Accepted manuscript doi:
10.1680/jadcr.18.00172
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Advances in Cement Research

Accepted manuscript doi:
10.1680/jadcr.18.00172
Submitted: 08 September 2018
Published online in ‘accepted manuscript’ format:
11 February 2019
Manuscript title: Performance of limestone calcined clay blended cement-based concrete against carbonation
Authors: Mohammad S. H. Khan, Quang Dieu Nguyen and Arnaud Castel
Affiliation: Centre for Infrastructure Engineering and Safety, School of Civil and
Environmental Engineering, The University of New South Wales, Sydney, NSW 2052,
Australia.
Corresponding author: Mohammad S. H. Khan, Centre for Infrastructure Engineering and
Safety, School of Civil and Environmental Engineering, The University of New South Wales,
Sydney, NSW 2052, Australia. Tel.: +61401343968
E-mail: jehin_154@yahoo.com
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Accepted manuscript doi:
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Abstract
This work at aims investigating the carbonation resistance of limestone and calcined clay blended cement-based concrete. Two limestone and calcined clay concretes with an average 28 days compressive strength of about
36MPa were considered. Limestone and calcined clay (with a ratio of 2:1) were blended with a General Purpose
(GP) cement. The GP cement substitution rates considered were 30% and 45%.A low grade calcined clay was used with about 50% amorphous phase. Accelerated and natural carbonation tests were performed. Mercury intrusion porosimetry (MIP) and X-ray diffraction (XRD) were carried out, assisting in analysing the experimental results. Results show that the early age compressive strength is only marginally affected by the limestone and calcined clay substitution up to 45% and a significant refinement of the pore structure was observed compared to reference GP cement concrete. The resistance of concrete against carbonation is reducing when increasing the GP cement substitution rate. Overall, this study shows that limestone and calcined clay blend used as simple substitution of General Purpose cement in concrete can provide adequate protection against carbonation induced steel reinforcement corrosion if the Ordinary Portland Cement content in the mix is at least
60%.
Keywords : Calcined clay; limestone; LC3; carbonation; pH; durability
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Accepted manuscript doi:
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Introduction
The production of Portland cement is responsible for 5% to 7% of the worldwide man-made
CO
2
emission, of which 50% is from the chemical process and 40% from burning fuel
(Worrell et al. 2001; Damtoft et al. 2008). The utilization of supplementary cementitious materials (SCMs), such as fly ash and Ground Granulated Blast Furnace Slag (GGBFS) which allows to decrease the quantity of Portland cement used in concrete, has been a rational approach to reduce the overall carbon footprint of construction industry.
Among promising SCMs, calcined clays have received considerable attention due to their global availability. Clay structure is a combination of silica and alumina sheets and aluminasilicates minerals representing 74% of the earth’s crust. Clay is available almost everywhere
(Antoni 2013), whereas supply of traditional SCMs (fly ash or GGBFS) is only limited to some areas of the world (Damtoft et al. 2008). Using calcined clay allows to improve the pore structure and the long term strength of concrete as a result of pozzolanic reactions between
AS
2
(Al
2
O
3
.2SiO
2
) and Portlandite (Siddique and Klaus 2009). However, using fly ash or calcined clay leads to a reduction in early age development of blended cement-based concrete strength (Menendez et al. 2003; Lothenbach et al. 2011), which is a concern for numerous applications.
Another issue with using pozzolanic SCMs is the reduction in resistance against atmospheric carbonation which can affect the durability of reinforced concrete structures. Indeed, steel is protected from corrosion (i.e. passivated) by the high alkalinity of concrete. However, concrete carbonation by atmospheric CO
2
leads to a reduction in the alkalinity of the concrete pore solution. When the carbonation front reaches the steel reinforcing bars embedded in concrete as the result of CO
2
diffusion through the concrete porosity, steel is depassivated and corrosion starts(Beckett 1983; Glass et al. 1991). In accelerated condition, several authors (Nicolas et al. 2014; Shi et al. 2016) reported a significant increase in carbonation depth of concrete containing 25% calcined clay compared to reference OPC concrete even though calcined clay concrete permeation was reduced. Indeed, using pozzolanic SCMs lead to a reduction in Portlandite hydrates quantity in the matrix because of the lower OPC content in the mix and the consumption of Portlandite by the pozzolanic reactions. As a result, the extent of chemical interaction (and binding of CO
2
) is reduced compared to OPC concrete
(Thomas and Matthews 1992) leading to an increase in CO
2
diffusion.
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Regarding the issue relating to early age strength development, a possible approach to address this problem is the addition of limestone in a ternary blended cement. Limestone powder provides an increase of the surface area for the nucleation of hydration products, leading to an acceleration of reactions, which is known as filler effect. Previous studies utilized isothermal calorimetry and compressive strength test to highlight the positive “filler effect” of finely powdered limestone in different particle size and OPC replacement rates
(Kumar et al. 2013; Kumar et al. 2013; Oey et al. 2013). Beside the physical filler effect, limestone also acts as an accelerator for Alite hydration resulting in the improvement of the early strength (Damtoft et al. 2008). Moreover, calcium carbonates of limestone powder can interact with aluminate hydrates, leading to the stabilization of ettringite and an increase in the total volume of hydration products. Indeed, SCMs with high alumina content such as calcined clays can produce significant amount of void-filling carboaluminate phases if blended with a large amount of limestone (T.Matschei et al. 2007; Weerdt et al. 2011;
Antoni et al. 2012; Vance et al. 2013). Recently, Kunther (Kunther et al. 2016) reported that blending metakaolin with limestone leads to the suppression of stratlingite and other phases such as monosulfate, hemicarbonate and monocarbonate. Antoni (Antoni et al. 2012) showed that a ratio of 2:1 of metakaolin and limestone powder provided better mechanical properties at 7 and 28 days than that of 100% OPC concrete for OPC replacement rates up to 45%.
Another investigation (Tironi et al. 2017) utilized a 3:1 proportion of kaolinitic calcined clay and limestone filler looking at OPC replacement rates up to 40%. Results revealed that limestone filler facilitates pozzolanic reactions particularly at early age leading to a significant reduction in the quantity of coarse pores in the matrix.
Shi et al. (Shi et al. 2016) investigated the carbonation resistance of LC3 mortar by extensive microstructure analysis. However, their study focused on pure grade kaolinite clays produced in the laboratory which overlooked the potential application of large-scale availability of lowgrade calcined clays (Badogiannis et al. 2005; Nicolas et al. 2013; Scrivener 2014; Tironi et al. 2014). This study investigates the carbonation resistance of LC3 concrete using a low grade calcined clay with about 50% amorphous phase.
Carbonation induced reinforcement corrosion is usually governing the design for durability in exposure classifications involving cycles of wetting and drying. Indeed, both concrete carbonation and steel corrosion require humidity and CO
2
diffusion in concrete is stronger during the drying phases (Castel et al. 1999). The typical concrete grade specified in outdoor
Advances in Cement Research

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10.1680/jadcr.18.00172 exposure is around 30 MPa characteristic compressive strength (i.e. about 35 to 40 MPa average compressive strength). In this study, two LC3 concretes with an average compressive strength of about 36 MPa were considered. Limestone and calcined clay (with a ratio of 2:1) were blended with a General Purpose (GP) cement. The GP cement substitution rates considered were 30% and 45%. Limestone and calcined clay blend is used as any other SCM in Australia by straight replacement of GP cement in the concrete mix without any optimisation of sulphate content or alkalinity of the blended cement (Antoni et al. 2012). This approach aims to reduce the time for limestone and calcined clay blend adoption in the industry. Accelerated carbonation tests were carried out using 1% CO
2
. Phenolphthalein indicator test was used to assess the carbonation depth of concrete exposed to natural and accelerated carbonation. pH profiles of LC3 concretes were measured. Mercury intrusion porosimetry (MIP) and X-ray diffraction (XRD) were carried out assisting in analysing the experimental results.
Experimental Program
Materials
The limestone calcined clay cement concrete used in the study is a ternary blend of General
Purpose (GP) cement, calcined clay and limestone. All raw materials were obtained from the industry. The GP cement complies with the Australia Standard AS 3792-2010. Importantly,
Portland cement content is about 90% in Australian GP cements. Indeed, 7% to 8% of mineral additions are allowed in GP cement in addition to 2% to 3% of gypsum. The limestone branded as Stone Dust was supplied by Boral Construction Materials Limited in
New South Wales, Australia. The calcined clay used was made using a flash calcination process and supplied by Argeco, France. Table 1 shows the chemical composition of all cementitious materials determined by X-ray fluorescence (XRF) and the mineralogical composition of GP cement determined by using XRD-Rietvelt analysis. XRD patterns of the calcined clay and the limestone are presented in Fig. 1. The only crystalline phases identified are quartz in the calcined clay and calcite in the limestone.
Calcined clay and limestone were analysed using a Hitachi S-3400N scanning electron microscope (SEM) at Mark Wainwright Analytical Centre, UNSW Sydney, Australia (Fig.
2). Gold coating was applied into samples prior to the analysis. The system configuration for
SEM analysis comprised working voltage of 20kV, probe current of 50 and working distance
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10.1680/jadcr.18.00172 of 10mm. The presence of spherical particles (Fig. 2(a)) with agglomerated particles of metakaolin on its surface (Fig. 2(b)) represents the main disparity between flash calcined clay and traditional calcined clay. Indeed, traditional calcined clay is mostly composed of few nanometres thick hexagonal particles. During the flash calcination, the clay melts in a hot gas steam, creating a significant quantity of spherical particles. Similar morphology can be noticed in other SCMs such as fly ash (San Nicolas et al. 2013). Argeco calcined clay can contain 10 to 15% of spherical particles (Cassagnabere et al. 2013; San Nicolas et al. 2013).
The particle size distribution and characteristics of GP cement, calcined clay and limestone powders are presented in Fig. 3 and Table 2, respectively. Laser diffraction technique by
Malvern Mastersizer 2000 instrument measuring particle sizes ranging from 0.01µm to
10mm was used to determine the particle size distribution (PSD). GP cement was dispersed into isopropanol to prevent any hydration whilst calcined clay and limestone were dispersed into water and ultrasonic was utilized to scatter powders into primary particles. GP cement showed the finest PSD and limestone had a significantly higher percentage of coarse particles with a D v90
being around 130µm. Calcined clay and limestone used are classified as coarse calcined clay and coarse limestone, respectively, based on previous studies (Bosiljkov 2003;
Vizcaíno Andrés et al. 2015).
The selection of calcined clays to be used as SCM is mainly based on their reactivity which is governed by the transformation of the kaolinite during the calcination process (Tironi et al.
2012; San Nicolas et al. 2013). XRD results revealed that there is no trace of kaolinite in the calcined clay (Fig. 1), which indicates that all the kaolinite was dehydroxylated during calcination. The dehydroxylation of kaolinite results in the formation of amorphous material with high reactivity in cement-based system (He et al. 1995; Tironi et al. 2012; San Nicolas et al. 2013). The quantity of amorphous phases after the calcination process is an important factor as well. The amorphous content of the calcined clay used in this study is 50.9%, obtained from XRD-Rietveld refinement (Table 3), which classifies this calcined clay as lowgrade (Badogiannis et al. 2004; Badogiannis et al. 2005; Badogiannis and Tsivilis 2009;
Vizcaíno et al. 2015). XRD-Rietveld results are consistent with the high silicon dioxide content obtained by XRF analysis (Table 1). Indeed, Table 1 shows that the SiO
2
content represents over 70% of the total weight of calcined clay and the SiO
2
/Al
2
O
3
molar ratio is 5.3, which is higher than that of kaolinite (SiO
2
/Al
2
O
3
=2) due to the presence of quartz in large quantity. Calcined clay with high amorphous phase (over 90%) typically contains 50-55%
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SiO
2
and 40-45% Al
2
O
3
. In conclusion, an increase in silicon dioxide content with a decrease in aluminium oxide content reflects a decrease in amorphous phase (Badogiannis et al. 2005;
Tironi et al. 2012; San Nicolas et al. 2013; Ramezanianpour 2014).
Fine aggregate is Sydney sand with specific gravity of 2.65 and water absorption of 3.5%.
Crushed basalt supplied from Dunmore quarry in New South Wales, Australia was utilized as coarse aggregate. Its characteristics comprised specific gravity of 2.8, maximal nominal size of 10mm and water absorption of 1.6%. To obtain saturated surface dry (SSD) condition, all aggregates were first oven dried at 105 o
C for 24h to eliminate any moisture content, then the exact amount of water required was added prior to concrete casting.
Concrete Mix Design and Batching Procedure
Three concretes with an average compressive strength of about 36 MPa were considered: one reference concrete using only GP cement, labelled GPC-30 and two LC3 concretes labelled
LC3-30 and LC3-45. The LC3-30 and LC3-45 mixtures are defined by the rate of GP cement replacement being 30% and 45% with the ratio 2:1 by mass of calcined clay and limestone.
However, as mentioned previously, Australian GP cements contain about 90% of OPC. As a result, the OPC content of LC3-30 and LC3-45 binders is about 60% and 45% respectively.
All mix designs are given in Table 4 with aggregate mass in saturated surface dry (SSD) condition.
Specimens cast were standard cylinders with 100 mm diameter and 200 mm height. A vibrating table was used to compact the concrete. After surface finishing, all moulds were covered by using lids to prevent surface from moisture loss.
Curing Condition
All concrete specimens were demoulded after one day. Then, all specimens were placed into a lime saturated water bath continuously for 7 days. After that, the specimens were stored in the controlled room at a fixed temperature of 23±2 o
C and relative humidity of 50% until the testing dates.
Testing Program
Mechanical Properties
The compressive strength was measured at 7, 14 and 28 days using three 100mm diameter standard cylinders following standard AS1012.9 (ASTM C39). The modulus of elasticity was measured using three duplicates at 28 days in accordance with AS1012.17.
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Carbonation test
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To carry out the carbonation test, after 28 days, 50 mm thick specimens were extracted from the middle section of 100×200 mm standard cylinders. The top and bottom 25 mm parts of each cylinder were disregarded.The remaining 150mm height segment was then cut into three
50 mm samples. 50 mm discs were sealed using aluminium tapes along the perimeter leaving top and bottom sides exposed for CO
2
diffusion. For natural carbonation, the specimens were kept in a controlled environmental room. For accelerated carbonation, the specimens were placed in a carbonation chamber with a CO
2
concentration of 1%. For both accelerated and natural tests, the temperature was 23 o
C and the relative humidity was 55%.
Concrete pH profiles were obtained by combining water extraction method (Haque and
Kayyali 1995) and pore solution extraction method (Jr. and Diamond 1981). For water extraction method, concrete powder was sampled every 1mm over 25mm depth of concrete specimen by using a Profile Grinder PF-1100 purchased from Germann Instruments. The powder was then mixed with de-ionised water with a solid to liquid ratio of 1:1 and the pH of the solution was measured using a pH probe. Water extraction method does not allow to accurately measure the pH (Haque and Kayyali 1995). As a result, the trend provided by the pH profile is accurate but not the pH values. To calibrate the pH trend obtained from the water extraction method, pore solution was extracted from uncarbonated paste samples
(having the same mix design and curing condition as the concrete specimens) (Jr. and
Diamond 1981). The pH of the extracted pore solution was directly measured by using a calibrated pH probe and then compared to the one obtained by using the water extraction method for uncarbonated specimen. The difference between these two pH values were then added to all water extracted pH values to obtain the calibrated pH profile.
Moreover, to assess the carbonation depth, each specimen was split and 1% phenolphthalein indicator was sprayed on the fractured surface.
Identification of crystalline phases
The crystalline phases in uncarbonated and carbonated LC3 samples were identified by using
X-ray diffraction (XRD). 50 mm cube specimens of LC3 pastes were used having the same mix design and similar curing conditions as LC3 concretes. The paste samples were ground to powder which were then analysed using X-ray diffractometer Phillips X’Pert Pro Multipurpose (MPD) system housed at the Mark Wainwright Analytical Centre at the University of
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New South Wales, Australia. This used Cu-Kα radiation with wavelength of 0.15418 nm and operated at 45 kV and 40 mA, scan range 5-65 o
and 0.026
o
2θ step size. The scan results were interpreted using the software package HighScore Plus for phase identification.
Mercury intrusion porosimetry
The pore structure of both LC3 and GPC paste specimens was analysed by mercury intrusion porosimetry (MIP) which was conducted at Particle & Surface Sciences Pty. Limited,
Sydney, Australia by using AutoPore IV 9500 V1.09. MIP analysis utilized contact angle of
130o and surface tension of 0.485 N/m.
Results and discussions
Mechanical properties of concrete
The average compressive strength after, 7, 14 and 28 days are provided in Table 5 for all concretes tested. The 28-day compressive strength of LC3-30 and LC3-45 was 35.5MPa and
36.3MPa respectively, very similar to that of GPC-30. Results show that the 7-day compressive strength was only marginally affected by the limestone and calcined clay substitution. After 14 days, the compressive strength of LC3 concretes is equivalent or higher than that of reference concrete. Moreover, little difference is observed between the compressive strength of LC3-30 and LC3-45 at all age suggesting that the level of GP cement replacement up to 45% is not affecting the performance of LC3 concrete.
The elastic modulus after 28 days of GPC-30, LC3-30 and LC3-45 was 28.6GPa, 29.3GPa and 30.7GPa respectively suggesting that limestone and calcined clay marginally improve the modulus of elasticity of concrete. Authors (Qian and Li 2001; Khatib and Hibbert 2005;
Justice and Kurtis 2007)observed an increase in the elastic modulus associated with the utilization of calcined clay. However, another study (Bonavetti et al. 2000) reported that limestone addition caused a reduction in the elastic modulus. Consequently, it appears that calcined clay eliminates the detrimental effect of limestone on concrete elastic modulus.
MIP results
Fig. 4 shows the MIP results for uncarbonated LC3 and GPC pastes measured at 28 days.
LC3 allows eliminating most of the coarse pores with size ranging between 0.08μm to 0.5μm.
Similar porosity refinement has been reported in the literature (Antoni et al. 2012; Shi et al.
2016; Tironi et al. 2017). Fig. 4 further shows that the total porosity of LC3-30 and GPC-30 mixes was similar, being 31.3% and 28.5% respectively. The total porosity of LC3-45 was
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10.1680/jadcr.18.00172 only 24.4% although its compressive strength was similar to that of LC3-30 and GPC-30 mixes.
Identification of carbonation products in LC3 concrete
Fig. 5 shows the XRD patterns of LC3-30 paste exposed to natural and 1% accelerated carbonation. A considerable amount of portlandite was consumed after 4 weeks of 1% accelerated carbonation and 30 weeks of natural carbonation. In both cases, ettringite and hemicarboaluminate started to carbonate. Fig. 6 shows the XRD patterns of LC3-45 paste exposed to natural and 1% accelerated carbonation. Portlandite, ettringite and hemicarboaluminate phases completely disappeared and peak height of calcite significantly increased due to both natural and 1% accelerated carbonation. XRD results suggest that calcite was the only carbonation products in both LC3 mixes.
Concrete pH profiles
Fig. 7 shows the calibrated pH profiles of uncarbonated LC3 concretes. The pH obtained from pore extraction and water extraction methods was 12.9 and 12.2 respectivelyfor LC3-30 and 12.7 and 12.2 respectively for LC-45. A difference of 0.7 and 0.5 were added to all water extracted value for LC3-30 and LC3-45 respectively to obtain the calibrated pH profiles of both uncarbonated and carbonated concretes. The calibrated pH of uncarbonated GPC-30 was about 13.2. The concrete pH reduces steadily when increasing the calcined clay and limestone content. This can be attributed to the reduction in both alkali metal (sodium and potassium) and hydroxide ions concentration due to the partial replacement of Portland cement and the decline in Ca(OH)
2 content due to the pozzolanic reactions.
Figures 8 and 9 show the pH profiles of LC3-30 and LC3-45 respectively after up to 8 weeks of exposure to 1% accelerated carbonation. For all exposure periods, results show that both carbonation front penetration and pH reduction in LC3 concrete increase when increasing the
GP cement substitution. For LC3-30, the lowest pH value obtained after 8 weeks of exposure was about 10.7. Carbonation seems complete at pH 10.7 for LC3-30. 10.7 was measured over a depth in the concrete of about 8mm. For LC3-45, Fig. 9 shows no clear zone with constant pH after 8 weeks of exposure. But, the pH ranges between 9.1 close to the surface to about 10 up to 11mm depth in the concrete showing that the pH of carbonated LC3-45 is significantly lower than that of LC3-30. Carbonation front penetration was greater as well.This is
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10.1680/jadcr.18.00172 consistent with XRD analysis results (Fig. 6) showing that portlandite, ettringite and hemicarboaluminate phases completely disappeared in carbonated LC3-45 after only 4 weeks of exposure.
Fig. 10 shows the comparison between the pH profiles obtained on LC3 and reference GPC concretes after 4 weeks of 1% accelerated carbonation. The minimum pH of carbonated LC3-
LC3-30 concrete, measured close to the surface, is slightly lower than that of GPC reference concrete. But, it is worth noticing that the pH of GPC-30 is initially (before carbonation) higher than that of LC3-30. As a result, the pH reduction due to carbonation of GPC concrete and LC3-30 is similar. However, the maximum pH reduction measured on LC3-45 is much higher than that of all other concretes. It is important to remind that the OPC content of LC3-
45 is only about 45% which can explain the poor resistance of LC3-45 against carbonation.
This higher pH reduction may impact the corrosion propagation phase as well. The influence of OPC replacement rate on steel reinforcement corrosion in carbonated LC3 concrete is currently under investigation at UNSW Australia. Fig. 11 shows the comparison between the pH profiles obtained on LC3 concretes after 30 weeks of natural carbonation. Results confirm the poor performance of LC3-45 compared to LC3-30. The minimum pH measured close to the surface is again much lower for LC3-45 than for LC3-30.
Carbonation depth and carbonation rate
Figs. 12 and 13 show photos of the phenolphthalein indicator test results obtained on concrete specimens exposed to 1% CO
2
for 4 weeks and 8 weeks respectively. All carbonation depths deduced from phenolphthalein indicator test together with the scatters are reported in Fig. 14. The carbonation depths reported are the average values of 15 to 20 measurements at different locations at least 10mm away from the coated side surface of the specimens.
Overall, results confirm that the resistance of LC3 concrete against carbonation is reducing when increasing the GP cement substitution rate. Shi et al (Shi et al. 2016) reported similar results on LC3 mortar. Resistance against carbonation is not entirely governed by the pore structure of the concrete which is refined in LC3 concrete compared to reference GPC-30
(Figure 4). The chemical interactions between diffusing CO
2
and the matrix greatly influence carbonation. Portlandite content is significantly reduced in LC3 concrete because of the lower quantity of OPC in the mix and the consumption of Portlandite by the pozzolanic reactions.
As a result, the extent of chemical interactions binding CO
2
is reduced compared to GP
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10.1680/jadcr.18.00172 cement concrete leading to an increase in CO
2
diffusion. For LC3-30 with 60% OPC content, the increase in carbonation depth compared to GPC-30 is 17% and 30% after 4 weeks and
8weeks of accelerated carbonation respectively. For LC3-45 with only 45% OPC content, the increase in carbonation depth compared to GPC-30 is 70% after both 4 weeks and 8weeks of accelerated carbonation respectively.
Phenolphthalein indicator test results are consistent with pH profile results, offering similar trends. Based on the results obtained after 8 weeks of exposure to 1% CO
2
, phenolphthalein indicator seems to slightly overestimate the average carbonation depths deduced from pH profiles by 2 to 3mm. Therefore, phenolphthalein indicator test is a reliable tool to conservatively assess the carbonation depth of LC3 concrete.
In natural condition, only LC3-45 carbonation depth is reported in Fig. 14. For LC3-30 and
GPC-30, only few millimetres of slightly faded purple colour was observed which could not be considered as fully carbonated regions. This agrees with pH profiles reported in Fig.
11showing that LC3-30pH remains superior to 11.5. Carbonation results obtained in natural condition confirm the vulnerability of LC3-45 to carbonation.
Fig. 15 shows the carbonation depth of concrete versus the square root of the exposure time
(in weeks) to 1% CO
2
. A linear correlation between carbonation depth and the square root of the exposure time is usually assumed for Portland cement-based concrete. Results confirm that there is no difference for LC3 concrete.
The concrete pH profiles, carbonation depth and carbonation rate are inconsistent with MIP results which presented a reduction in coarse pores leading to a refined pore structure and higher degree of capillary condensation (Shi et al. 2016). The high carbonation rate of LC3 concretes in this study can be explained by the increase of porosity during the carbonation process. Previous studies using MIP technique indicated that LC3 concretes obtained an increase in the mercury intruded porosity after exposure to carbonation whereas OPC system presented an overall reduction in the porosity (Shi et al. 2016; Shah et al. 2018). The dissimilarity in the development of total porosity can be attributed to the different amount of
Portlandite in different system. Calcium carbonate is the precipitation product of Portlandite after carbonation, which can fill in the empty pore of microstructure due to higher solid volume than Portlandite and lead to a reduction in total porosity of plain GP cement concrete.
However, LC3-based system showed the considerably low amount of Portlandite in
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10.1680/jadcr.18.00172 comparison with GPC-30, which is presented in previous study using the same mix design
(Nguyen et al. 2018). Due to the lack of calcium hydroxide, an extensive carbonation of C-S-
H or other hydration phases (ettringite, hemicarboaluminate, etc.) could occur in LC3 concrete and result in an increase in porosity and higher carbonation rate.
In environmental condition where carbonation induced reinforcement corrosion is governing the design for durability (i.e. outdoor exposure involving no chloride or other aggressive species), standard specifications such as Australian Standards are usually about 30MPa characteristic compressive strength and 30mm concrete cover. Based on this assumption, the time required for the carbonation front to reach the reinforcement can be extrapolated from
Figure 15 linear correlation curves in accelerated conditions. For GPC-30, LC3-30 and LC3-
45, the time required for the carbonation front to reach the reinforcement would be about 88,
55 and 31 weeks respectively (i.e. 37% and 65% reduction for LC3-30 and LC3-45 respectively). GPC-30 is a reference concrete complying with most of current international
Standards for this type of exposure condition to protect steel reinforcement against carbonation induced corrosion for 50 years in natural condition. Assuming a similar reduction in time for steel depassivation in accelerated and natural conditions, the service life of the structure would be reduced to 32 years and 17 years for LC3-30 and LC3-45 respectively. A possible way to specify a 50 years life time for LC3-30 and LC3-45 concrete structure is to increase the concrete cover. Only 8mm increase would be enough for LC3-30. However,
21mm increase in concrete cover would be required for LC3-45.
This study shows that limestone and calcined clay blend used as simple substitution of
General Purpose cement in concrete should be able to provide adequate protection to steel reinforcement if the OPC content in the mix is at least 60% (30% GP cement substitution).
Only a marginal increase in concrete cover is required to achieve a similar service life as that of reference Portland cement-based concrete. For higher GP cement substitution, LC3 concrete is not suitable in exposure condition where carbonation induced steel reinforcement corrosion can be an issue. Interior exposure condition or permanently submerged condition could be more appropriate for reinforced concrete applications.
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Conclusions
In agreement with the literature, results show that the early age compressive strength is only marginally affected by the limestone and calcined clay substitution up to 45% and a significant refinement of the pore structure was observed in LC3 concrete compared to reference Portland cement concrete.
XRD results have shown that Portlandite, ettringite and hemicarboaluminate phases disappear under natural and 1% accelerated carbonation and calcite is the only carbonation product in both LC3 mixes.
The results have further shown that the resistance of LC3 concrete against carbonation reduces when increasing the GP cement substitution rate. For LC3-30, the increase in carbonation depth compared to reference concrete is 17% and 30% after 4 weeks and 8 weeks of accelerated carbonation respectively. For LC3-45, the increase in carbonation depth compared to reference concrete is 70% after both 4 weeks and 8 weeks of accelerated carbonation respectively, indicating a significant reduction in carbonation resistance which correlates well with the XRD results.
This study shows that only a marginal increase in concrete cover is required when using LC3-
30 concrete to achieve a similar service life as that of reference Portland cement-based concrete. For higher GP cement substitution, LC3 concrete is not suitable in exposure condition where carbonation induced steel reinforcement corrosion can be an issue. Interior exposure condition or permanently submerged condition could be more appropriate for reinforced concrete applications.
Acknowledgement
This research project was supported by the Australian Research Council through ARC
Discovery Project DP160104731.The experiments were conducted in the materials and structures laboratory in the School of Civil and Environmental Engineering at the University of New South Wales. The assistance of the laboratory staff is acknowledged here.
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AS 1012.17 (1997). Methods of testing concrete: Determination of the static chord modulus of elasticity and Poisson's ratio of concrete specimens. Standards-Australia. .
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Na
2
O
K
2
O
TiO
2
SO
3
Loss on ignition
(LOI)
Accepted manuscript doi:
10.1680/jadcr.18.00172
Table 1.
Chemical composition of cementitious materials
Chemical composition
SiO
2
GP cement
(wt.%)
19.74
Calcined clay
(wt.%)
70.42
Limestone
(wt.%)
0.36
Cement mineralogical composition
(wt.%)
C
3
S 51.2
Al
2
O
3
4.70 22.34 0.11 C
2
S 16.5
Fe
2
O
3
CaO
MgO
2.98
64.62
1.48
2.34
0.49
0.16
0.1
57.51
0.29
C
3
A
C
4
AF
Gypsum
6.5
7.3
0.9
0.21
0.64
0.31
2.24
3.18
0.1
0.19
1.1
0.02
1.76
-
-
-
-
42.61
Bassanite
Calcite
Amorphous
0.6
5.2
11.7
Table 2.
Characteristics of cementitious materials
Particle Size Distribution [µm]
Material
GP cement
Calcined clay
Limestone
D v10
1.32
2.50
7.74
D v50
8.48
21.19
43.35
D v90
25.26
64.75
130.46
Table 3.
Mineral composition of calcined clay determined by XRD-Rietveld analysis
Minerals
Quartz
Amorphous
[wt.%]
49.1
50.9
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Table 4.
Concrete mixes
Materials (kg/m
3
)
Coarse aggregate
Fine aggregate
Total binder
GP cement
Calcined clay
Limestone
Water
GPC-30
1265
545
380
380
0
0
210
LC3-30
1221
620.8
388
271.6
77.6
38.8
174.5
LC3-45
1221
620.8
388
213.4
116.4
58.2
174.5
Table 5.
Average compressive strength
Age (days)
7
14
28
Average compressive strength (MPa)
GPC-30 LC3-30
30.9
33.6
36.0
26.1
33.4
35.5
LC3-45
26.0
33.8
36.3
Advances in Cement Research

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Fig. 1. XRD patterns of metakaolin and limestone
Fig. 2. SEM images: (a) calcined clay (b) spherical particle of calcined clay and (c) limestone
Fig. 3. Particle size distribution of GP cement, calcined clay and limestone
Fig. 4: MIP resultfs for LC3 and GPC pastes
Fig. 5: XRD of LC3-30 pastes
Fig. 6: XRD of LC3-45 pastes
Fig. 7: pH profiles of uncarbonated LC3 concretes
Fig. 8: pH profiles of LC3-30 concrete exposed to 1% accelerated carbonation
Fig. 9: pH profiles of LC3-45 concrete exposed to 1% accelerated carbonation
Fig. 10: Comparison between pH profiles of LC3 and reference GPC concretes after 4 weeks of 1% accelerated carbonation
Fig. 11: Comparison between the pH profiles of LC3-30 and LC3-45 after 30 weeks of natural carbonation
Fig. 12: Phenolphthalein indicator test results after 4 weeks of 1% accelerated carbonation
Fig. 13: Phenolphthalein indicator test results after 8 weeks of 1% accelerated carbonation
Fig. 14: Carbonation depth of concrete
Fig. 15: Carbonation depth of concrete versus the square root of exposure time to 1% CO
2
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
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Advances in Cement Research
Accepted manuscript doi:
10.1680/jadcr.18.00172