Geopolymers as an alternative to Portland cement: An overview
Portland cement manufacture emits 5–7% CO2, which is responsible for global warming. Geopolymers minimize CO2 emission and may be a partial alternative to Portland cement in the building industry. The geopolymer technology gives solution to the utilization of industrial byproducts (waste) containing aluminosilicate phases with little negative impact on environment. Geopolymer cements are mainly produced by using secondary raw materials such as fly ash, metakaolin, calcined clays, zeolite etc. by the activation of alkali/alkali silicate solutions.
Combination of different source materials containing aluminosilicate and alkali solutions with optimization of curing temperature, alkali concentrations, additives, Na2O/SiO2 ratio etc. gives geopolymer cements of high mechanical and durability properties. Due to their high mechanical properties and environmental benefit, geopolymer cement and concrete appear as a future prospective construction material and have applications in different areas.
Geopolymers were used in many ancient constructions and the building units of geopolymer consist primarily TO4 tetrahedrons (where T is Si or Al) . The concrete made from geopolymer cements has good engineering properties and appears to be a good alternative to conventional concrete. Geopolymeric concrete is a kind of green concrete which produces significant environmental benefits, which cannot only effectively recycle solid waste and save a lot of resources, but also can reduce CO2 emissions. It may solve the problems of cement industries and issues related to environmental pollution. This article reviews the process of geopolymerization, heat curing methods, characterization techniques, models, properties of geopolymer concretes as well as reviews comprehensively the literature to provide insights into the potential application of this material in the construction industry.
Manufacturing of Portland cement consumes huge amount of energy and raw materials and at the same time emits lot of CO2 responsible for global warming. Global annual cement production is expected to be 5.9 billion tons with more than 4.8 billion tons CO2 production by 2020. This situation will be alarming and hence there is an urgent need to minimize CO2 emissions from the cement industries. Two ways may be adopted: (i) Portland cement can partly be replaced by supplementary cementitious materials to save energy, raw materials and reduce CO2 emissions and (ii) producing clinker free cement.
In recent years alkali activated binders/geopolymer cement and concrete appear to be an alternative to conventional concrete. Alkali activated binders are made by alkali activation of aluminosilicate source materials such as ground granulated blast furnace slag (GGBFS), fly ash (FA), metakaolin (MK) etc. These binders may solve the problems of construction industry and the waste generation. Since Ca rich source materials particularly slags are limited, therefore more attention is given towards low calcium source materials producing geopolymers. Joseph Davidovits in 1978 coined the term “geopolymer” for the first time. This is an inorganic polymer formed through polycondensation reaction of certain waste material containing aluminosilicate, with alkalis. Geopolymers are amorphous to semi-crystalline 3-dimensional aluminosilicate framework structures formed by the combination of [SiO4]4− and [AlO4]5− tetrahedra. The structure maintains electrical neutrality as a result of aluminium substitution for silicon in the tetrahedral layer by the available alkalis such as Na+. The setting and hardening mechanism of geopolymer is not completely understood. The process of geopolymerization may occur by dissolution of raw materials containing aluminosilicate in alkali solutions leading to the formation of monomers of aluminate and silicate. These are then converted to oligomers and subsequently to geopolymers. Water is used up during dissolution and released in polymerization.
The geopolymerization process consists of dissolution and reorganization, condensation, and polymerization. The dissolution and reorganization of aluminosilicate form several types of oligomers; oligomers connect and form large polymers. When oligomers connect, the OH groups at their end meet and release water by sharing an oxygen atom.
More info: Elsevier
