File Name: geopolymer chemistry and applications joseph davidovits .zip
New state-of-the-art materials designed with the help of geopolymerization reactions are opening up new applications and procedures and transforming ideas that have been taken for granted in inorganic chemistry. High temperature techniques are no longer necessary to obtain materials which are ceramic-like in their structures and properties. This new generation of materials, whether used pure, with fillers or reinforced, is already finding applications in all fields of industry.
- Geopolymer cement
- Geopolymers based on natural and synthetic metakaolin - A critical review
- Geopolymers based on natural and synthetic metakaolin - A critical review
Although metakaolin reacts in alkaline as well as in acidic medium, the present issue focusses exclusively on the alkaline route. The acidic phosphate-based technology will be presented in another issue, in the next future. It was manufactured with an advanced technology in a flash calciner instead of being roasted in a rotary kiln or a vertical multiple-hearths oven.
We discovered that this metakaolin was reacting very well with soluble alkali silicates. I recognized the potential of this discovery and presented an Enveloppe Soleau for registration at the French Patent Office.
It was the first mineral resin ever manufactured. However, we had to test at least 10 different metakaolin brands in order to find the right product, which would react as a geopolymeric precursor, in alkaline medium.
Indeed, at that time, the bulk of the various metakaolins was used essentially as fillers in the paper making and plastic industry. Its specific chemical reactivity towards alkalis remained confined in the production of very special products, namely synthetic zeolites, especially the type Zeolite A. Both metakaolins reacted weakly compared to the metakaolin we had been working with in France.
We recognized that we had had luck when starting the geopolymer research, in Saint-Quentin. We had tested the right source of metakaolin, from the beginning. And we became aware of one major parameter in geopolymer science, namely the calcining temperature of the geological kaolinitic clays.
This new structural approach has profound consequences with regard to a better understanding of geopolymerization mechanisms. In particular, according to the reaction:. Metakaolin is not alumina! Other researchers who continued to use the general term metakaolin did not adopt this label.
It will be used all throughout this review. In calcination carried out in a rigid vertical multiple-hearths calciner, a sufficiently low water vapour pressure is maintained during the entire roasting process, providing the desired chemical reactivity Al in 5-fold coordination. Same for products manufactured in a flash calciner. This is not the case for metakaolins obtained in a rotary kiln, commercialized as Portland cement additives.
Unfortunately, this later product is more and more used in geopolymer research because it is easily available. This raises new concerns in terms of reactivity and reproducibility of the results obtained with this raw material essentially tailored for Portland cement applications, not for geopolymer technologies. The present special issue, acknowledging the abundance of publications on metakaolin-based geopolymers, tries to collect the best of the scientific production for the understanding of geopolymer resins, binders and cement properties.
Our review will also pinpoint those papers where the authors overlooked the important parameters mentioned above. The geopolymer chemistry was invented 40 years ago because we had the luck to get the right geological raw material and the appropriate calcination process. Today, there exist new methods based on the synthesis of alumino-silicates.
Therefore, this review is split into two subthemes, namely:. Our first contact with MK in was very exciting. We had prepared a mixture of 1 kg powder with 0. We had discovered one of the major properties of MK, namely its powerful exothermicity and reactivity in alkaline medium.
This characteristic was mentioned in all our earlier published papers, for example Davidovits , , It helped us to develop a standard method for quality control and selection of reactive raw materials. This technique was successfully used during our selection of the US metakaolin brands mentioned in the introduction. Despite this highly propagated property, the amount of research carried out and the number of published papers on this topic is surprisingly low.
The paper by Xiao Yao et al. It confirms the various parameters involved in the development of this exothermicity, namely: curing temperature, alkalinity ratio SiO 2 :Na 2 O. The reactivity and exothermicity are closely related to the calcination method of the raw kaolinitic clay source. What we find in the literature is totally different, so that it is hard to compare any results deriving from these researches. The following non-exhaustive list is representative of what we came across during this critical review of publications; it gives the calcination temperature and the time yet we do not know if it includes the heating and cooling times.
One called M is calcined in a rotary kiln and characterized by rounded massive aggregates of lamellar particles.
The second, called MS, calcined in a flash kiln, is made up of fine lamellar particles with lower agglomeration. Granulometric distributions are broad and multimodal for both metakaolins. M like the brand MetaMax and others are essentially commercialized as additive to Portland cement. We mentioned in the introduction the beneficial influence of this flash calcined metakaolin in the discovery of geopolymer science. UK, now Imerys. Highly refined kaolin clay is calcined in a vertically oriented multiple hearth furnace.
The material is moved by mechanical rakes across each hearth and then drops to the next hearth below. Each hearth has separate temperature controls and, unlike with rotary kilns, the time that the material is inside the furnace and the temperature gradient that the material is exposed to is precisely controlled ensuring the consistent production of high- purity highly reactive metakaolin.
Approximately one-half of these well-defined aluminium sites are octahedral, one-quarter are tetrahedral, and the remainder are either tetrahedral, or some other regular site with chemical shift intermediate between tetrahedral and octahedral.
It was against the view of the main stream of scientists at that time. The Al 4 line remains with the same intensity. Nonetheless, studies on the calcination time at definite temperatures are seldom. None of the papers cited above are mentioning the reason why the calcination was run at a particular temperature and during a given time. However, when calcination time is prolonged, these reactive species, especially Al 5, decrease in intensity, due to the metakaolinite-spinel-mullite transformation.
Another important issue in metakaolin-based geopolymerization relates to its reaction mechanism. At the beginning of geopolymer research Davidovits, and afterwards for at least 25 years, it was assumed that the geochemical syntheses occurred through hypothetical oligomers dimer, trimer. Further polycondensation of these hypothetical building units provided the actual structures of the three-dimensional macromolecular edifice.
Review papers published at the First Geopolymer Conference in , and at the second, 11 years later, in , could not present scientific details describing the actual reaction mechanism Davidovits, , , see the page Science at the Geopolymer Institute.
The most important contribution to this issue is the paper by North and Swaddle Indeed, it was discovered that the polymerization of oligo-sialates was taking place on a time scale of around milliseconds, i. At room temperature, or higher, the reaction is so fast that it cannot be detected with conventional equipment. They successfully detected five solute species, two linear molecules and three cycles:. The hypothetical oligomers set forth in geopolymer synthesis were no longer virtual molecules.
As a matter of fact, they exist in soluble forms and are stable in concentrated solutions at high pH. For these cement scientists, the geopolymerization reaction involves a 3-step process, namely dissolution—reorientation— polycondensation.
However, the setting and hardening of geopolymer cement is so rapid that this 3-step process almost takes place at the same time. Therefore, it is impossible to isolate the three steps by experimental study, yielding no better understanding of the details of each step until now. In fact, the suggested dissolution mechanism of metakaolinite should be similar to the one set fourth in Duxson and Provis paper for alumino-silicate glass.
But, we already know that the monomers immediately polymerize into this same ortho-sialate in solution, according to Swaddle. In other words, there is no isolated monomeric Al and Si. Supporting information to this claim is provided by another important paper by Bauer and Berger They followed the entire reaction of kaolinite in KOH solutions and examined the dissolution rate of kaolinite in high pH KOH solutions 0.
They found that the aqueous concentrations of Si and Al increased linearly with log t whatever the temperature and the KOH concentration was. More, the amounts of Si and Al are identical with time, i. Since the s, in all papers, the concentrations of Al in solutions are analysed colorimetrically with an UV-visible spectrophotometer, using the Catechol violet method.
Then, separately, Si concentrations are measured with the Molybdate blue method. Applied to the oligo-sialate molecule, this method gives 1 Al for 1 Si in solution, even when there is no monomeric Si and Al. Despite the strong evidence towards the formation of the oligo-sialate molecule, several authors like Buchwald et al. Like many others, they do not take into account the macrocospical structure of the raw material metakaolin, per se.
The calcination of kaolinite does not destroy the lamellar structure of the tabular shaped metakaolinite pellets. The alkaline attack starts on the outer faces of the metakaolinite particle. It continues, layer by layer, from the edges to the inside. This is a very important feature, which induces two different geopolymerization mechanisms. We discovered the implications of this structural parameter by chance. Our PhD student M.
This means that both geopolymer phases, namely nepheline and albite detected by X-ray diffraction, are in solid solution, on the nano scale. Duxson et al.
The authors suggest that the various silicate units Q 0 , Q 1 , Q 2 , Q 3 present in the alkali-silicate solution have a templating function during geopolymerization. Their elastic modulus study on MKbased geopolymer suspensions confirms the 2 step reaction mechanism mentioned above.
In the second phase, the geopolymer hardens with a second rapid increase of the elastic modulus. Accordingly, the geopolymerization of MK with Na,K -silicates results from two reaction mechanisms taking place in the following order:.
MKbased geopolymers have a variety of applications. They are new binders and resins for coatings and adhesives, fibre composites, waste encapsulation and new cement for concrete. The wide variety of applications includes: fire-resistant materials, decorative stone artefacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock refractories, bio-technologies materials for medicinal applications , foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin systems, radioactive and toxic waste containment, arts and decoration, cultural heritage, archaeology and history of sciences.
Geopolymers based on natural and synthetic metakaolin - A critical review
Description Description. The present invention relates to a new type of geopolymer cement for construction. Prior art. There are two types of cement: hydraulic cements and geopolymer cements. Geopolymeric or geopolymeric cements result from an alkaline activation mineral polycondensation reaction, called geosynthesis, as opposed to traditional hydraulic binders in which curing is the result of hydration of calcium aluminates and calcium silicates.
Geopolymers based on natural and synthetic metakaolin - A critical review
On 24 Oct In Books. Update on November 29, Thousands of books sold since all over the world. Half are sold to libraries and public laboratories, and half to the industry.
New state-of-the-art materials designed with the help of geopolymerisation reactions are opening up new applications and procedures and transforming ideas that have been taken for granted in inorganic chemistry. High temperature techniques are no longer necessary to obtain materials which are ceramiclike in their structures and properties. These materials can polycondense just like organic polymers, at temperatures lower than deg.
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It is a more environmentally friendly alternative to conventional Portland cement. Room temperature hardening is more readily achieved with the addition of a source of calcium cations, often blast furnace slag. Geopolymer cements can be formulated to cure more rapidly than Portland-based cements; some mixes gain most of their ultimate strength within 24 hours. However, they must also set slowly enough that they can be mixed at a batch plant, either for precasting or delivery in a concrete mixer. Geopolymer cement also has the ability to form a strong chemical bond with silicate rock-based aggregates. The production of versatile, cost-effective geopolymer cements that can be mixed and hardened essentially like Portland cement would represent a game changing advancement, revolutionizing the construction of transportation infrastructure and the building industry.
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