Title: Quantitative Phase Development of crystalline, nano-crystalline and amorphous phases during hydration of OPC blended with siliceous fly ash
Abstract: Ambitious efforts driven by political and environmental considerations to reduce carbon dioxide emission are currently present, amongst other branches in the construction material industry as well. One possible solution concentrates on the replacement of cement by supplementary cementitious materials like fly ash or granulated blast furnace slag. Due to its high amorphous phase content and the related reactivity potential fly ash seems well suited for being used in cement or concrete. Unfortunately fly ash shows a very slow reaction leading, compared to a neat cement sample to a retardation of the hydration process in general. The reaction mechanism and the influence of fly ash on the early hydration reaction of cement in particular are of great importance in order to enable an efficient use of fly ash in cementitious materials. In this thesis a siliceous fly ash from a coal combustion plant in Germany was used to replace 50 wt.% of cement in order to investigate the influence on the early hydration cement. Of special interest were the development and the quantification of phases being nano-crystalline or even amorphous. In order to follow the hydration in-situ XRD analysis and heat flow calorimetry was used. To account for effects related to the sole physical presence of particle reference samples of cement blended with 50 wt.% quartz powder were prepared as well.
In a first step a quantification technique was successfully implemented enabling the determination of absolute phase quantities in order to quantify amorphous phase content. This so called G-factor method was developed further, making a tracking of nano-crystalline phases like C-S-H during the hydration possible. Even the direct quantification of the fly ash glass in dry samples is possible. However it also turned out that a hydrating fly ash – cement sample appears too complex at the moment to allow a selective quantification of all the amorphous phases present. This especially pertains to the very slow reacting fly ash glass. Despite the slow reaction of fly ash a clear influence on the reactions of the cement can be found within the first 2 days of hydration. The addition of fly ash leads to a retardation of both the aluminate and the silicate reaction, representing the main cement reactions. This effect is stronger if the particle size of the fly ash added is decreased. One explanation for the retardation is the adsorption of Ca2+ ions to the surface of fly ash particles leading to an insufficient amount of available calcium ions. Additional SO3 provided by the fly ash promotes the retardation of the aluminate reaction and therefore is strongly dependent on the fly ash chemistry. A blending with quartz powder triggers the cement reactions by offering suitable nucleation sites. In further investigations it was proofed, that these findings are valid for technical and laboratory prepared cements as well.
By tracking the pozzolanic reaction of the fly ash via the quantification of portlandite it becomes apparent that the active reaction of fly ash starts within the first 7 days of
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hydration. After 7 days one can find reaction edges due to dissolution around fly ash particles. As the availability of calcium ions at the slowly dissolving fly ash particles is hindered by the dense hydration products of the cement reaction, the formation of aluminum rich C-(A)-S-H gel takes place. This gel is present as an inner product (replacing the solid fly ash particle) showing a low density and a high water amount as scanning electron microscopy (SEM) investigations proofed.
Publication Year: 2015
Publication Date: 2015-01-01
Language: en
Type: article
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