A Novel Method for Monitoring Hydration Process of Cement Paste

Material

Hashem Almattarneh1,a, Abdullah Alwadie2,b, Ahmad Malkawi 3,c

and Muhd Fadhil Nuruddin4,d 1,2,3

Faculty of Engineering, Najran University, Saudi Arabia

4Faculty of Engineering, University of Technology PETRONAS, Malaysia

ahmalmattarneh@nu.edu.sa,

basalwadie@nu.edu.sa,

cabmalkawi@nu.edu.sa,

dfadhilnuruddin@petronas.com.my

Keywords: Cement; Hydration; Dielectric.

Abstract. A new measurement system is developed to monitor the early hydration of cementitious

materials based on measured dielectric properties of the material in low electromagnetic frequency

range. The objectives of this paper were to evaluate the changes in the electromagnetic properties

for samples with different fly ash content and to establish the reliability of the measurement

technique by comparing with results obtained by traditional method such as thermal method that is

either time consuming or impractical. The method adopted in the present experimental work is a

parallel plate electrode system (PPES). The suggested monitoring device for concrete hydration and

strength development is based on the relationship between the electromagnetic properties such as

dielectric constant, loss factor and the strength development during hydration process and curing

time. In this research the electromagnetic properties of concrete is found to be dependent on the

hydration and strength of concrete. Therefore the development of microstructure and concrete

compressive strength can be determined by monitoring its electromagnetic properties in the

frequency range of 1 to 100 kHz.

Introduction

Hydration of cement based materials is an important process. The development of strength and

low permeability is a result of hydration that involves a reaction between water and the anhydrous

compounds present in the cement. Therefore, the degree of hydration can be monitored to identify

the strength either it is higher or lower [1]. Corresponding to time, hydration will occur

continuously and it will never cease. To what extent the hydration process has preceded is known as

degree of hydration. These physical and chemical reactions are still very complicated to understand.

Such hydration process has attracted a considerable attention and has been studied by many

methods, like the commonly used Vicat needle, universal testing machine, scanning electronic

microscope (SEM), X-ray diffraction (XRD) and differential thermal analysis. However, most of

these methods are conducted on small samples which needs special preparation process and

considered to be cost and time consuming process. Moreover, the accuracy of these methods

depends largely on the experience and skill of the person performing it, and such methods cannot

provide a continuous information about hydration. Therefore, alternative techniques, which are

accurate and non-destructive, are highly needed to monitor the hydration process of cement-based

materials [2].

Non-destructive testing (NDT) techniques have attracted considerable attention for the concrete

behavior characterization. Electromagnetic is one of the most important techniques. In most cases,

electromagnetic techniques were used to monitor the hydration process of cement-based materials.

[3,4,5,6]. It is easy to perform and provides correlation between electromagnetic and mechanical

properties of dispersible systems such as rock, clays and other soils which have microstructures

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similar to that of concrete. The hydration of various types of cement based materials have been

characterized using electromagnetic technology by several researchers [7,8, 9, 10].

In this paper, electromagnetic system is developed for monitoring the evolution of materials

properties in the early stage of hardening. The monitoring device for concrete hydration and strength

development is based on the relationship between the electromagnetic properties such as dielectric

constant, loss factor and the strength development during hydration process and curing time. The

suggested system offers the following advantages: relatively simple technique, bridges the gap

between sophisticated basic examination and traditional test, no time lost, non-destructive, the

electromagnetic properties of concrete control by parameters which are impossible to measure in

other ways and easy to adapt for use in the field. Preliminary results show good correlation between

electromagnetic parameters and the best current knowledge concerning microstructure of concrete.

Experimental

To evaluate the hydration of cement paste, Portland cement paste was prepared using cements

Type I and w/c ratio 0.5. The hydration of cement paste was monitored using three different

methods. These methods are bound water, calorimeter and dielectric method using PPES. Another

set of paste was prepared by partial replacement of Portland cement with fly ash. The replacement

was of 20% by weight. The hydration processes were monitored using the dielectric method only.

The mixing of the cement pastes was complied with ASTM C305-91, in the room temperature

and ambient humidity. The measurement was not conducted under isothermal conditions, and the

samples were compacted into rigid Perspex molds for electrical measurements.

Measurement System and Testing Molds

The adopted device to monitor the hydration of cementitious material is known as parallel plate

electrode system (PPES). This system involves sandwiching a thin sheet of material between two

electrodes to form a capacitor. The device consists of a source of electromagnetic signal and a

transmission line forming an electrical circuit. The cementitious material sample forms a component

of this circuit. Then the setup device will measure the permittivity of the circuit. Using the Lump

Model, the impedance of the cementitious material can be measured and the dielectric properties of

cementitious material can be deduced.

All impedance measurements were acquired using the Hewlett Packard 4263B LCR meter. The

measurements were conducted in frequency of 100 kHz. Calibration is an important technique to

prepare the back ground reading of the LCR meter. There are two steps of calibration which are

open and short calibration. Impedances of cable and plates, fringing and system without material

(air-filled) were measured. The molds were made by Perspex and PVC pipes. Copper plates were

used as electrodes. The diameter of the copper plates was 120mm, the model circuit of the PPES

system and the testing molds are shown in Fig. 1

Figures 1. The model circuit of the PPES system and the testing molds.

334 Structural, Environmental, Coastal and Offshore Engineering

Experimental Results

Hydration by Bound Water

The bound water of cement paste made with w/c of 0.5 was determined. The degree of hydration

)(t was calculated using Eq. 1. The results are shown in Fig. 2.

potW

tWt

)()( (1)

where W(t) is the bound water at time t and Wpot is the potential bound water liberated if all cement

hydrated.

It is difficult to prevent water evaporation from cement paste during experiment; therefore the

small change of bound water could not be measured for long period of time using gravimetric

method. The degree of hydration using bound water showed higher values in the early age than

expected. It was also time consuming, tedious, and costly.

Figures 2. Degree of hydration of cement paste measured during the first 24 hours using bound

water method.

Hydration by Calorimetric Method

The temperature of cement paste was monitored and the results are shown in Fig. 3. The

responses of heat evolution over curing time are shown in Fig. 4a. The degree of hydration of

cement paste was calculated using Eq. 2. The degree of hydration is calculated using the model

proposed by [11]. The results are shown in Fig. 4b the experimental results match the model. Since

it is difficult to measure the temperature of specimens after one day of the experiment and almost

three days in similar other studies reported by [11, 12].

potQ

tQt

)()( (2)

where Q(t) is the liberated heat at time t and Qpot is the potential heat liberated if all cement

hydrated.

Applied Mechanics and Materials Vol. 567 335

Fig. 4 shows that calorimetric method indicates the foure different stages of hydration named as

(I, II, III, and IV). These stages represent the different ammount of heat evolution and rate at each

stage as a result of changing chemical reaction of cement paste.

Figures 3. Temperature released from cement paste measured during the first 24 hours.

(a) (b)

Figures 4. Monitor hydration process using heat evolution from cement paste measured during

the first 24 hours, (a) cumulative temperature released, (b) degree of hydration

Hydration by Dielectric Properties

The complex impedances of the PPES containing Portland cement paste were measured during

the first 24 hours. The results the real part of impedance (Resistance = R) and the imaginary part of

the impedance (Reactance = X) are presented in Fig. 7. Continuous increasing of the resistance and

decreasing of the conductance was noticed during the progress of the hydrations of cement paste.

This could be as a result of reducing free water in the paste and bound the charges in the

microstructures of cement paste. The calculated dielectric constant and loss factor of cement paste

from the measured impedances are presented in Fig. 8. The results indicate that the dielectric

properties could be used to monitor the hydration process and clearly explain the four different

stages of hydration designated as I, II, III, and IV.

Two cement pastes of varying fly ash content were tested. When water was first added to cement,

the dielectric constant of the paste was found to be as high as 4000 because of the release of a large

amount of Ca++

and OH- ions into the water. The presence of these ions resulted in the paste

becoming highly polarizable and conductive. As expected, the resistivity at that point of time was

very small (stage I). However, as hydration progressed the free water in the paste started changing to

adsorbed water and bound water. Due to the increasing formation of the CSH gel, the polar

molecules started getting bound and unable to polarize. Calcium hydroxide, the main source of ions,

336 Structural, Environmental, Coastal and Offshore Engineering

started to crystallize. For these reasons, the dielectric constant showed a rapid decrease (stage

II).The decrease in conductivity was also indicated by a tremendous increase in the resistivity.

At about 5 hours after casting, there was an increase in the loss factor (stage III). At this point,

there was a release of lime from the C3S grains due to further hydration of the alite phase. The

resulting increase of ions in the water increased the dielectric constant.

After this, the concrete hardened and the polar molecules found it difficult to move, resulting in

less polarization and a decrease in the dielectric constant (stage IV). The dielectric constant has been

observed to be higher for cement paste containing fly ash, as expected.

(a) (b)

Figures 7. The measured impedance (r) of cement paste measured during the first 24 hours, (a)

real part, (b) imaginary part.

(a) (b)

Figure 8. Dielectric properties of cement paste measured during the first 24 hours, (a) dielectric

constants, (b) loss factors.

Conclusions

Monitoring the hydration process by bound water and calorimetric methods is difficult to conduct

and the results are sensitive to adiabatic and none adiabatic conditions and environmental changes.

These methods also cannot be used to define the stages of hydration clearly. The measured

impedances and the calculated dielectric constants and loss factors in the very early age of cement

paste can be used to monitor the early hydration of cementitious material. In addition, the dielectric

properties can be used to indicate the different stage of hydration of cement paste. Furthermore, the

measured electromagnetic properties can be used to indicate the degree of hydration of Portland

cement.

Applied Mechanics and Materials Vol. 567 337

Acknowledgement

This work was supported by a grant “No. PCSED-015-12 Development near and far field

electromagnetic sensors” from Deanship of Scientific Research and promising center of electronic

sensors, Najran University, Kingdom of Saudi Arabia.

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