The development of new, sustainable, low-CO2 construction materials is essential if the global construction industry is to reduce the environmental footprint of its activities, which is incurred particularly through the production of Portland cement. One type of non-Portland cement that is attracting particular attention is based on alkali-aluminosilicate chemistry, including the class of binders that have become known as geopolymers. These materials offer technical properties comparable to those of Portland cement, but with a much lower CO2 footprint and with the potential for performance advantages over traditional cements in certain niche applications. This review discusses the synthesis of alkali-activated binders from blast furnace slag, calcined clay (metakaolin), and fly ash, including analysis of the chemical reaction mechanisms and binder phase assemblages that control the early-age and hardened properties of these materials, in particular initial setting and long-term durability. Perspectives fo...

Geopolymers and Related Alkali-Activated Materials

A recent increase in the use of ecofriendly, natural fibers as reinforcement for the fabrication of lightweight, low cost polymer composites can be seen globally. One such material of interest currently being extensively used is basalt fiber, which is cost-effective and offers exceptional properties over glass fibers. The prominent advantages of these composites include high specific mechano-physico-chemical properties, biodegradability, and non-abrasive qualities to name a few. This article presents a short review on basalt fibers used as a reinforcement material for composites and discusses them as an alternative to the use of glass fibers. The paper also discusses the basics of basalt chemistry and its classification. Apart from this, an attempt to showcase the increasing trend in research publications and activity in the area of basalt fibers is also covered. Further sections discuss the improvement in mechanical, thermal and chemical resistant properties achieved for applications in specific industries.

A short review on basalt fiber reinforced polymer composites

This paper presents a review of alkali-activation technology, moving from the atomic scale and chemical reaction path modelling, towards macroscopic observables such as strength and durability of alkali-activated concretes. These properties and length scales are intrinsically interlinked, and so the chemistry of both low-calcium (‘geopolymer’) and high-calcium (blast furnace slag-derived) alkali-activated binders can be used as a starting point from which certain engineering properties may be discussed and explained. These types of materials differ in chemistry, binder properties, chemical structure and microstructure, and this leads to the specific material properties of each type of binder. The secondary binder products formed during alkali-activation (zeolites in low-Ca systems, mostly layered double hydroxides in alkali-activated slags) are of significant importance in determining the final properties of the materials, particularly in the context of durability. The production of highly durable concretes must remain the fundamental aim of research and development in the area of alkali-activation. However, to enable the term ‘highly durable’ to be defined in a satisfactory way, the underlying mechanisms of degradation—which are not always the same for alkali-activated binders as for Portland cement-based binders, and cannot always be tested in precisely the same ways—need to be further analysed and understood. The process of reviewing a topic such as this will inevitably raise just as many questions as answers, and it is the intention of this paper to present both, in appropriate context.

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Geopolymers and other alkali activated materials: why, how, and what?

Fly ash-based geopolymer: clean production, properties and applications

The role of the concentration and the nature of aluminium in the creation of hierarchical porosity in both commercial and synthesized MFI zeolites have been investigated through controlled mesoporosity development by desilication in alkaline medium. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range 25-50 has been identified; this leads to an optimal mesoporosity centred around 10 nm and mesopore surface areas of up to 235 m(2) g(-1) while preserving the intrinsic crystalline and acidic properties. At lower framework Si/Al ratios the relatively high Al content inhibits Si extraction and hardly any mesopores are created, while in highly siliceous ZSM-5 unselective extraction of framework Si induces formation of large pores. The existence of framework Al sites in different T positions that are more or less susceptible to the alkaline treatment, and the occurrence of re-alumination, are tentative explanations for the remarkable behaviour of Al in the desilication process. The presence of substantial extra framework Al, obtained by steam treatment, inhibits Si extraction and related mesopore formation; this is attributed to re-alumination of the extraframework Al species during the alkaline treatment. Removal of extraframework Al species by mild oxalic acid treatment restores susceptibility to desilication, which is accompanied by formation of larger mesopores due to the enhanced Si/Al ratio in the acid-treated zeolite.

Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent.

The cracking of 2-methyl-2-butene was carried out as a model reaction of C4+ olefins cracking by using of various zeolite and zeotype catalysts to obtain light olefins, such as ethylene and propylene. Only ZSM-5 and ZSM-11 zeolites with 10-MRP with intersecting channels and acid sites concentration ranging between 0.03 and 0.1 mmol/g exhibit high and stable selectivity to C2–C4 olefins at high conversion level. Active and selective catalysts were prepared by three different methods: direct synthesis of H-ZSM-5 with high Si/Al ratio, steaming of H-ZSM-5 and “partial” ammonium ion exchange of Na-ZSM-5. Neither H-(Al)ZSM-5 with high concentration of acid sites nor H-(Fe)ZSM-5 are suitable catalysts as they catalyze hydrogen-transfer reactions yielding paraffins and aromatics, showing the importance of low concentration and high strength of acid sites for cracking activity-selectivity. High yield of butenes in the reaction products indicates that pentenes cracking in 10-MRP proceeds via formation of voluminous carbenium ion intermediates demanding sufficient space inside of zeolites channel. Therefore, the presence of intersection of 10-MRP is very important. While in ZSM-5 and ZSM-11 zeolites the low amount of active sites mainly situated in the pores intersection is sufficient to achieve high activity-selectivity in the case of mono-dimensional 12-MRP zeolites such as mordenite and ZSM-12 and also with 10-MRP ferrierite much higher total acid concentration is necessary, as vast of the acid sites, situated out of the main channel do not take part in the reaction. AlPO-11 materials with narrow mono-dimensional channel exhibit low conversion with majority of β-scission cracking mechanism independently on the acid sites concentration.

Cracking of pentenes to C2–C4 light olefins over zeolites and zeotypes: Role of topology and acid site strength and concentration

Geopolymer based catalysts—New group of catalytic materials

Ion exchange of Na+ ions in a geopolymer by Cs+, NH4+, and Co2+ was used to characterize the accessibility of the inner volume of a Na-geopolymer (Si/Al 1.8) and the size of the rings controlling the mobility of cations in the geopolymer matrix. Ultraviolet (UV)—visible (vis) spectroscopy of Co2+ ions in the Co2+-exchanged geopolymer was used to characterize the local geometry of the Co2+ ions and the regularity and shape of local structures accommodating these Co2+ ions in the geopolymer matrix. Na-geopolymers exhibit ion-exchange properties similar to those of zeolites. All Na+ ions balancing Al atoms of the network of the as-prepared geopolymer could be replaced (ion-exchanged) by NH4+ or Co2+ ions. By analogy to zeolites, this indicates that all the Al atoms are accessible through rings containing at least eight Al and Si atoms. Co2+ ions in the geopolymer network are accommodated in two regular types of deformed six-membered or eight-membered rings. As indicated by the lower extent of Cs+ exchange, only one-half of the Al atoms are simultaneously accessible through 10-membered or larger rings. This indicates that neither regular alumosilicate six-membered rings nor the hexagonal prisms typical of some zeolitic structures are present in the geopolymer network.

Oleg Bortnovsky

Papers

Cracking of pentenes to C2–C4 light olefins over zeolites and zeotypes: Role of topology and acid site strength and concentration

Geopolymer based catalysts—New group of catalytic materials

Metal Ions as Probes for Characterization of Geopolymer Materials

Structure of Al–Lewis Site in Beta Zeolite Active in the Meerwein–Ponndorf–Verley Reduction of Ketone to Alcohol

Exchange of Co(II) ions in H-BEA zeolites: identification of aluminum pairs in the zeolite framework