Synthesis and Characterization of a Hybrid Cement Based on Fly Ash, Metakaolin and Portland Cement Clinker

Abstract

Hybrid cement has become one of the most viable options in the reduction of CO 2 emissions to the environment that are generated by the cement industry. This could be explained by the reduction of the content of clinker in the final mixture and substitution of the remaining percentage with supplementary cementitious materials with the help of an alkaline activation. Following that, properties that are provided by an Ordinary Portland Cement and of a geopolymer are mixed in this type of hybrid material and could be achieved at room temperature. Thereafter, the main objective of this research was to synthesize hybrid cements reducing the clinker content of Portland Cement up to 20% and use metakaolin and fly ash as supplementary cementitious materials in different proportions. The mixtures were alkaline activated with a mixture of sodium silicate and sodium hydroxide, calculating the amounts according to the percentage of Na 2 O that is present in each of the activators. The samples were then characterized using Compressive strength, X-ray diffraction, Fourier Transform Infrared Spectroscopy, and Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy. The results indicated that the hybrid cements have similar mechanical properties than an Ordinary Portland Cement, and they resulted in a dense matrix of hydration products similar to those that are generated by cements and geopolymers.Materials 2020, 13, 1084 2 of 15 that serve as partial or total substitutes are called supplementary cementitious materials (SCMs) [4], of which the best known are ground granulated blast furnace slag (GGBFS), fly ash (FA), silica fume (SF), and calcined clays. The use of these materials can be a viable solution for the reduction of CO 2 emissions [5]. Current cement consumption is projected to be double by 2050, and the use of these materials could highly reduce CO 2 emissions to the environment [4]. Following that, a model of binders is currently being studied, which consists of adding to the mix of supplementary materials a very low portion of CaO in the form of clinker, to obtain a setting at room temperature [1]. During the past decades, alkali-activated and blended cement have attracted strong interest worldwide, due to their advantages of low energy cost, high strength, good durability, and sustainability. A major incentive for the further development of such cements is generated by the great quantity of annual generation of wastes, which cause a need to find new uses for them. The main raw materials used for this proposal are FA, GGBFS, and metakaolin (MK), in which activators, like water glass and sodium hydroxide, are used. Among the alkaline cements, two large groups are known: (a) calcium and silicon-rich activated materials (Me 2 O-MeO-Al 2 O 3 -SiO 2 -H 2 O system); here, the activation of GGBFS (SiO 2 + CaO > 70%) corresponds, under relatively mild alkalinity conditions [6]. A hydrated calcium aluminosilicate gel (C-A-S-H) is the main hydration product generated, similar to the gel generated in the hydrati...

Fly Ash-Based Geopolymer Building Materials for Green and Sustainable Development

“…However, many fly ash-based geopolymers previously reported in the literature for different purposes and applications could be improved by applying the formulations developed in this study. These formulations could be used to improve the fly ash-based geopolymer coating for fire resistance materials [ 32 ], the immobilization of hazardous substances [ 25 ] and backfilling materials [ 71 ], low micro-filling effect (low porosity) materials [ 24 ], absorbent materials [ 44 ], or asphalt binders [ 43 ].…”

“…Kaur et al [ 21 ] indicated that approximately 1.2 times more compressive strength could be obtained when a combination of NaOH and Na 2 SiO 3 was added to fly ash, compared to the use of NaOH only. The compressive strength increased as NaOH concentration increased from 8 to 16 M. Thus, the increase of NaOH concentration caused a significant acceleration of dissolution reactions, promoting the early stages of the geopolymerization process [ 24 , 25 ]. A maximum compressive strength of 40.42 MPa was obtained when fly ash was activated with 16 M NaOH and mixed with a sand to fly ash ratio of 3:1 [ 21 ].…”

“…The properties of geopolymers are very similar to OPC when formed under appropriate conditions [ 46 ]. However, there is a strong difference between Portland cement hydration and alkali activation of fly ash [ 24 , 25 ]. Alkali activation is similar to the chemistry involved in the synthesis of large groups of zeolites [ 11 ].…”

Hybrid Materials Based on Fly Ash, Metakaolin, and Cement for 3D Printing

“…A geopolymer hybrid is a combination of geopolymer and OPC or other binders, in order to produce material that combines the positive properties of OPC with the properties of alkali-activated materials or geopolymers. Hybrid OPC-geopolymer concrete has a lowered carbon footprint and improved ambient temperature curing while maintaining the positive properties of heavy metals immobilization, generally good mechanical properties, and high durability [50,51]. In the case of hybrid OPC-geopolymer concrete, generally of high FA content (70-90%) and low OPC content (10-30%), the clinker reaction products and reaction products from the glass phases of FA coexist.…”

The hygrothermal performance of concrete with coarse aggregates made of recycled concrete pavements

“…Moreover, various waste products have been used to partially replace the original compounds that concretes are made of. These efforts are especially evident in the case of cement which is partially substituted by blast furnace slag [12,13], silica fume [14,15], fly ash [16,17] or other less energy-demanding material alternatives that exhibit certain pozzolanic properties. However, the primary resource savings can also be achieved in the case of other compounds such as aggregates.…”