Effect of fly ash on the pore structure of cement paste under a curing period of 3 years
Introduction
The hydration of cement goes along with continuous evolution of the microstructure of cement paste and reduction of porosity. The transport properties of cement paste are mainly controlled by their microstructure development [1]. In cement paste blended with fly ash the pozzolanic reaction of fly ash consumes calcium hydroxide and changes the pore structure of paste [2], [3]. Although the effect of fly ash on the pore structure of blended cement paste has been studied by many researchers, however, most research results are limited to a short curing period, e.g. 3 months [4], [5], [6], [7]. This is insufficient to explain the transport properties of Portland cement-fly ash binary systems at later ages. In our previous work [8], the pore structure of cement paste blended with fly ash was studied at the curing age up to 2 years. The influences of fly ash on the total porosity and the critical pore size of cement paste were discussed, however, other pore parameters related to the transport properties of cement paste, such as capillary porosity, ink-bottle porosity, etc., were not involved.
In this study, attempts were made to investigate the microstructure development of blended cement paste, from solid phase to pore phase, at ages up to 3 years. The solid phases are observed by environmental scanning electron microscope (ESEM). The pore structure of blended cement paste is determined by mercury intrusion porosimetry (MIP).
Section snippets
Materials
Portland cement (CEM I 42.5 N) used in the experiments was produced by ENCI, the Netherlands. The fly ash is a low-calcium fly ash (Class F fly ash). Table 1 shows the chemical composition of the Portland cement and fly ash as determined by X-ray fluorescence (XRF) spectrometry method. The mineral composition of Portland cement is calculated with the Bogue equation [9], as C3S (67.1%), C2S (5.9%), C3A (7.8%) and C4AF (9.6%). The glass phase of the fly ash is 72.81%.
Mixture compositions
Three kinds of mixtures are
Solid phases of cement paste blended with fly ash
Fig. 2 shows BSE images of cement paste blended with fly ash at different ages. For comparison the images obtained from Portland cement paste are shown in Fig. 3. It is easy to distinguish the phase of reaction products, pores and unreacted cement and fly ash (spherical shape) from their grey levels in the images. From the BSE images the following observations can be made:
- 1)
As observed from Fig. 2, in blended cement paste hollow fly ash particles with a thin shell can easily be identified,
Conclusions
In this paper the effect of fly ash on microstructure development of blended cement paste, especially the development of its pore structure, is studied at ages up to 3 years. From the experimental results and discussions in this paper the following conclusions have been drawn.
- (1)
BSE images show that at ages up to 3 years blended cement paste is more porous than Portland cement paste. In blended cement paste (30% fly ash; w/b = 0.4) some circular voids surrounded by reaction products are observed at
Acknowledgement
I hereby express gratitude for the financial support of the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20161001), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 16KJB430018), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Changjiang Scholars and Innovative Research Team in University (No. IRT_15R35). The support from the Microlab, Faculty of Civil Engineering and Geosciences,
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