Nicht nur unter technischen, sondern zunehmend unter Aspekten der Wirtschaftlichkeit und Nachhaltigkeit wird die Erforschung von alternativen Bindemitteln und Zementersatzstoffen mit ausgeprägten reaktiven Eigenschaften verstärkt vorangetrieben. Hierbei spielen neben bereits seit längerem bekannte Betonzusatzstoffe wie z.B. Flugasche, Silikastaub und Hüttensand auch weitere aufbereitete Sekundärrohstoffe bzw. Reststoffe eine zunehmend wichtige und dem Gedanken des Stoffkreislaufs entsprechende Rolle für den Betonbau. In diesem Zusammenhang werden in den vergangenen Jahren immer wieder alkalisch-aktivierte Bindemittel und Geopolymer-Bindemittel genannt. Um diese alternativen Bindemittel für die Herstellung von Beton einsetzen zu können, ist es zwingend notwendig, verallgemeinerbare technologische und bemessungsrelevante Grundlagen bereitzustellen. Da die mit alkalisch-aktivierten Bindemitteln erzielbaren Betoneigenschaften in hohem Maße von den verwendeten Ausgangsstoffen abhängen, erscheint es zunächst sinnvoll zu sein, eine allgemeingültige Begriffsdefinition zur Eingruppierung dieser alternativen Bindemittel im Vergleich zu konventionellen, hydraulischen Bindemitteln vorzuschlagen. In dem Beitrag wird daher eine Klassifizierung von alkalisch-aktivierten Bindemitteln und Geopolymer-Bindemitteln sowie gleichzeitig eine Abgrenzung zu hydraulischen Bindemitteln unter Berücksichtigung der festigkeitsbildenden Reaktionsprodukte und der Hydraulizität vorgestellt.
Proposal for the classification of alkali-activated binders and geopolymers
Research into alternative binders and cement substitutes with distinctively reactive properties is being pursued not only from a technical but increasingly also from an economic and sustainable point of view. Concrete additives which have been known for many years such as fly ash, silica fume and blast furnace slag, though also other processed secondary raw materials and residues are playing an increasingly important role for concrete construction along with the idea of life cycles. Alkali-activated binders and geopolymers have been repeatedly named in this connection over the past years. Generalised technological and design-relevant principles have to be provided in order to be able to use these so-called alternative binders to produce concrete. Since the properties of concrete that can be achieved with alkali-activated binders largely depend on the raw materials that are used, it would initially appear sensible to suggest a general definition of terms to classify these alternative binders as opposed to conventional, cementitious binders. The article therefore presents a classification of alkali-activated binders as well as geopolymers and at the same time a differentiation to conventional binders taking into account the strength-forming texture and hydraulicity. On the basis of the documented research and own experimental tests, it could be shown that the chemical and mineral phase compositions of the reactants/products of various binders affect the strength-forming texture and can thus have a significant influence over the final properties of the concrete too. It could also be shown that binders can be classified according to their water-binding capacity. The wide range of possible raw materials for the alkali activation, in light of the use of alkali-activated binders in concrete, necessitates the general and unambiguous classification of, as well as differentiation between, the different binders. In the case of AAB, a classification into Ca-rich and low-Ca AAB is expedient to achieve consistent results and express generalised recommendations. It is recommended that geopolymers are classified as a subgroup of AAB with a low Ca-content since only these systems are able to form aluminosilicate compounds and thus live up to the properties described in the references. If considering reaction-chemical and binder-specific production as well as appropriate curing parameters, AAB offer properties that are at least equivalent to those of conventional binders. It can generally be said that in order to encourage the use of alternative binders as well as mixed cements in concrete, new concrete technology assessment concepts will have to be provided in which the performance of the concrete is no longer evaluated solely on the basis of one parameter such as the compressive strength. Rather, the reaction mechanism of the relevant systems has to be understood and used specifically for certain structural or permanent requirements. So-called “performance-based concrete concepts” in particular will play a leading role in future compared to the current prescriptive specifications. However, these will require corresponding test methods and bases of valuations with adequate statistical substantiation.