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Materials and Structures

, Volume 48, Issue 6, pp 1703–1716 | Cite as

Reaction and microstructure of cement–fly-ash system

Original Article

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.

Keywords

Reaction Microstructure Fly ash Cement Diffusion 

List of symbols

Variables

b

Binder mass (kg/m3)

c

Cement mass (kg/m3)

fb

Mass fraction of blend material (kg/kg)

fC

Mass fraction of cement (kg/kg)

fFA

Mass fraction of fly ash (kg/kg)

hc

Constant (m2/kg)

ht

Time-dependent parameter

kA

Efficient factor

kd

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)

t0

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

tI–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)

KD

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)

Sb

Specific surface area of blend material (m2/kg)

Seff

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)

Greek symbols

α

Reaction extent (–)

α

Reaction extent of neat cement (–)

αC

Reaction extent of cement (–)

αFA

Reaction extent of fly ash (–)

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

Reaction extent changes by the heterogeneous nucleation (–)

\(\epsilon\)

Efficacy function (–)

Notes

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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Copyright information

© RILEM 2014

Authors and Affiliations

  1. 1.Institute of Building Materials, Civil Engineering DepartmentTsinghua UniversityBeijingPeople’s Republic of China
  2. 2.Key Laboratory of Civil Engineering Safety and Durability of China Education MinstryBeijingPeople’s Republic of China

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