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Reaction and microstructure of cement–fly-ash system

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Abstract

The reaction and microstructure of the cement–fly-ash system were investigated in the present study. The reaction of cement–fly-ash system was described by a diffusion-controlled reaction process, leaving aside the nucleation and growth and boundary phase reaction processes at an earlier stage. The extent of hydration and the microstructure of hydrates were investigated by employing thermogravimetry, nitrogen adsorption/desorption analysis and scanning electron microscopy. A descriptive model of the reaction processes of the cement–fly-ash system was proposed to determine the importance of each process. In summary, (a) the blending of fly ash augments the local water-to-cement ratio and affects the heterogeneous nucleation of hydrates, promoting the hydration rate and extent; (b) the rapid hydration of cement coats the fly ash, retarding the dissolution and reaction of fly ash; (c) the continual dissolution of active silica-aluminate etches the surface of fly ash; and (d) the diffusion coefficient is estimated as 10−19 m2/s.

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Abbreviations

b :

Binder mass (kg/m3)

c :

Cement mass (kg/m3)

f b :

Mass fraction of blend material (kg/kg)

f C :

Mass fraction of cement (kg/kg)

f FA :

Mass fraction of fly ash (kg/kg)

h c :

Constant (m2/kg)

h t :

Time-dependent parameter

k A :

Efficient factor

k d :

Diffusion coefficient (m2/s)

\(\Updelta m_{{\rm Wn}}\) :

Mass loss of non-evaporable water (g/g)

\(\Updelta m_{{\rm CH}}\) :

Mass loss of calcium hydroxide (g/g)

t :

Reaction time (s or day)

t 0 :

Time for the diffusion-controlled reaction (s or day)

t I–D :

Time for the reaction transition (s or day)

w :

Water mass (kg/m3)

C :

Reaction constant by nucleation and growth processes

CH:

Calcium hydroxide (g/g)

\(\hbox{CH}_{(\infty,\hbox{C})}\) :

Calcium hydroxide by cement with complete reaction (g/g)

\(\hbox{CH}_{(\infty,\hbox{FA})}\) :

Calcium hydroxide by fly ash with complete reaction (g/g)

K D :

Rate coefficient of diffusion process (1/s)

LOI:

Ignition loss (g/g)

LOIC :

Ignition loss of cement (g/g)

LOIFA :

Ignition loss of fly ash (g/g)

R :

Radius of cementitious particles (m or μm)

S b :

Specific surface area of blend material (m2/kg)

S eff :

Effective specific surface area (m2/kg)

Wn:

Non-evaporable water (g/g)

\(\hbox{Wn}_{(\infty,\hbox{C})}\) :

Non-evaporable water by cement with complete reaction (g/g)

\(\hbox{Wn}_{(\infty,\hbox{FA})}\) :

Non-evaporable water by fly ash with complete reaction (g/g)

α :

Reaction extent (–)

α′:

Reaction extent of neat cement (–)

α C :

Reaction extent of cement (–)

α FA :

Reaction extent of fly ash (–)

\(\Updelta\alpha_{\rm HN}\) :

Reaction extent changes by the heterogeneous nucleation (–)

\(\epsilon\) :

Efficacy function (–)

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Acknowledgments

The research is supported by general financial grant from the China Postdoctoral Science Foundation (No. 2012M520288) and China national major fundamental research grant (973 Program, No. 2009CB623106).

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Authors and Affiliations

  1. Institute of Building Materials, Civil Engineering Department, Tsinghua University, Beijing, 100084, People’s Republic of China

    Qiang Zeng

  2. Key Laboratory of Civil Engineering Safety and Durability of China Education Minstry, Beijing, 100084, People’s Republic of China

    Kefei Li

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  1. Qiang Zeng
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Correspondence to Qiang Zeng.

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Zeng, Q., Li, K. Reaction and microstructure of cement–fly-ash system. Mater Struct 48, 1703–1716 (2015). https://doi.org/10.1617/s11527-014-0266-y

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