Microstructure development in mixes of calcium aluminate cement with silica fume or fly ash
04 Feb 2009-Journal of Thermal Analysis and Calorimetry (Springer Netherlands)-Vol. 96, Iss: 2, pp 335-345
TL;DR: The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcine hydrate to cubic form, which is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength.
Abstract: The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared spectroscopy (FTIR) have been used to assess the microstructure of these formulations.
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TL;DR: In this paper, the compressive strength and microstructure of various alkali-activated binders at elevated temperatures of 300 and 600°C were investigated using XRD, SEM and FTIR techniques.
Abstract: This paper reports the results of the compressive strength and microstructure of various alkali-activated binders at elevated temperatures of 300 and 600 °C. The binders were prepared by alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 50/50, 10/90 and 0/100 wt.%. Specimens free of loading were heated to a pre-fixed temperature by keeping the furnace temperature constant until the specimens reached a steady state. Then the specimen was loaded to failure while hot. XRD, SEM and FTIR techniques were used to investigate the microstructural changes after the thermal exposure. The fly ash-based specimen shows an increase in strength at 600 °C. On the other hand, the slag-based specimen gives the worst high-temperature performance particularly at a temperature of 300 °C as compared to ordinary Portland cement binder. This contrasting behaviour of binders is due to their different binder formulation which gives rise to various phase transformations at elevated temperatures. The effects of these transformations on the compressive strength are discussed on the basis of experimental results.
137 citations
TL;DR: In this article, the microstructural and chemical changes of calcium aluminate cement (CAC)-based UHPC exposed to high temperatures were investigated, which led to a significant increase in micro-pores.
Abstract: This study investigated the microstructural and chemical changes of calcium aluminate cement (CAC)-based UHPC exposed to high temperatures. Upon exposure to 100 °C, C3AH6 was formed by the dehydration of CAH10. A further increase in the exposure temperature to 450 °C resulted in the formation of a new phase C12A7, which is attributed to the dehydration of C3AH6 and AH3. The compressive strength of UA50 and UA70 increased significantly due to the formation of C-A-(S)-H gel resulting from the further hydration of anhydrous CAC and silica fume upon exposure to 450 °C. The hydration reaction of CAC in UHPC led to a significant increase in the micro-pores (
90 citations
TL;DR: It is demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA, and fulfilled the compressive strength and leachability requirements for on-site reuse.
Abstract: Landfill disposal of municipal solid waste incineration fly ash (MIFA) presents significant environmental and economic burden. This study proposed a novel and high-efficiency approach for stabilisation/solidification (S/S) of MIFA by phosphate-modified calcium aluminate cement (CAC). Experimental results showed that the presence of Pb (the most leachable metal contaminant in the MIFA) retarded the early-stage reaction of CAC, resulting in an extension of setting time and a significant decline of compressive strength of CAC pastes. The incorporation of phosphate additives (10 wt% of binder), especially for trisodium phosphate, in CAC system effectively mitigated the negative impact of Pb on the CAC reaction and reduced the Pb leachability. Elemental mapping results illustrated that Pb2+ coordinated with phosphate to generate insoluble precipitates (e.g., Pb3(PO4)2). The S/S treated MIFA samples fulfilled the compressive strength and leachability requirements for on-site reuse. Overall, this study demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA.
81 citations
TL;DR: In this article, a thermal energy storage (TES) in concrete as solid media for sensible heat storage is proposed to improve the cost and efficiency of solar thermal electricity (STE) plants.
Abstract: A concept for thermal energy storage (TES) in concrete as solid media for sensible heat storage is proposed to improve the cost and efficiency of solar thermal electricity (STE) plants. Mortar and concrete mixes were designed with calcium alumina cement (CAC) blended with blast furnace slag (BFS), using aggregates of different sources and size for stability performance after long term at high temperature. Seventy-five thermal cycles of 24 h length, within the temperature range 290 °C to 550 °C, have been used to simulate the expected operating conditions of TES. The dehydration processes at microstructural level have been evaluated and correlated with mechanical properties. Dehydration processes and consecutive heat/cool cycles induce changes in concrete at micro- and macro-level. The stabilization of damage with the charge/discharge heat cycles for thermal fatigue depends significantly on the aggregate type used. CAC is a suitable binder to use in thermal energy storage systems able to maintain its properties under repetitive heat cycles.
70 citations
TL;DR: In this article, the Spanish Ministry of Science and Innovation (MOSI) and FEDER funded Project GEOCEDEM BIA 2011-26947 were used to support this study.
Abstract: The authors are grateful to the Spanish Ministry of Science and Innovation for supporting this study through Project GEOCEDEM BIA 2011-26947, and to FEDER funding.
65 citations
Cites background from "Microstructure development in mixes..."
...[13], bands appear within the 625oC to 875oC temperature range....
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...[13] introduced into the cement system appropriate additions of spent catalyst FCC [16], and silica fume or fly ash [13] to stabilise CAC hydrates and to avoid, or to at least reduce, their conversion and negative effects on CAC composites....
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...[13], they occur at 3,660, 900 and 550 cm , and overlap that of typical CAC hydration products and the newly-formed gel....
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...region [13], which is difficult to identify by FTIR....
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...Several papers [13,16,18] have also addressed CAC hydration with water in the presence of reactive silica-rich compounds (slag, silica fume, fly ash, etc....
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References
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TL;DR: In this paper, the authors discuss the use of calcium aluminate cements in hydraulic structures, including resistance to acid attack and particularly to biogenic corrosion and abrasion resistance.
Abstract: Calcium aluminate cements have a radically different chemistry to Portland cements. Due principally to their higher cost, they do not compete directly with Portland cements. Nevertheless, concretes based on these cements have very high performance in specific applications. Two of these are discussed in this article: resistance to acid attack and particularly biogenic corrosion and abrasion resistance in hydraulic structures. Such applications extend the range of applications for cementitious materials.
281 citations
TL;DR: The main interaction of CaCO 3 with the cement involves the phase C 3 A resulting in the formation of hydrated carboaluminate, a similar compound to that obtained in the 3CaO·Al 2 O 3 -CaSO 4 ·2H 2 O system as discussed by the authors.
Abstract: The main interaction of CaCO 3 with the cement involves the phase C 3 A (3CaO·Al 2 O 3 ) resulting in the formation of hydrated carboaluminate, a similar compound to that obtained in the 3CaO·Al 2 O 3 –CaSO 4 ·2H 2 O system. In this work gypsum partially substituted by CaCO 3 was studied during hydration and the phases formed as consequence of the competition between CaCO 3 and CaSO 4 ·2H 2 O for the C 3 A at early ages were characterized. Calcium mono- and tricarboaluminato formation (phases Afm and Aft, carbonate) during early C 3 A hydration are discussed as well as the influence that limestone presence causes in the system. Infrared spectroscopy (FT-IR) technique confirmed the existence and formation of the phases reported during the early hydration step.
189 citations