s target. Cocciopesto was one of the first recycled material used because the fines under 45 micron had some good reactivity with lime and coarser pieces worked as crushed rocks but more lightweight.
Self healing is process that involves unreacted material that restart working after cracking and thanks humidity (water), crystalizing again creating insoluble minerals. But this results can be achieved in several ways and Romans probably learned how to do that from the Egyptians that for first developed ancient Geopolymer recipes with prolonged durability to resist chemical alteration.
Roman cement used straw as fiber reinforcement and old Geopolymers too, but the first was still hydraulic material while the second was polymeric but completely mineral based so after centuries they can be confused. But mechanical performances where very different. Pozzolanic reaction starts when lime reacts with amorphous silicon aluminate sources so isn’t fast hardening, but is much faster than hydraulic lime setting (to have a comparison).
If you want to accelerate pozzolanic reaction you can use some basic additive but never could be compared to Portland cement strength development.
Instead also Old Geopolymers were quite fast hardening because time was a key factor even in the past. Look at the time needed to build the Pantheon and consider the tools and what Romans could use at that time…their work was amazing and super fast. Bricks aren’t cooked (dried under sunlight like nowadays used for clay-bricks) but are less water absorbent and more dense compared to modern bricks, just because different chemistry.
Today Roman cement recipes are still used in Italy but reaction depends on several factors even the water used for mixing. At Roman age only weak buildings lime based were done using sweet water. Roman cements can be obtained even or better if working with seawater and Geopolymers too.
Romans never used silica fumes because it didn’t exist (come from arc furnace flame > 2000°C). Even Metakaolin is more modern material but could be obtained during reaction if they use clay and add some heating (normally were the chemicals sodium and potassium rich that come from the ashes of burning wood). They used for sure natural pozzolana because was humid and already rich about amorphous minerals so all mixing and reactions were faster and easier. But Roman Empire was huge so cannot be used everywhere exactly the same recipe of Roman cement. Romans were super smart for their age to understand and move trough the empire just the necessary to replicate the system.
Today there is already the knowledge about this way to build but the limit is the costs of raw materials because durable doesn’t mean cheap. Even Romans knew that because only important constructions were done using more durable materials. Ratio between pozzolanic raw materials and Ca(OH)2 is variable but every time you need much more pozzolanic materials (calcination and micronization costs) and additives to accelerate the CSH crystallization. Those are the issues.
Best regards
Alex Reggiani
I met Prof Davidowitz when he came to South Africa. One question i discovered when looking at the Pyramids. The shells were random in the mix. Stone couldn't have been crushed.
However, if they could reform their poured mix, I think they could have liquified the rock.
Dear Gordon,
about pyramids I’m agreed with you. But Roman Cement was little different because raw materials in Italy come from other sources. We haven’t big and long rivers and we haven’t big tides due to big rainstorms so Romans learned how to adapt the recipes. I don’t understand well what you are meaning with liquifying rocks but this technique could be close to what has been used for acid Geopolymers in South America. Check on the site of Geopolymer Institute there is a paper dedicated at www.geopolymer.org
Actually I can assure that reproducing the ancient Roman cement it’s possible but it isn’t cheap. Almost every places in the world have the raw materials to do that but of course mechanical strength will be affected on the quality and reactivity of them.
Best
Alex
Thank you. I sold silica fume from SA back in the 1990's and presented a paper on metokaolin to an ATC
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