Effect of fly ash on the pore structure of cement paste under a curing period of 3 years

doi:10.1016/j.conbuildmat.2017.03.182

Construction and Building Materials

Volume 144, 30 July 2017, Pages 493-501

https://doi.org/10.1016/j.conbuildmat.2017.03.182 Get rights and content

Highlights

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After 1 year the total porosity of blended cement paste decreases only slightly.
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After 90 days, fly ash results in the formation of more small capillary pores.
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After 28 days, fly ash does not change the critical pore width of the pastes.
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The reacted hollow fly ash particles provide extra space for the accommodation of reaction products.

Abstract

The service life and durability of concrete structures strongly depend on the transport properties of the concrete. These transport properties are highly related to their microstructure, especially the pore structure of the hydrated cement paste. In this paper the microstructure development of cement paste and cement paste blended with fly ash is investigated, from solid phase to pore phase, at a long-term curing period up to 3 years. The solid phases are observed by environmental scanning electron microscope. The pore structure of blended cement paste is determined by mercury intrusion porosimetry. The results indicate that the addition of fly ash increases the total porosity of cement paste, not only at early ages, even at curing age of 3 years. The voids in hollow fly ash particles act as ink-bottle pores with progress of the pozzolanic reaction of fly ash, resulting in the increase of the total porosity of blended cement paste.

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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Effect of fly ash on the pore structure of cement paste under a curing period of 3 years

Highlights

Abstract

Introduction

Section snippets

Materials

Mixture compositions

Solid phases of cement paste blended with fly ash

Conclusions

Acknowledgement

Cem. Concr. Res.

Constr. Build. Mater.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Constr. Build. Mater.

Cem. Concr. Res.

Cem. Concr. Res.