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REHYDRATION OF CEMENT FINES: A TG /CALORIMETRY STUDY

Sérgio C. Angulo (1,2); Mário S. Guilge (2); Valdecir A. Quarcioni (2); Raphael

Baldusco (2); Maria A. Cincotto (1)

(1) Department of Construction Engineering, University of Sao Paulo (USP), Brazil

(2) Institute for Technological Research (IPT), Sao Paulo state, Brazil

Abstract

Research on new sources of cementitious materials is required to reduce global

antropogenic CO2 emissions of cement industry. Cementitious powders from construction and

demolition waste can be dehydrated and rehydrated and present some residual strength

according to some initial studies. Systematic studies including characterization techniques

(thermogravimetry and calorimetry) are lacking for a better understanding of this material.

This paper aims to investigate rehydration of cement powders using these techniques, in order

to establish relations between rehydrates water content, heat of rehydration and achieved

compressive strength. The compressive strengths of treated cement fines were related to heat

of rehydration and rehydrates water content.

1. INTRODUCTION

Cement production is the industrial sector responsible for ~5% of antropogenic CO2

emissions in the world [1]. Since the cement production tends to double in the next 20years,

this sector would represent more than 30% of global antropogenic CO2 emissions in 2050.

Research on new sources of cementitious materials and additions are required for future.

Portland cement is constituted of 95-97% (in mass) of clinker and 3-5% (in mass) of

gypsum ( - CaSO4.2H2O) [2]. Clinker contains mostly alite – C3S (3CaO.SiO2) and

belite - C2S (2CaO.SiO2), but it also includes around 25% of calcium-aluminate phases, such

as C3A (3CaO.Al2O3) and C4AF (4CaO.Al2O3. Fe2O3). In contact with water, such crystalline

phases form the compounds described below.

In short, hydration reactions can be described as follows. Tricalcium-aluminate, in

presence of gypsum, is the first phase to be hydrated near the cement’surface grains forming

ettringite ( ) and calcium monosulfoaluminate just after the contact cement-water.

After the induction period of the hydration, silicate hydrate (C-S-H) and portlandite (CH) are

formed followed by the complete formation of ettringite as result of the total consumption of

sulfate. Between 20 and 30 hours, most of ettringite is converted to calcium

monosulfoaluminate ( ). In-situ X-ray difraction and thermogravimetry (TG) over

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time are powerful techniques for understanding early cement hydration over time. It is now

well understood and validated by thermodynamic model [3].

Figure 1 – Thermodynamic modelling of Portland cement hydration [3].

As reported by Lothenbach et al. [3], after 28 days of hydration, the paste’ volume is 45%

of C-S-H, 25% of CH, and 20% of calcium sulfolauminates phases, remaining near 5% of

anhydrous calcium silicate phases (C3S and C2S). Alite is more reactive than belite, that may

persist unhydrated over month or years. Even a long term hydrated cement may contain

certain amount of belite phase, still active in cementitious waste. When cementitious waste is

milled as powder (< 100µm) it will present certain residual strength, as reported by [4]–[6].

Recently, some authors have been studied dehydration and rehydration of cementitious

powders in order to recover this residual strength [7]–[9]. The environmental advantage of

this type of recycled cement is the absence of decarbonation of the raw materials in the

process, avoiding CO2 emissions during its production.

Different techniques have been applied aiming to analyse the obtained product. Shui et al.

[8] studied the dehydration of cementitious powders and pure cement until 800ºC. They

identified by X-ray diffractometry (XRD) that ettringite disapears after thermal treatment at

200ºC, followed by an increase of intermediate silicate after 500ºC. The characteristics peaks

of this intermediate silicate are close to that of C2S. Compressive strengths from 4.7 to 8.3

MPa were achieved with the rehydratin of cementitious powders. Using nuclear magnetic

ressonance (NMR), Alonso and Fernandes [7] also observed that, after 200ºC, this

intermediate silicate starts to be formed and increases near 500ºC, being completed formed at

750ºC.

Guilge [6] also confirmed the rehydration of cementitious powders using complementary

characterisation techniques (XRD, thermogravimetry – TG, and isothermal calorimetry).

Initial results of this dissertation are presented here. In this paper the investigation of recycled

cement powders rehydration was made by complementary techniques thermogravimetry and

calorimetry. A comprehensive study using thermogravimetry (TG) is necessary to check the

recovering ability of C-S-H and other hydrates (C-A-H, ettringite, etc) in different

dehydration temperatures. Other relevant aspect is the use of isothermal calorimetry to follow

rehydration reactions through heat released. Relationships among rehydrates content, heat of

rehydration and achieved compressive strength were investigated here.

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2. MATERIALS AND METHODS

An Ordinary Portland Cement (95% of clinker) was hydrated, dehydrated at 300 °C, 500

°C and 650 °C and rehydrated. Thermogravimetry (TG) and Isothermal Calorimetry (IC) were

used to evaluate dehydration and rehydration of these materials, as well as by compressive

strength essays at 28 days.

2.1 Materials

The cement (C) of the study contained 95% of clinker in mass (CPV-ARI Brazilian

standard type). The chemical composition of the cement type CPV-ARI is shown in Table 1.

Table 1 – Chemical composition of the cement. Oxides SiO2 CaO Al2O3 Fe2O3 SO3 MgO K2O Na2O LOI Content (%) 18.9 62.2 5.4 2.8 4.2 1.9 1.0 0.21 3.8

2.2 Methods

Hydrated cement (HC) specimens (cylinders of 5 x 10 cm) were prepared with

water/cement ratio of 0.48 (kg/kg) and cured during 28 days (at room temperature with RH

100%). After the curing period, the specimens were dried at 40°C for 24 h, crushed and

milled as powders (< 150 µm sieve aperture).

HC powders were then submitted to dehydration in a controlled heating condition up

to each selected temperature: 300, 500, 650 °C. The furnace was heated at a rate of 10

°C/min, starting from room temperature. When the desired temperature was reached, the

powders were kept in this condition during 2 h. The whole heating scheme [6], adapted from

Shui’s procedure [8], is shown in Fig. 1. Dehydrated cement (DC) powders were then

submitted a rapid cooling with a fan to room temperature and covered with a plastic film and

kept into a desiccator until characterization tests. They were named DC300, DC500 and DC650,

respectively.

Figure 2 - Heating scheme of HC [6], [8].

HC and DC powders (DC300, DC500 and DC650) were then rehydrated with

water/cement ratio of 0.48, moulded in specimens (cylinders of 5 x 10 cm) and cured during

28 days. After 28 days, the rehydrated cement (RC) specimens were dried at 40°C for 24 h,

crushed and milled again as powders (< 150 µm sieve aperture). The powders were named

RC, RC300, RC500 and RC650, respectively.

Cooling abrupt to room temperature

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Thermogravimetric (TG/DTG) analyses were conducted with HC, DC300, DC500,

DC650, RC, RC300, RC500 and RC650 samples, using a TA Instruments equipment, model SDT

2960. The essays were carried out under the following conditions: 50 mg sample were heated

in an alumina crucible 110µl without cover, nitrogen gas flow of 50 ml/min ultrapure; heating

flow of 10°C/min, and temperature interval from 10°C to 1,000°C. Table 2 shows the thermal

events of hydrated cement considered for the DTG’s interpretation [2].

Table 1 – Thermal events of hydrated cement for DTG’s interpretation [2].

Thermal events Temperature range

(°C)

Dehydroxilation of C-S-H 0 – 400

Dehydroxilation of ettringite (AFt) 70 – 140

Dehydroxilation of calcium sulfoaluminates (AFm) 185 – 200

Dehydroxilation of hydrogarnet - C3AH6 250 – 550

Dehydroxilation of hydrotalcite- [M0,75A0,25(OH)2] 0,125H0,5 330-430

Dehydroxilation of portlandite – Ca(OH)2 430 – 550

Decarbonation of calcite - CaCO3 550 - 900

To monitor rehydration process, isothermal calorimetric (IC) analyses were carried out

with the C, HC, DC300, DC500, DC650 samples, at 25°C, using equipment Thermometric TAM

AIR with 8 channel, with 16s frequency. The mass used was 10 g each sample and data were

collected during 72 hours.

Compressive Strength (CS) tests of C, RC, RC300, RC500 and RC650 were made using

Brazilian standard procedure (NBR 7215: 1997): 1:3 proportion in mass (cement:quartz

reference sand); water/cement ratio of 0.48; four layers of material manually compacted; and

compressive strength tests with four specimens (5 x 10 cm) at 28 days.

3. RESULTS

3.1 TG/DTG analyses

DTG results of HC, DC300, DC500, DC650 are presented in Fig. 3. The first thermal

event (temperature range from 0 to 330ºC) is related to the presence of ettringite (discrete

peak near 95ºC), C-S-H (peak near 120ºC) and calcium monosulfoaluminate (discrete peak

near 180ºC). All these events are clearly noticeable in HC. For DC300, DC500, DC650 samples,

the presence of ettringite and calcium monosulfoaluminate is not observed anymore, only a

progressively reduction of the amount of remained combined water of C-S-H.

The second thermal event (temperature range from 330 to 430ºC) is related to brucite’s

prsence (Mg(OH)2). This event is only observed for HC and DC300. Due to higher dehydration

temperature, this event is not more observed for DC500 and DC650 samples.

The third thermal event (temperature range from 430 to 550ºC) is related to

portlandite’s dehydration (Ca(OH)2). The event is clearly observed to HC and DC300, but

partially observed to DC500 and DC650 samples, since the temperature of thermal treatment

partially decomposed this compound.

The fourth event (temperature > 550ºC) is related to calcite’s presence (CaCO3). This

event is observed for HC, DC300, and DC500. A slightly reduction of this compound occurred

for DC650 sample.

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Figure 3- Dehydration of HC samples.

Table 3 shows the mass losses of HC, DC300, DC500, DC650 samples for each of these

events. A gradual reduction of combined water of C-S-H is noticed when temperature of

dehydration is increased. A reduction of portlandite is also observed for DC500 and DC650

samples; probably forming higher amounts of dehydrated CaO compounds, easily rehydrated

in presence of humidity at room condition (Fig. 3). Dehydration removes 55%, 91% and 94%

(in mass) of the combined water of ettringite, C-S-H, C-A-H and calcium

monosulfoaluminates, respectivelly.

Table 3 – Mass losses of HC and DC samples in the TG/DTG temperatures’ ranges.

Samples Thermal event Temperature

range (ºC)

Mass loss

(%)

HC Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 15.30

Dehydroxilation of hydrotalcite 330-430 1.95

Dehydroxilation of portlandite – Ca(OH)2 430-550 4.31

Decarbonation of calcite - CaCO3 550-1,000 4.71

DC300 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 6.87

Dehydroxilation of hydrotalcite 330-430 2.69

Dehydroxilation of portlandite – Ca(OH)2 430-550 6.57

Decarbonation of calcite - CaCO3 550-1,000 5.38

DC500 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 1.35

Dehydroxilation of hydrotalcite 330-430 0.00

Dehydroxilation of portlandite – Ca(OH)2 430-550 3.88

Decarbonation of calcite - CaCO3 550-1,000 6.25

DC650 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 0.95

Dehydroxilation of hydrotalcite 330-430 0.00

Dehydroxilation of portlandite – Ca(OH)2 430-550 2.54

Decarbonation of calcite - CaCO3 550-1,000 4.46

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Figure 4 compares DTG’s of the rehydrated samples (RC, RC300, RC500 and RC650)

with the hydrated cement (HC). HC and RC are quite similar. The combined water of C-S-H

is partially recovered for all dehydration temperatures, as well as the water of brucite, and

portlandite. Difference may be attributed to non-recovery of ettringite. RC650 also recovered

less amount of water if compared with the other temperatures of dehydration (Table 4).

Figure 4- Rehydration of DC samples.

Table 4 – Mass losses of RC samples in the TG/DTG temperatures’ ranges.

Samples Thermal event Temperature

range (ºC)

Mass loss

(%)

RC Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 14.6

Dehydroxilation of hydrotalcite 330-430 2.35

Dehydroxilation of portlandite – Ca(OH)2 430-550 4.26

Decarbonation of calcite - CaCO3 550-1,000 5.09

RC300 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 15.20

Dehydroxilation of hydrotalcite 330-430 2.22

Dehydroxilation of portlandite – Ca(OH)2 430-550 4.46

Decarbonation of calcite - CaCO3 550-1,000 4.82

RC500 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 14.4

Dehydroxilation of hydrotalcite 330-430 1.73

Dehydroxilation of portlandite – Ca(OH)2 430-550 4.95

Decarbonation of calcite - CaCO3 550-1,000 5.04

RC650 Dehydroxilation of Aft, C-S-H, C-A-H, Afm 25-330 12.50

Dehydroxilation of hydrotalcite 330-430 1.65

Dehydroxilation of portlandite – Ca(OH)2 430-550 4.45

Decarbonation of calcite - CaCO3 550-1,000 4.64

Rehydration recovers 54%, 85% and 75% (in mass) of the combined water of

ettringite, C-S-H, C-A-H and calcium monosulfoaluminates, respectively. Thermal treatment

at 500ºC achieved the best condition in terms of cement rehydration.

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3.2 Isothermal Calorimetry (IC)

Figure 4 presents the isothermal calorimetry results. Hydration of conventional cement (C)

generated roughly 360 J/g. Rehydration of HC sample generated 49 J/g, probably due to

hydration of some residual anhydrous calcium silicates. Rehydration of DC samples (DC300,

DC500, DC650) generated less amount of heat: 120, 235 and 177 J/g, respectively. The most

reactive sample was obtained by the thermal treatment at 500ºC. High heat liberation in the

first minutes may be attributed to the presence of CaO in the samples. The rehydration

phenomena occurred in the first 30 hours, faster than the hydration phenomena. This finding

corroborates with that found by Shui et al. [8], [9].

(a) (b)

Figure 4- Heat flux and cumulative heat released during rehydration of DC samples.

3.3 Compressive strength (CS)

The average compressive strength of RC, RC300, RC500 , RC650 and HC were 1.50, 4.40,

7.60, 4.20 and 42.0 MPa, respectively (Table 5). Shui et al. achieved similar strength for a

dehydrated cement at 400-500ºC.

Table 5 – Compressive strengths of RC samples.

Samples Compressive Strength (MPa).

spec 1 spec 2 spec 3 spec 4 Average Std. Dev.

RC 1.4 1.4 1.6 1.4 1.5 0.1

RC 300 4.1 4.6 4.3 4.5 4.4 0.2

RC 500 7.4 7.8 7.2 7.9 7.6 0.3

RC 650 4.0 4.1 4.6 4.2 4.2 0.3

3.4 Correlation between the parameters

Figure 5a shows a correlation between rehydrates content (% in mass) and heat of

rehydration (J/g). The higher rehydrates content is the higher the heat of (re)hydration is.

Similar conclusion can be obtained between heat of rehydration and compressive strength for

a fixed value of water/cement ratio (0.48) (Figure 5b).

0

1

2

3

4

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6

0 10 20 30 40 50 60 70

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DC 650

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250

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CHCDC 300DC 500DC 650

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Figure 5-Rehydrates water content x Heat of rehydration (a) and

Heat of rehydration x compressive strength (b).

3. CONCLUSIONS

Dehydration and rehydration of Portland cement fines was quantitatively determined by

thermogravimetry and calorimetry. The compressive strengths observed were related to heat

of rehydration and rehydrates water content. Restricted to this study and type of cement

applied, the most appropriate temperature for dehydration of cement fines in terms of

reactivity and strength was 500ºC.

ACKNOWLEDGEMENTS

This research was developed and funded by IPT. Authors also thank Intercement/BNDES

for technical partnership and financial support on the on-going research project (Development

of technology for cement production from CDW) motivated by this initial research.

REFERENCES

[1] WBCSD e IEA, “Cement Technology Roadmap: Carbon emissions reductions up to 2050”, World

Business Council for Sustainable Development; International Energy Agency, Report, 2009.

[2] H. F. Taylor, Cement Chemistry, 2o ed. London: Thomas Telford.

[3] B. Lothenbach, G. Le Saout, E. Gallucci, e K. Scrivener, “Influence of limestone on the hydration

of Portland cements”, Cem. Concr. Res., vol. 38, no 6, p. 848–860, jun. 2008.

[4] M. Arm, “Self-cementing properties of crushed demolished concrete in unbound layers: results

from triaxial tests and field tests”, Waste Manag., vol. 21, no 3, p. 235–239, jun. 2001.

[5] C.-S. Poon, X. C. Qiao, e D. Chan, “The cause and influence of self-cementing properties of fine

recycled concrete aggregates on the properties of unbound sub-base”, Waste Manag., vol. 26, no

10, p. 1166–1172, jan. 2006.

[6] M. S. Guilge, “Desenvolvimento de ligante hidráulico a partir de resíduos de cimento hidratado e

de tijolo cerâmico”, Dissertação de Mestrado, Instituto de Pesquisa Tecnológica, São Paulo, 2011.

[7] C. Alonso e L. Fernandez, “Dehydration and rehydration processes of cement paste exposed to

high temperature environments”, J. Mater. Sci., vol. 39, no 9, p. 3015–3024, maio 2004.

[8] Z. Shui, D. Xuan, H. Wan, e B. Cao, “Rehydration reactivity of recycled mortar from concrete

waste experienced to thermal treatment”, Constr. Build. Mater., vol. 22, no 8, p. 1723–1729, ago.

2008.

[9] Z. Shui, D. Xuan, W. Chen, R. Yu, e R. Zhang, “Cementitious characteristics of hydrated cement

paste subjected to various dehydration temperatures”, Constr. Build. Mater., vol. 23, no 1, p. 531–

537, jan. 2009.

R² = 0,95

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(a) (b)