Sensing pH Changes in Cement Hydration Processes

The pore solution, or condition of the liquid phase in cementitious systems, is of special relevance in gaining a thorough knowledge of the dissolution-precipitation mechanisms that drive hydration reactions. Apart from the concentration of many ions (e.g. Ca²⁺, Si_aq, Al_aq, SO₄^2-, K⁺, Na⁺), the pH value of the pore solution impacts the degree of saturation and stability of various phases and hydration products.

Image Credit: Sidorov_Ruslan/Shutterstock.com

Concrete admixtures such as retarders, superplasticizers, and setting accelerators can influence the pH development in cementitious systems. These are often utilized in wet shotcrete applications to produce quick setting and high early strength.

Alkali-free setting accelerators based on Al₂(SO₄)₃ are frequently employed due to their low influence on final strength and excellent safety profile. The fundamental mechanism of Al₂(SO₄)₃ accelerators is the quick and vast production of ettringite following their intermixture. As a result, the pore solution’s chemical composition and pH value are subject to fast changes once the accelerator is added.

Al³⁺-ions (liberated from the accelerator) are instantly transformed into Al(OH₄)^- and Al(OH)₃ aquo-complexes in the highly alkaline pore solution. Here, they combine with SO₄^2- (from the accelerator as well as the pore solution) and Ca²⁺ (from the pore solution) to generate ettringite. The pore solution’s pH is significantly reduced due to these processes, which consume OH^- ions.

In strongly diluted suspension with high water/binder (w/b) ratios >10, or after high pressure “squeezing” operations, state-of-the-art pH analysis in cementitious systems is routinely done. Techniques to squeeze out the pore solution of samples at realistic w/b ratios (~0.5) are, on the other hand, associated with a complex test setup.

Furthermore, the reported data have a poor temporal resolution, only representing the condition of the pore solution at certain moments of hydration. Important changes in the pore solution may be ignored or not properly mapped if hydration events occur very fast, as in “accelerated” systems.

A new approach for in situ pH monitoring during the hydration of cementitious systems means of optical sensors and innovative pH indicators with high apparent pK_a values were recently described in a study published in the journal Cement and Concrete Research. The proposed approach was further refined in the current study for the field of accelerated systems when reactions occur at a high rate and are followed by rapid temperature rises.

In situ pH measurements provide a better understanding of the events that occur immediately after the setting accelerator is intermixed. Early and later phases of shotcrete hydration require a correct reaction sequence during the initial minutes of hydration to achieve the necessary mechanical qualities. To optimize the mixes and their characteristics, a thorough understanding of the changes occurring in the system, both in the fluid and solid phases, is essential.

Methodology

CEM I 52.5R (CEM I) and CEM I 52.5N SR0, both conforming to EN 197-1, were utilized in the studies (CEM SR0). XRD and subsequent Rietveld studies were used to identify the mineralogical composition of both type of cement, as indicated. Table 1 shows the results.

Table 1. Mineralogical composition of CEM I and CEM SR0. Source: Briendl, et al., 2022

A commercial alkali-free Al₂(SO₄)₃ based suspension was employed as a setting accelerator (characteristics of the accelerator are listed in Table 2.

Table 2. Accelerator characteristics, according to the supplier. Source: Briendl, et al., 2022

Figure 1 shows the test setup for continuous in situ pH monitoring.

Figure 1. Test setup for continuous pH measurement in paste samples. Figure was drawn with Sketch Up Pro 2021 software tool and integrated 3D warehouse database. Image Credit: Briendl, et al., 2022

Epifluorescence examination (ChemiDoc MP, blue light with 530/28 emission filter, 0.01 second exposure period) and subsequent visualization using Image Lab v5.2 software were used to visualize the accelerator distribution.

Results

The sample homogeneity following accelerator intermixture was critical in the tested systems since the pH values were monitored by ~1 mm² sensors, which had a limited spatial resolution.

After 8 or 30 seconds of mixing, Figure 2 shows the distribution of the accelerator in samples with 11% accelerator.

Figure 2. Epifluorescence photographs of CEM I paste samples, produced with 11% accelerator after 8 and 30 seconds of mixing. Fluorescein-Na was added to the accelerator as tracer material. Image Credit: Briendl, et al., 2022

The time-resolved pH development of cement pastes made with CEM I and CEM I SR0 without the accelerator (0–~690 seconds) and with accelerator in the range of 3–20 wt% dose (bwc.) is shown in Figure 3(a–f) and Figure 4(a, b).

Figure 3. pH and temperature development (bold lines = arithmetic mean (n = 3), colored areas displays the standard deviation) of the pore solution. (a–c): CEM I and (d–f): CEM SR0. Accelerator (acc.) dosage was varied from 3 - 11% bwc. Note that pH values in the grey colored area are out of calibration for the in situ test method. Lime green colored areas display the time spans, when the sample was mixed. For better visibility, the timespan around accelerator addition (675 - 760 seconds after water addition) is additionally augmented in each graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.). Image Credit: Briendl, et al., 2022

Figure 4. Effect of a high accelerator (acc.) dosage (20% bwc.) on the pH development and sample temperature of a: CEM I and b: CEM SR0. Note that pH values in the grey colored area are out of calibration for the in situ test method. Lime green colored areas displays the time spans, when the sample was mixed. For better visibility, the timespan around accelerator addition (675 - 760 seconds after water addition) is additionally augmented in each graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.). Image Credit: Briendl, et al., 2022

The pH values reported by strong acid titration in the CEM I sample were lower than those found by optical measurement; however, they were comparable to or higher in the CEM SR0 sample. Precipitation of hydration products throughout sample preparation of CEM I samples is predicted because samples of both cement types were produced in the same way. Carbonation effects are predicted to be minimal due to the limited time between pore solution extraction and measurement.

Conclusion

Optical in situ measurements were used to explore the influence of an Al₂(SO₄)₃ setting accelerator on the pH development of cementitious systems in this work. Pore solution extraction and subsequent pH studies were done for comparison. As a result of the findings, the following conclusions may be drawn.

In situ pH measurements in accelerated cement pastes at a real water/binder ratio were done for the first time (0.5). The results provide a more thorough understanding of the mechanisms that occur during and after the addition of Al₂(SO₄)₃ setting accelerators to cementitious matrices.

The lower the pH decreased in the initial seconds after the accelerator was mixed, the greater the accelerator dosage. The pH did not drop below the ettringite stability threshold within the range of practical, relevant accelerator doses (~5–10 wt% bwc.). The subsequent rapid breakdown of the cement clinker phases buffered the system, preventing the pore solution’s pH from dropping any further.

The pH values obtained using optical sensors were comparable to those obtained using the traditional method of liquid phase separation under high pressure and subsequent electrochemical potential or titration to evaluate the pH of pore solutions.

A slight change to the system in shotcrete applications where ettringite production regulates the quick setting and early strength development might have negative consequences. As a result, this “simple” technique might be used to forecast and analyze probable deviations or retardation effects from chemical admixtures.

To overcome the existing limits of the proposed sensor system, future research should focus on creating sensor dyes with a pH sensitivity in the region of very high pH values (>13.3). In addition, the invention and application of other types of luminescent sensors for the simultaneous analysis of additional ions in solution (e.g., Ca²⁺, Na⁺, etc.) will help in completing the view of the fluid phase in these cementitious systems.

Journal Reference:

Briendla, L. G., Grengg, C., Müller, B., Koraimann, G., Mittermayr, F., Steiner, P. and Galan, I. (2022) In situ pH monitoring in accelerated cement pastes. Cement and Concrete Research, 157, p.106808. Available Online: https://www.sciencedirect.com/science/article/pii/S0008884622000990.

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