Chemical mapping of cement pastes by using confocal Raman spectroscopy

Abstract

In the present study, chemical mapping on the hydration process of cement paste with water-to-cement (w/c) ratio of 0.60 was implemented by Raman spectroscopy (RS). The RS was applied to study the paste from 12 hours after mixing to 28 days. Cement ingredients and hydration products, including calcium silicates (C₃S and C₂S), calcium hydroxide (CH), and ettringite, were quantitatively studied. From the research, it is observed that calcium silicates were consumed gradually with the increased hydration age. Calcium hydroxide increased and tended to cluster in the pore vicinity during hydration. Ettringite was found to form on the surface of the unreacted particles, which was shown as a mixed zone of ettringite and calcium silicates in the maps. It is concluded from the study that chemical mapping was an effective method to assist in visualizing particle dispersion and connection on top of the quantitative analysis.

Keywords

chemical mapping / Raman spectroscopy / calcium silicates / calcium hydroxide / ettringite

Introduction

The development of concrete properties is determined by paste hydration. Traditional methods applied to study hydration process are either focused on the chemical changes or the morphology of components in the cement pastes, such as XRD and SEM [ 1– 4]. The information about the effects of chemical reactions on the distribution and connection of paste compound is missing, which is required for resolving the hydration mechanisms. In this study, Raman spectroscopy (RS) is used to chemically map the pastes to reveal the relationships between the hydration reactions and microstructural development of the cement paste.

In a Raman spectroscopy system, the sample is usually illuminated with a laser. The laser interacts with molecules in the sample, resulting in the energy of laser photons being shifted up or down. The shift in energy, called Raman shift (with a unit of cm^-1), provides information about the vibrational modes of a specific molecule. Because each molecule has its unique energy shifts, this allows chemical identification of compounds within a sample.

In cement chemistry, Raman spectroscopy has been used to characterize clinker phases and hydration products, including tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), tetracalcium aluminoferrite (C₄AF), gypsum (CaSO₄·2H₂O), calcium hydroxide (CH), calcium silicate hydrate (C-S-H) gel, ettringite (AFt), and monosulfate (AFm), etc [ 5– 10]. The hydration process of each individual clinker phase (such as C₃S and C₃A) has also been studied by this technique [ 11– 13]. Recent researches reported that RS can be used as an in situ and real-time method to study the hydration process of Portland cement pastes during the setting period [ 14, 15]. It was also found that the RS was able to detect the dehydration process of gypsum phases in cement manufacturing [ 16]. All these studies suggest that the RS can be a powerful technique in exploring the cement hydration mechanisms. However, little has been done on chemically mapping of the hydration progress. Mapping is a method that can simultaneously measure the chemical and microstructural aspects for cement hydration. The obtained chemical maps are able to provide the link between the chemical and microstructural evolutions.

In this research, the protocol of chemical mapping on cement paste was developed. Paste with w/c ratio of 0.60 was mapped at the ages from 12 hours after mixing to 28 days. The obtained chemical maps were used to quantify the fractional amount of calcium silicates (i.e., C₃S and C₂S) and CH in the pastes at each studied age. The distribution and connection of calcium silicates, CH, and ettringite at different hydration ages were also explored.

Experimental procedures

Materials and sample preparation

Cement used was Type I Portland cement. The chemical composition of the cement is listed in Table 1. The Blaine surface area of the cement particles was 400.8 m²/kg, and mean particle size was 13 µm. Cement paste with w/c ratio of 0.60 was properly mixed following ASTM C305 and transferred to sample holders with a size of around 20 mm × 20 mm × 10 mm. The sample holders were slightly tapped to remove the entrapped air and covered with layers of plastic sheets to prevent moisture loss. When hardened, the samples were demolded and cured under lime saturated water with a constant temperature of 22±2°C till the designated ages of 12 h, 3 days, 7 days and 28 days. At each age, 3 samples were studied. The hydration of each sample was stopped by using isopropanol and the sample surface was ground slightly right before carrying out the Raman chemical mapping measurement.

Raman spectroscopy experimental configuration

Raman spectroscopy used was a Renishaw Invia micro-Raman spectrometer with a CCD detector. Samples were illuminated with a 633 nm He-Ne laser. The laser power was about 4 mw on the sample surface. Each spectrum was acquired using the sync mode from 300 to 2000 cm^-1. All the spectra were obtained with a 50 × magnification objective. The laser spot had a diameter of 4 µm with this magnification. Before each test, calibrations were done by utilizing the 520.5 cm^-1 line of a silicon wafer. Data acquisitions were carried out by WiRE3.4 software.

For each paste, mapping was carried out on an area of 100 µm× 100 µm having a total of 625 points spreading in a grid matrix on the sample surface, shown as in Fig. 1. The number of points in the studied area was determined by the size of laser spot (4 µm). This is to ensure the coverage of the measured area. After setting up the measuring area, Raman spectrometer collected spectra sequentially from point 1 to point 625. Thus from one mapped area, there were 625 spectra acquired.

Data analysis

To construct a chemical map of cement paste, the composition of each measured point needs to be identified. This was achieved by assigning the peak for each individual paste component in the Raman spectrum, according to the results presented by the researchers [ 5– 16]. The characteristic peaks for the different compounds are listed in Table 2. One may notice that compared to other components, CH has a much broader peak range assigned at shifts from 1400 to 2000 cm^-1. This assignment was done by the researchers through the Raman measurement on the reagent-grade CH crystals [ 14, 15]. Figure 2 shows the spectrum of pure CH powder. It is apparent that the most intensive peaks of CH crystals are within the range of 1400 to 2000 cm^-1. From the pattern, another weak peak was also detected at 355 cm^-1. This peak was used to identify calcium hydroxide in the previous work [ 11]. Its drastically lowered intensity compared to the peaks at higher shifts (1400–2000 cm^-1) in the current study can be caused by the insensitivity of the corresponding molecular vibration to the 633 nm laser used. Therefore, in this study, those strong peaks at 1400–2000 cm^-1 were utilized to characterize CH in the cement paste.

For each assigned component peak, chemical map was constructed by applying the “Signal to Baseline” function embedded in the WiRE3.4 software. The underlying technique of this function is the comparison of the area under the assigned peak in all the patterns acquired from the measured area. Peak area is directly reflecting the concentration of the compound in the measured points. To distinguish different phases in the cement paste by mapping, each studied component was assigned with a different color. In each obtained chemical map, all the measured points had the assigned color with the brightness proportional to the respective peak area, even for those points that only had scattered intensity values rather than real peaks in the range of the assigned Raman shifts. These points were displayed in the chemical image with fairly low brightness. Therefore, color brightness threshold was set to remove these points from the chemical map, which classified each measured point into two categories: either it contained the compound or it did not. This means if a point generates no peak or only negligible peak for the studied compound, the point will be deemed not to contain that compound and its color will be white in the chemical map. On the contrary, the points having higher brightness than the defined threshold due to the presence of strong peak will be displayed as the assigned color with the maximum brightness in the map. Consequently, from the final processed chemical images, the presence of the chemical species in all the measured points can be identified, which can be further utilized to observe the distribution and connection of the components in the cement pastes.

Results and discussion

Examples of Raman pattern and chemical map for paste

Figure 3 shows an example of spectrum from the mapped area (Fig. 1) for the cement paste with w/c ratio of 0.60 at the hydration age of 12 h. For illustration purpose, this exemplified spectrum was background corrected by using the cubic spline interpolation method [ 15]. During map creation process, background correction was not required. This is because maps were created on the basis of peak area, which did not change whether the background was subtracted or not. According to Fig. 3, obvious peaks were found at the shift of 833, 887, 991, 1005, and 1400–2000 cm^-1, which were assigned to C₃S, C₂S, ettringite, gypsum, and CH, respectively (Table 2). In the present study, chemical maps on calcium silicates (C₃S+ C₂S), ettringite, and CH were constructed separately. The gained single component maps were also merged to form composite maps. An example of the obtained composite map is shown as in Fig. 4. In this figure, the colors yellow, green, and blue are representing calcium silicates, ettringite, and CH, respectively. It can be seen from this map that the colors were overlapped with each other at some measured points. This is because the spectra from these points showed the simultaneous presence of multiple components (e.g., Figure 3), instead of only one single phase. The details of mapping for each phase are discussed in the following sections.

Chemical maps for C₃S+ C₂S

The maps for each single component were studied first. Figure 5 shows the chemical maps for calcium silicates from 12 hours to 28 days. The detailed area fractions of the silicates at the different ages are illustrated in Fig. 6. It can be seen from both figures that calcium silicates decreased consistently with the increased age. According to the curve (Fig. 6), the fraction was found to decrease more rapidly at the first few days. Then this decreasing rate slowed down and the silicates content only decreased slightly until 28 days. The maps also indicated that from 12 hours to 28 days, there were less and less small cement particles. This can be observed by comparing the number of small spot (4 µm) at each specific age. This complies with the widely accepted concept that smaller particles have higher hydration rate than bigger particles.

Chemical maps for CH

For the same paste, the maps for calcium hydroxide are shown in Fig. 7. The average area fractions of CH calculated from the chemical maps are displayed in Fig. 8. According to these figures, CH content was found to increase with the hydration progress. The rapid increase of CH fraction at the first few days was observed from the fraction curve, which is corresponding to the faster consumption of calcium silicates during this period (Fig. 6). Thereafter, the increasing rate of CH slowed down and its content only increased lightly until 28 days. It is also noticeable from the chemical maps that CH particles were smaller and more dispersive at the early age. They became bigger and more connected with each other at later ages.

Composite maps for calcium silicates, CH, and ettringite

Composite maps were utilized to investigate the distribution and connection of the components in the paste microstructure. Figure 9 shows the composite maps obtained for cement paste at the hydration age of 12 hours to 28 days. It can be seen from the maps that the fraction of ettringite (green color) is much less than calcium silicates (yellow color) and CH (blue color) at all the ages. The majority of ettringite formed on the surface of calcium silicates, which is shown as an overlapped zone of colors yellow and green on all the maps. On the contrary, there are very few overlapped areas between CH and calcium silicates. CH was mainly produced at the locations away from unreacted cement particles as big clusters (sometimes bigger than 20 µm). Thus it is postulated that CH was more likely to precipitate in the pores rather than on the surface of cement grains.

Conclusions

In this work, Raman spectroscopy was used to chemically map the hydration process of cement pastes with w/c ratio of 0.60 from 12 hours to 28 days. It was found that the content of calcium silicates decreased consistently during the measuring period, while CH content kept increasing and precipitated as big particles in the pores. The chemical maps also showed that ettringite had much less area fraction than calcium silicates and CH, and it mainly formed on the surface of unreacted cement particles. Future work will be focused on the systematic study on chemical maps of cement pastes with various w/c ratios, mainly including the quantitative area fraction analysis and comparison between the chemical maps and SEM analysis.

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