Ageing of Portland Cement Concrete Cured under Moist Conditions
Zhuing Yu1*, Guang Ye1, 2, Klaas van Breugel1, Wei Chen3(1) Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands
(2) Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Ghent, Belgium
(3) School of materials science and engineering, Wuhan University of Technology, P.R. China
Abstract:
Deterioration of microstructure in cement concrete will cause changes in the transport properties of the concrete. Transport properties at different ages of the concrete provide information about the microstructural changes of the material. A way to measure the transport properties, i.e. the chloride diffusion coefficient, in the laboratory is rapid chloride migration (RCM) Test. With this test, the resistance of concrete to chloride ingress is evaluated. Changing losing resistance represents deterioration and ageing of concrete. In this study, RCM tests were carried out for Portland cement concrete cured in moist conditions at various curing ages up to 3 years. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis were performed to identify the reaction product, like delayed ettringite. The experimental results show that the resistance of Portland cement concrete to chloride ingress decreases when ettringite is formed in concrete. Based on the result it is concluded that delayed ettringite formation contributes to the deterioration of Portland cement concrete, and should thus be considered as a relevant factor in the ageing process of concrete structures at moist curing condition.
Keywords: Ageing of concrete; DEF; Portland cement concrete; RCM Test;
1 Introduction
Ageing of concrete leads to the deterioration in its properties (a reduced capacity to withstand loads, increased permeability to water, decreased resistance to chemical attack, etc.) and the reduction of service life of concrete structure. There are various factors resulting in the ageing of concrete, e.g. thermal effects, efflorescence, chemical attack and fatigue. Curing of concrete is a pre-requisite for the hydration of the cement in concrete [Jackson 1996]. To ensure the continuous hydration process, the concrete is normally required to be cured under appropriate temperature and moist conditions. A proper curing greatly contributes to reduce the porosity and cracking of concrete due to shrinkage, and thus to achieve high strength [Neville 1995] and good resistance to physical or chemical attacks in aggressive environments [Safiudeen 2007]. In theory, the resistance of Portland cement concrete to chloride ingress increases with the increase of curing age under most conditions. However, after a certain curing ages (months), the resistance of Portland cement concrete to chloride ingress turns to decrease [Tang 1996; Obla 2003; van Dalen 2005; Audenaert 2007; Gailius 2008; Yu 2013]. This phenomenon did not cause its attention by researchers. In order to investigate the long-term performance of Portland cement concrete, it is necessary to explore the possible reasons why the chloride resistance of Portland cement concrete decreases at later curing age. In a previous study [Yu, et al 2013], some aspects, like leaching of calcium hydroxide, delayed ettringite formation (DEF), and transformation of low density calcium silicate hydrate (CSH) into high density CSH were investigated. It was suspected that the DEF in Portland cement concrete is the most possible reason which makes Portland cement concrete deteriorating when exposed to moist conditions.
*Corresponding author: Zhuqing Yu, PhD student, Delft University of Technology;
In this study, a further investigation is carried out to explore the DEF occurring in Portland cement concrete. The rapid chloride migration (RCM) tests are performed and the chloride migration coefficient (DRCM) of the Portland cement concrete is determined at different curing ages up to 3 years. Two techniques are used to identify the formation of ettringite, viz, scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis.
2 Materials and Test Methods
2.1 Materials
Portland cement (CEM I 42. 5 N), aggregate, and tap water were used in preparing concrete mixtures. Graded river sands with a maximum grain size of 4 mm and gravel with a 16 mm maximum grain size are used as fine and coarse aggregates, respectively. The chemical compositions of the Portland cement are shown in Table 1.
Table 1 Chemical compositions of the Portland cements used
Chemical composition
(% by mass) SiO2 Al2O3 CaO Free-CaO Fe2O3 P2O5 K2O MgO SO3 Na2O Calcite CEM I 42.5 N 20.36 4.96 64.4 0.6 3.17 0.18 0.64 2.09 2.57 0.14 1.45
2.2 Mix proportions
Portland cement concrete was casted with three water/cement (w/c) ratios, which were 0.4, 0.5, and 0.6. A more detailed description of mixture proportions is given in Table 2.
Table 2 Mixture proportions of Portland cement concrete (kg/m3)
Mixture CEM I 42. 5N Tap water Masonry sand 0/2 Sand 0/4 Gravel 4/16
OPC04 390 156 178 498 1103
OPC05 390 195 178 498 1103
OPC06 390 234 178 498 1103
2.3 Test methods and sample preparation
The rapid chloride migration (RCM) test was conducted to determine the chloride migration coefficient of Portland cement concrete at different curing ages. The concrete specimens were cast in a cylindrical mold with a diameter of 100 mm and height of 300 mm after mixing for 2 minutes. After 24 hours, the specimens were demoulded and immediately cured in a lime saturated bath (100% relative humidity and 20°C ± 1°C temperature). At the curing age of 28 days, 91 days, 180 days, 1 year, 2 years and 3 years, three slices with the diameter of 100 mm and a thickness of 50 mm were sawed perpendicularly to the axis of the cylindrical concrete specimen. According to the NT Build 492 standard [NT Build 492], a corrected voltage and appropriate test duration were chosen. After the migration test, all specimens were split and sprayed with 0.1 M silver nitrate (AgNO3) solution to determine the penetration depth of chloride. The average penetration depth, Xd of chloride was calculated and the non-steady-state
X-ray diffraction (XRD) analysis was used to identify the hydration products of Portland cement concrete. XRD measurements were carried out on a Philips X’ pert diffract meter system with Cu/ȽǤ ͷͲιȋʹɅȌǡͲǤͲʹιȋʹɅȌ dwell time of 2 s per step. In the procedure of the sample preparation for XRD testing, the specimens were frozen by immersion into liquid nitrogen for 5 minutes in order to stop hydration. Afterwards, the specimens were then dried in a freeze-dryer until a constant weight loss (0.05%) was reached. The dried specimens were gently ground by pestle until the particle size was smaller than 125 μm.
Scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDS) technique was used to capture secondary electron (SE) images, and to determine the chemical composition of the concrete. The concrete samples were split into several small pieces by hammer and dried in a vacuum machine until a constant weight loss (0.05%) was reached. After drying, carbon coating was applied on the surface of samples to create a conductive layer in order to inhibit charging and improve the SE signal.
3 Experimental results
3.1 Chloride migration coefficient of Portland cement concrete
Figure 1 shows the chloride migration coefficient (DRCM) of Portland cement concrete with three different w/c ratios over 3 years of curing. As can be seen from Figure 1, the w/c ratio has a clear influence on the DRCM which rises from initially (28 days) 13×10-12 m2/s to 28 ×10-12 m2/s with increasing w/c ratio. At each curing age, high w/c ratio exhibits negative effects on the resistance of Portland cement concrete to chloride ingress.
However, the values of DRCM for each type of Portland cement concrete are fluctuating in this curing period. In Figure 1, with the w/c ratio of 0.4 or 0.5, the DRCM decreases gradually over the curing age from 28 days to 180 days, and then increases to a peak value around 1 year followed by a further decline. With a high w/c ratio of 0.6, likewise, an initial ongoing decrease in DRCM is observed from 28 days to 90 days. Afterwards, the DRCM keeps an upward trend up to the curing age of 3 years. After 3 years, the DRCM of Portland cement concrete with the w/c ratio of 0.6 appears to get downtrend slowly. The development of the DRCM of Portland cement concrete with time illustrated in Figure 1 has a similar trend with the experimental results presented in other researches [Tang 1996; Obla 2003; van Dalen 2005; Gailius 2008; Audenaert 2007].
3.2 XRD results of Portland cement concrete
Figure 2 shows the XRD patterns of three samples of Portland cement concrete (w/c=0.4, 215 days; w/c=0.4, 2 years; w/c=0.6, 2 years). It is clear that except quartz (SiO2), portlandite (CH), calcite (CaCO3), and monocarboaluminate (Mc) are identified, the peak of ettringite is also detected in the XRD patterns of the Portland cement concrete with different w/c ratios at curing age of 2 years. However, the peaks of ettringite and monocarboaluminate are not found in XRD pattern of the sample at curing age of 215 days. It indicates that the formation of ettringite is a slow process in Portland cement concrete.
E: Ettringite; Q: SiO2; CH: Ca (OH) 2; CO: CaCO3; Mc: Monocarboaluminate
Figure 2 XRD patterns of Portland cement concrete
3.3 SE images of Portland cement concrete
Figure 3 shows the SE image of Portland cement concrete at curing age of 2 years. The needle-shaped crystals are widely recognized as ettringite. An EDS analysis is made at the spot of red square. The EDS spectrum with element characteristic peaks is shown in Figure 4. The molar ratio of Al: S is around 1.38:1, which is in the range from 0.67 (molar ratio of Al: S of ettringite) to 2 (molar ratio of Al: S of monosulfatealuminate, AFm). It indicates that the selected area probably includes ettringite and AFm together. Furthermore, micro-cracks with a width of 3-5 μm are also observed in the same concrete sample as shown in Figure 3. Ettringite and micro-cracks are also clearly evidenced in the Images collected from other sample with w/c=0.6 at curing age of 3 years.
Molar ratio: Al : S=1.38:1 39.1 0.24 0.44 K 55.85 0.60 1.56 Fe 24.31 0.48 0.55 Mg 26.98 1.83 2.30 Al 40.08 18.67 34.97 Ca 32.07 1.32 1.98 S 28.09 7.64 10.03 Si 16.00 49.98 37.37 O 12.01 19.25 10.80 C Atomic mass At % Wt % Element Molar ratio: Al : S=1.38:1 39.1 0.24 0.44 K 55.85 0.60 1.56 Fe 24.31 0.48 0.55 Mg 26.98 1.83 2.30 Al 40.08 18.67 34.97 Ca 32.07 1.32 1.98 S 28.09 7.64 10.03 Si 16.00 49.98 37.37 O 12.01 19.25 10.80 C Atomic mass At % Wt % Element
Figure 5 SE images of Portland cement concrete (w/c=0.6 at the curing age of 3 years) A: 1000×; B: 2000×; C:
2000×; D: 1000×
4 Discussions
Based on the results of RCM test (see Figure 1), it is clear that the chloride migration coefficient of the Portland cement concrete turns to increase around 180 days, which indicates a decreased resistance to chloride ingress. The change of the resistance to chloride ingress is highly associated with the microstructural development of Portland cement concrete.
Delayed ettringite formation (DEF) is defined as the delayed formation of mineral ettringite, which is a normal product of early hydration of cement, within cement paste system of hardened concrete [Hime 1996]. It is related to heterogeneous expansion and manifests cracks and loss of strength of concrete. The DEF increases the risk of secondary forms of deterioration, such as chloride ingress, and impairs the long-term performance of concrete. From the experimental results presented in Figure 2 to Figure 5, it can be seen that the DEF occurs in the Portland cement concrete accompanying with micro-crack initiation when the concrete is cured under moist conditions.
Normally, the formation of delayed ettringite requires a new source of sulfate ions in the pore solution, 1) external sulfate (environmental sulfate from water or soil); 2) internal sulfate (from
steam curing, and forms again when the concrete is then kept wet or moisture [Fu 1994; Collepardi 2001]. In this research, the samples of Portland cement concrete were cured in moist conditions, viz. sulfate-free environment and no steam curing. The sulfate sources related to “external sulfate” and “thermal decomposition of ettringite” are excluded.
Except the presence of sulfate ions, there are other two factors will lead to the formation of delayed ettringite in concrete, 1) in wet (moist) curing condition; 2) the presence of limestone powder (CaCO3) in concrete [Klemm 1990]. Under the moist conditions, CaCO3 can slowly dissolve and subsequently react with any calcium monosulfoaluminate hydrate or calcium aluminate hydrate to form AFm carboaluminate phase, which is a more stable product due to its higher insolubility (see Table 3). The chemical reaction of CaCO3 is shown as Equation (1) [Klemm 1990; Taylor 1997]. As illustrated in Table 1, a small amount of CaCO3 was blended into cement clinker as the minor additional constituent. The peaks of ettringite and monocarbonate (Mc) presented in Figure 2 confirm the occurrence of the Equation (1). Furthermore, from a thermodynamic point of view, the reaction of Equation (1) can happen easily since ettringite is more stable than monosulfoaluminate (AFm) due to the lower solubility of ettringite (see Table 3).
2 3 4 2 3 2 2 3 3 2 2 3 4 2 3 3 12 ( ) 2 18 2 3 11 ( ) 3 3 32 ( )
CaO Al O CaSO H O AFm CaCO H O CaO Al O CaCO H O Mc
CaO Al O CaSO H O Ettringite
o
(1)
Table 3 Solubility products of Calcite, AFm, AFt and Ettringite [Zhang 1980]
Salt Solubility products at 25 °C
Calcite CaCO3 8.7 ×10
-9
Monosulfoaluminate (AFm)
3CaO Al O 2 3
CaSO412H O2 1.7 ×10 -28
Monocarboaluminate (Mc)
3CaO Al O 2 3
CaCO311H O2 1.4 ×10-30 Ettringite (AFt)
3CaO Al O 2 3
3CaSO432H O2 1.1 ×10
-40
5 Conclusions
In this paper, based on the experimental results it is proved that the delayed ettringite formation occurs in Portland cement concrete when limestone powder is presented and the concrete is cured under moist conditions. The DEF results in the change of microstructure of concrete and increase of DRCM at later curing age.
6 Acknowledgement
We hereby express our gratitude to our technicians and colleagues at the Microlab, at Delft Technical University and financial support from China Scholarship Council (CSC).
7 References
[1] Neil Jackson, Ravindra K. Dhir (1996) Civil Engineering Materials. Fifth Edition. Published by Palgrave Macmillan, Basingstoke, Hampshire. 534 pages, ISBN-13: 978-0333636831.
[2] Safiuddin Md, Raman S.N. Zain M.F.M (2007) Effect of different Curing Methods on the Properties of Micro Silica Concrete. Australian Journal of Basic and Applied Science, 1 (2): 87-95,
[3] Neville, A. M. (1995) Properties of concrete, 4th and final Ed., Longman’s, London.
[4] Tang, L. (1996) Electrically accelerated methods for determining chloride diffusivity in concrete, Magazine of Concrete Research. 48(176):173-179.
[5] Karthik H. Obla, Russell L. Hill, Michael D. A. Thomas, Surali G. Shashiprakash, Olga Perebatova. (2003) Properties of Concrete Containing Ultra-Fine Fly Ash, ACI Materials Journal. September-October 426-433.
[6] Sander M. van Dalen (2005) Experimenteel onderzoek naar de RCM-methode (In Dutch). Master Thesis. Delft University of Technology.
[7] Katrien Audenaert, Veerle Boel, Geert De Schutter (2007) Chloride migration in self compacting concrete, in F. Toutlemonde et al. (eds.) CONSEC’07 Tours, France Concrete under Severe Conditions : Environment & Loading.
[8] Albinas Gailius, Marta Kosior-Kazberuk (2008) Monitoring of Concrete Resistance to Chloride Penetration, Materials Science (MEDŽIAGOTYRA). Vol. 14, No. 4.
[9] Zhuqing Yu, Guang Ye, Martin Hunger, Reinier van Noort (2013) Discussion of the evolution of the chloride migration coefficient of Portland cement concrete tested by the Rapid Chloride Migration (RCM) test at long-term curing periods up to 5 years. CONSEC'13 - 7th International Conference on Concrete under Severe Conditions – Environment and Loading. 23 - 25 September 2013, Nanjing.
[10] NT Build 492 (1999) “Concrete, mortar and cement-based repair materials: chloride migration coefficient from non-steady-state migration experiments”. UDC 691.32/691.53/691.54. Approved 1999-11.
[11] William G. Hime (1996) Delayed Ettringite formation-a concern for precast concrete, PCI Journal. 26-30. [12] FU, Y. et al. (1994) 6LJQL¿FDQFH RI pre-existing cracks on nucleation of secondary ettringite in Steam
Cured Past. Cement and Concrete Research, vol. 24, pp. 1015/1024.
[13] M. Collepardi. (2001) Ettringite Formation and Sulfate Attack on Concrete. ACI special publication. 2001 ;200 : 21-38.
[14] Klemm, Waldemar A., Adams, Lawrence D. (1990) An Investigation of the Formation of Carboaluminates, in P. Klieger and R. D. Hooton (Eds.) published in Carbonate Additions to Cement, ASTM STP 1064, American Society for Testing and Materials, Philadelphia, 1990, pp. 60-72.
[15] H. F. W. Taylor (1997) Cement Chemistry, Academic Press, London.
[16] Zhang, F., Zhou, Z., and Lou, Z., Solubility product and stability of Ettringite. Proceedings, Seventh international congress on the chemistry of cement. Vol. II, Paris, 1980, 88-93.
