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- Geopolymers (2) (entfernen)
Geopolymers are promising alkali activated materials with a wide range of possible applications like heat-resistant coatings and adhesives or concretes with lower C02 emissions than conventional Portland cement-based systems. Conventional ("two-part") geopolymers are synthesized by an activation of aluminosilicate feedstocks like metakaolin or blast furnace slags with highly alkaline solutions like sodium hydroxide solutions and/or water glass solutions.
A more recent approach are so called one-part geopolymers, where the alkaline activator is provided in solid form, so that only water has to be added to initiate the geopolymerisation reaction. The handling of alkaline solutions is avoided, which possesses several advantages compared to conventional geopolymers, in particular regarding safety and health issues, and, thus, it improves economic and social acceptance of these binders. However, the new formulations are less exhaustively studied and the knowledge about conventional geopolymers cannot simply be transferred to this new class of binders.
One-part geopolymers, synthesized from three different silica sources and sodium aluminate, were investigated regarding their microstructure and the evolution of their compressive strength on curing at slightly elevated temperatures. The effect of thermal treatment was studied up to 1000 °C and for the investigation of the chemical durability specific samples were treated with sulfuric acid (H2SO4, pH = 1).
Depending on the silica feedstock, different degrees of reaction were observed. One of the silicas had a significantly higher reactivity. For two of the silicas, significant amounts of zeolites occurred as reaction products besides geopolymeric gel, whereas the more reactive silica source lead to the formation of a rather fully condensed geopolymeric network.
The composites indicated promising behavior on heating in so far as no distinct shrinkage step occurred in the temperature range of dehydration. Up to 400 °C the residual strength of those mixes increased. Above 800 °C the samples underwent new phase formation. After exposure to sulfuric acid (pH 1; 70 d) specimens showed a residual compressive strength of about 77 % of the reference, indicating high acid resistance.