A new method to characterize cement hydration degree - low field nuclear magnetic resonance technique

01

Research background

The hydration degree of cement refers to the ratio of the hydration amount of cement particles to the complete hydration amount of cement in a certain time. In the pure cement system, because the cementing material is only cement, its hydration degree is the hydration degree of the entire sample. At home and abroad, the hydration degree test methods include chemical combined water method, CH quantitative test method, hydration thermal method and hydration dynamic simulation.

However, these methods have the disadvantages of complicated operation, time consuming and oversimplified model, which makes the characterization of cement hydration degree not accurate and objective. Therefore, it is necessary to develop a simple and accurate characterization method to study the hydration degree of cement.

It has been shown that low field NMR technology has high sensitivity to different degrees of free moisture in cement slurry. In this study, using water as a probe can analyze the relaxation information of water in the slurry and characterize the microstructure of cement slurry during hydration process, which makes it possible to study the hydration degree of cement by using low-field nuclear magnetic resonance technology.

In this study, a method for characterizing cement hydration degree was developed based on low-field nuclear magnetic resonance technology. Through regression analysis and quantitative test results of hydration heat and XRD, the hydration degree model was determined to analyze the characteristics of cement hydration degree under different curing temperatures.

02

Experimental design and method

The part of experimental design is very important, like the soul of an article, I suggest that everyone should take a closer look and draw inspiration from it.

Raw materials and sample preparation

In order to reduce the influence of paramagnetic substances on the results, white cement was selected in this study. Polycarboxylic acid superplasticizer was used to adjust the fluidity of cement slurry. The water-cement ratio of the cement slurry is 0.25,0.35 and 0.45, and the curing system experienced by the sample is shown in Figure 1.

FIG. 1 Maintenance system

The curing methods of specimens involved in this work are divided into standard curing and steam curing, the latter including 40℃ constant temperature steam curing and 60℃ constant temperature steam curing. Note: Taking the sample name of N2025 as an example, "20" represents the curing temperature and "25" represents the water-cement ratio of the sample.

Nuclear magnetic resonance analysis of slurry relaxation time

In this study, the nuclear magnetic resonance equipment (PQ001 series) produced by Shanghai Niumai Technology Co., LTD., and the built-in Sirt software was used to calculate the transverse relaxation time. The typical transverse relaxation time distribution is shown in Figure 2:

Figure 2. Typical transverse relaxation time distribution of cement slurry

Previous studies have found that there are many different relaxation peaks in the early transverse relaxation time spectrum, including gel pore peak, main peak, transition peak and surface water relaxation peak. As time goes by, the main peak gradually leftwards, and the gel pore peak and the main peak merge. In this work, the relaxation time of the main peak is mainly used to analyze the hydration degree of cement.

Hydration heat method was used to analyze hydration degree

According to relevant studies, the ratio of the integral heat release at time t to the total theoretical heat release when cement is fully hydrated can be used as the hydration degree of cement at time t.

The specific formula is:

Where, is the hydration degree of cement at time t, Qt is the integral heat release of cement at time t, and Qmax is the theoretical total heat release of cement at complete hydration.

Calcium hydroxide quantitative test method by X-ray diffraction

This study mainly uses quantitative XRD method to calculate the content of calcium hydroxide. The hydration degree of cement is proportional to the amount of CH generated by cement hydration. The hydration degree of cement can be obtained indirectly by determining the CH content in cement slurry. Concrete formula        Wt is the CH mass fraction in the hardened cement slurry at time t, which is calculated and determined by reference strength method. Wt is the CH mass fraction in the hardened cement slurry at full hydration, which is determined by fitting and predicting the CH content in the hardened slurry at 0, 1, 28, 200 and 400 days of hydration.

However, in the actual work, it was found that after 90 days of maintenance, the amount of CH changed very little, so the reference amount S (α-Al2O3) was added, and then the X-ray diffraction method was used to do it. The final calculation is as follows.

The calcium hydroxide content at a specified age can be calculated by formula 4 to analyze the degree of hydration of the cement.

03.Analysis of experimental conclusion

01.Transverse relaxation time distribution of cement slurry under different curing systems