Geopolymers in Construction

The lecture gives a short introduction to the technology of what is commonly called geopolymerisation. This technology,which has been studied on scientific level mainly since the last decade, permits the transformation at low temperature and energy consumption of certain inorganic resources into other materials with useful mechanical and thermal properties. The focus will be put on the use as building materials, since this is the application which has received most attention during our research at Free University Brussels (VUB). Since it is not the aim of giving a complete overview, the geopolymers based on the reaction of various solid aluminosilicates (like metakaolinite or fly ash) with concentrated aqueous alkali hydroxide or silicate solution will only touched briefly, and focus will be put on
the less well known synthesis starting from kaolinite clay.
First some clarification will be given regarding terminology. Different kinds of polymers, based on a carbon backbone,can be distinguished: natural polymers, biopolymers, synthetic polymers. But polymer material structures may exist,based on other atoms as backbone: silicon or phosphorous, for example. Often these are called geopolymers,indicating that they are originating from geological resources. Most synthetic polymers (plastics) are however alsosynthesised from geological resources. Mineral polymers seems to be a better terminology, but is not covering allpossible resources. Inorganic polymers is maybe the best choice, in contrast with organic polymers (which are all based on carbon).
Rock forming minerals have been used during the civilisation as durable building materials, mostly under the form of blocks of various sizes and shape. Weathered minerals such as clays can also be used as building materials, but due to their particulate nature they lack durability as a building material. Better stability can be obtained by firing clay minerals, at the expense however of a high energy demand. Is there however another way to transform a two dimensional mineral structure into a three dimensional one, without this temperature process, but through chemical reactions? A clue can be found in zeolite synthesis: under hydrothermal conditions below 100°C, synthetic zeolites can be obtained by digesting kaolinite clay in a solution of caustic soda.
When the zeolite forming reactions take place under drastically reduced water content around the plasticity limit of the clay (this is possible because of the high solubility of soda), the growing three dimensional network structures are interpenetrating in a similar way as the calcium silicates in traditional cement, leading to strength build-up and stability under humid conditions. The process is simple, has a low energy demand, and does not contribute to CO2 emission, all contrary to ordinary Portland cement.
As an example, properties will be given of the reaction product between a natural Jordanian kaolinite with caustic soda, using techniques as XRD, TGA, SEM/EDX, MAS NMR and mechanical compression test, as depending from composition and curing conditions. Examples of application as building material will be given.