Carbonation front in cement paste detected by T 2 NMR measurements using a low field unilateral magnet
- 285 Downloads
Unilateral magnetic resonance was used to obtain the CPMG T 2 decay at different positions along 6 cm long cement paste samples. The aim was to detect the carbonation front based on changes in the pore structure caused by accelerated carbonation and to compare the results with the phenolphthalein test. Cement pastes at water-to-cement ratios of 0.60, 0.50, and 0.40 were prepared using ordinary Portland cement. After moist curing and conditioning at 65% RH and 35 °C, the pastes were subjected to accelerated carbonation with 4% by volume CO2 at 65% RH and 35 °C. Carbonation increases the magnetic resonance T 2 lifetime compared with the control and compared with the noncarbonated region of the samples. A reduction in porosity and changes in the pore size distribution are caused by carbonation. The position with the highest rate of T 2 change was consistent with the fully carbonated front determined by the phenolphthalein test.
KeywordsCarbonation Magnetic resonance Cement paste T2 relaxation time Pore size distribution BET
Carbonation of concrete results from the chemical reaction of CO2 with calcium hydroxide (CH) and calcium silicate hydrate (CSH), the main cement hydration products. Carbonation is a surface driven process since CO2 in air diffuses into the concrete cover. Carbonation reduces the pH of the pore solution from 12.6–13.5 to <9 . This negatively affects the protection of the reinforcing steel provided by a thin passivating layer of oxide formed on the steel surface.
Carbonation also reduces the total porosity of ordinary Portland cement (OPC) and alters the pore size distribution. The volume of calcium carbonate formed is 11–12% greater than the specific volume of calcium hydroxide. A reduction in total porosity increases the strength and brittleness of concrete . Ngala and Page  found that upon carbonation the proportion of large pores (diameter > 30 nm) increased only slightly for OPC, but was more significant for pastes containing fly ash and slag. In these blended cement pastes where CH is consumed by pozzolanic reactions, carbonation of CSH plays a significant role. Carbonation affects the CSH even at low CO2 concentrations by producing a porous silica gel with a very coarse pore structure. The silica gel pores are approximately 300 nm in radius . Therefore, the combined effect of carbonation of CH and CSH is a reduction of total porosity and coarsening of the pore structure .
The principal technique used to determine the carbonation depth, in the laboratory or in the field, is to spray a phenolphthalein solution over a freshly broken concrete surface . Phenolphthalein, a pH indicator, marks the depth of carbonation through a color change. However, since the colour change occurs at pH = 8.6, it underestimates the carbonation depth. Previous research using thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) has identified three spatial regions associated with carbonation, carbonated, transition or partially carbonated, and noncarbonated. It has been suggested that the fully carbonated depth (indicated by the phenolphthalein technique) is approximately half the depth of the leading carbonation front (fully + partially carbonated zones) measured by TGA, XRD or FTIR .
In this paper T 2 NMR relaxation times are measured with a three-magnet array unilateral NMR device to detect changes in porosity, pore size distribution, and the carbonation depth penetration in cement paste samples subjected to accelerated carbonation in a 4% by volume CO2 environment at 65% RH and 35 °C. A comparison is given to results with the carbonation front detected with the phenolphthalein test. The T 2 lifetime distribution, which is a proxy for the pore size distribution, is also compared to BET pore size distribution measurements.
2 Materials and method
Cement chemical composition
The CPMG decays at carbonated (1 cm) and noncarbonated (5 cm) regions of the vacuum saturated samples were processed by Inverse Laplace Transformation (ILT) to obtain the T 2 distribution, which is considered a proxy for the pore size distribution.
After testing, the samples were fractured in the longitudinal direction and the freshly exposed surface was immediately sprayed with a 1% phenolphthalein solution to estimate the carbonation depth . Samples from one specimen w/c = 0.60 and one with w/c = 0.50 at carbonated and noncarbonated regions were taken for N2 BET analysis to obtain the pore size distribution and the pore surface area. The samples were dried for 24 h at 105 °C to eliminate the evaporable water. N2 adsorption isotherms of the samples were measured using a Belsorp-Max instrument (BEL JAPAN, INC., Osaka, Japan). The adsorption isotherms were measured at 77 K in the relative pressure range p/p o = 10−8–0.997. Carbonated and noncarbonated vacuum-saturated samples were measured to determine porosity on a volume basis.
3 Results and discussion
Figure 2b shows that after vacuum saturating the sample, which precludes a time-based study, there is a major change in the CPMG decays at the same two positions as reported in Fig. 2a. As expected, the SNR (>200) increases for the same acquisition parameters due to the higher water content. The CPMG measurements now reveal a bi-exponential behavior dominated by the short T 2 lifetime component, which increases in duration in the carbonated region (1 cm) compared to the T 2 lifetime in the noncarbonated region (5 cm). This suggests an increase in pore size of the hydrated cement paste [3, 4] and/or changes in the nature of the pore surface relaxivity .
Changes in the pore space during cement hydration , micro-cracking , and autogenous self-healing of cracks , are reflected in T 1 and T 2 relaxation times. Since carbonation changes the surface chemistry of the cement paste, the water molecules are now in contact with calcium carbonate particles that may not be as efficient in terms of relaxation as the original noncarbonated surface. The original surface has paramagnetic impurities (Fe2O3) which enhance relaxation. Dalas et al.  obtained values of the surface relaxivity (ρ 2) of 2.74 and 5.51 μm/s for synthetic calcite and C–S–H, respectively. These values suggest that in our case the observed change in T 2 lifetime may be affected not only by changes in pore size but also by changes in the surface relaxivity. The phenolphthalein test performed after the samples with w/c ratio of 0.60 were characterized by NMR, indicated an average carbonation depth of 2.5 cm. This is in agreement with the observed T 2 lifetime changes in Fig. 3a. These changes are the greatest between 2 and 3 cm, which is a transition zone between the carbonated and noncarbonated regions.
The main advantage of NMR for studying carbonation of cement-based materials compared to traditional techniques is that it is non-destructive. Samples may be studied by bulk or spatially resolved measurements. In this study, limited spatial resolution of the unilateral magnet (1 cm) means it is difficult to determine the actual shape of the carbonation front for the cement pastes with w/c ratios of 0.50 and 0.40. Improving the resolution to less than 1 cm would be possible by reducing the size or modifying the shape of the surface coil. Other alternative is by reducing the width of the sensitive spot produced by the three-magnet array. If resolution becomes higher, it will be possible to detect more precisely the carbonation front in low permeability materials (i.e. low water to cement ratio). For paste with a w/c of 0.60, as mentioned, there is a transition zone that is in agreement with the results obtained by Chang and Chen  using TGA, XRD, and FTIR. Their investigations allowed identification of three zones: fully carbonated, partially carbonated, and noncarbonated. They found the fully + partially carbonated zones extended approximately twice the depth indicated by the phenolphthalein test.
The initial CPMG signal intensity is proportional to the amount of water in the sample. Figure 3 also presents the signal intensity at various points along the sample. Contrary to the behavior observed in the T 2 lifetime, the signal intensity (water content) in the carbonated zone is lower than the signal in the noncarbonated zone. This means a lower porosity of the cement paste caused by carbonation, as expected. In ordinary Portland cement pastes, carbonation reduces the total porosity by less than 10% while slightly increasing the proportion of large pores (diameter > 30 nm) . The latter partially supports the increase in T 2 in the carbonated region observed in Fig. 3. It is known that carbonation not only affects the calcium hydroxide in the hydrated cement paste, but it also affects the CSH. A silica gel with a coarser pore structure, pores approximately 300 nm in radius, is produced when the CSH undergoes carbonation . For the three w/c ratios, the CPMG signal intensity in the noncarbonated region is similar to the signal intensity of the control samples.
N2 BET parameters and porosity on a volume basis for carbonated and noncarbonated cement pastes
Mean pore radius (nm)
Surface area (m2/g)
Pore volume (cm3/g)
Conditioning samples at 65% RH did not provide sufficient NMR signal to detect changes in T 2 lifetime. However, NMR measurements on vacuum saturated samples reveal significant changes in the porosity and the pore size distribution in the carbonated regions of the samples. The observed changes included an increase of T 2 lifetimes and decrease in NMR signal intensity in the carbonated region. The position with the highest rate of T 2 lifetime change is in agreement with the carbonation front indicated by the phenolphthalein test. The T 2 distributions indicated both pore size refinement due to precipitation of CaCO3 and pore size increase due to porous silica gel formation by carbonation of CSH. The BET results did not agree with the literature nor with the NMR measurements, probably because of the damage cause by oven drying the samples. Unilateral NMR can be used in the laboratory to non-destructively study the carbonation process of cement pastes.
5 Future research
Extension of the measurements to mortars and concrete subject to accelerated and natural carbonation will be pursued in the future.
P. Cano acknowledges Consejo Nacional de Ciencia y Tecnologia of Mexico (Grant Number 257348) and Instituto Politecnico Nacional of Mexico for the financial support for his sabbatical year at the UNB MRI Centre. B. J. Balcom thanks NSERC of Canada for a Discovery grant (Grant Number RGPIN-2015-06122) and the Canada Chairs program for a Research Chair in MRI of Materials (Grant Number 950-230894). P. Cano also acknowledges his students and colleagues from CIIDIR Unidad Oaxaca for preparing and carbonating the cement paste samples used for this research.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 1.Neville AM (1995) Properties of concrete, 4th edn. Longman Group, Harlow EssexGoogle Scholar
- 4.Bier TA, Kropp J, Hilsdorf HK (1989) The formation of silica gel during carbonation of cementitious systems containing slag cements. In: Proceedings of the 3rd international conference on the use of fly ash, silica fume, slag, and natural pozzolans in concrete, Trondheim, pp 1413–1428Google Scholar
- 13.Garcia-Naranjo JC (2012) Three-magnet arrays for unilateral magnetic resonance. PhD Thesis, University of New BrunswickGoogle Scholar
- 14.García-Naranjo JC, Guo P, Marica F et al (2014) Magnetic resonance core-plug analysis with the three-magnet array unilateral magnet. Petrophysics 55:229–239Google Scholar
- 20.Coates GR, Xiao L, Prammer MG (1999) NMR logging principles & applications. Halliburton Energy Services, HoustonGoogle Scholar
- 22.ASTM C 642-13 (2013) Standard test method for density, absorption, and voids in hardened concrete. ASTM International, West ConshohockenGoogle Scholar
- 23.Bu Y, Spragg R, Weiss WJ (2014) Comparison of the pore volume in concrete as determined using ASTM C642 and vacuum saturation. Adv Civ Eng Mater 3:308–315Google Scholar
- 25.Bentz DP (2005) Curing with admixtures beyond drying shrinkage reduction. Concr Int 27:55–60Google Scholar
- 30.Thiery M, Faure P, Morandeau A et al (2011) Effect of carbonation on the microstructure and the moisture properties of cement-based materials. In: De Freitas VP, Corvacho H, Lacasse MA (eds) XII DBMC international conference on durability of building materials and components. FEUP Edicoes, Porto, pp 1–8Google Scholar
This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.