REHYDRATION OF CEMENT FINES: A TG /CALORIMETRY...
Transcript of REHYDRATION OF CEMENT FINES: A TG /CALORIMETRY...
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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
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at fl
ux
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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.
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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
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[5] C.-S. Poon, X. C. Qiao, e D. Chan, “The cause and influence of self-cementing properties of fine
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[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.
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[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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