Characterizing the hydration process of cement based on synchronous thermal analysis method

Research on the characterization method of cement hydration progress - synchronous thermal analysis cement hydration progress is closely related to the strength of cement concrete materials. How to intuitively and effectively characterize cement hydration progress has been puzzling by researchers in this field. In this project, the content of various hydration products in the thermal analysis process of cement slurry and its variation law with age were obtained by simultaneous thermal analysis of the benchmark cement and the benchmark cement doped with early strength agent ZQ-1. By comparing the three DTG spectra of cement slurry mixed with early strength agent and blank benchmark cement slurry with maximum thermal weight loss on thermal decomposition, calcium hydroxide of hydration products with peak temperature between 400°C ~ 450°C can better characterize the effect of different admixtures on the hydration process of cement slurry. Early strength admixtures can better promote the early hydration of benchmark cement.


Introduction
Cement concrete is one of the most used and widely used building materials in the world.In 2019 alone, the global cement output reached more than 4 billion tons, of which 2.33 billion tons were in China.In the past three centuries, cement and concrete-based engineering structures have developed rapidly.They are widely used in civil construction, transportation, and marine development and have made great contributions to human civilization and construction.
Thermal analysis methods have the advantages of small sample size and easy operation and are widely used in geology, chemical industry, metallurgy, building materials, and other industries.Thermal analysis methods commonly used in cement hydration research include differential calorimetry (DTA), differential scanning calorimetry (DSC), and thermogravimetry (TG) [1~3] .The application of thermal analysis methods in cement hydration is mainly manifested in identifying the hydration products of cement and their content, studying the hydration rate and hydration degree of cement, determining the transition temperature of hydration products, and determining the thermal stability of samples.In recent years, thermal analysis combined with technology has been a new hotspot in cement hydration research.Among them, the thermal synchronous analysis method (TG-DSC combined technology) has been widely studied in cement hydration [4][5][6][7] .
In general, the thermal analysis process of cement hydration products can be characterized by nonisothermal differential heat, differential scanning calorimetry or thermogravimetry, and the changes of the endothermic peaks of hydrated calcium silicate (C-S-H) gel, calcium alum (AFt), Ca(OH)2 and CaCO3 can be clearly seen on the non-thermo and differential scanning calorimetric curves.The peak temperature on the DTG curve can be obtained due to the mass change caused by the heating process.
Then, the hydration mechanism of cement and the influence of different water-reducing agent types and dosages on cement hydration can be studied [8][9][10] .Cement hydration progress is closely related to the strength of cement concrete materials.Therefore, it is of great significance to systematically study how to use synchronous thermal analysis methods to characterize cement hydration progress.In this paper, we will conduct tests on cement slurries at different hydration ages based on synchronous thermal analysis to study how to quantitatively characterize the cement hydration process.

Instruments
The main equipment used in the experiment is shown in Table 1

Main raw materials
The main test raw materials used in the experiment are shown in Table 2.

Sample preparation.
According to the water-cement ratio of 0.4, the cement net slurry was prepared, and a small square test block with a side length of 2 cm was made, sealed, and placed in a standard curing room to cure until the required hydration age, and then the sample was taken out.0.5 g of the sample was taken from the center of the sample, crushed to no more than 2 mm in any direction, and soaked in about 25 ml of isopropanol (the mass ratio of isopropanol for immersion to cement net slurry sample is 50:1).It was then sealed, placed at low temperature in a 4 °C refrigerator for 24h and then taken out of the filtration.The filter residue is placed in a vacuum dryer to dry for 24 h.The dried specimen is sealed and stored, and the powder is taken out during the test and evenly screened for synchronous thermal analysis test.

Water reduction performance evaluation.
In the synchronous thermal analysis test, the nitrogen atmosphere was used.The nitrogen gas flow rate was set to 50 mL/min.The 5.0 mg±0.1 mg sample was weighed for testing.The heating rate selected in the thermal decomposition characterization experiment of the cement slurry hydration test product was 10°C/min.The selected temperature test range was 30°C~850°C.

The slurry of the benchmark cement
In order to better study the thermal decomposition TG-DTG behavior of cement slurry hydration products, samples of 3 h, 6 h, 12 h, 24 h, 3 d (72 h), 7 d (168 h) and 28 d (682 h) of the base cement hydration were prepared according to 2.3.1.The TG-DTG diagrams of the hydration products of the benchmark cement slurry at different ages were obtained by thermogravimetric testing, as shown in Figure 1.From the TG diagram and DTG diagram of the base cement hydration product in Figure 1, it can be seen that the main products of the base cement hydration DTG diagram include P1: 50°C ~ 200°C (weak binding water dehydration peak of hydration product C-S-H, AFm, and Aft dehydration peak in early hydration), P2: 400~450°C (Ca(OH)2 decomposition peak), P3: 550°C ~ 700°C (C-S-H bound water dehydration peak).Among them, the dehydration of hydration products and the production of Ca(OH)2 gradually increased with the increase of hydration time.The peak height of DTG gradually increased, and the C-S-H combined water dehydration peak had no obvious law with hydration time.
At the same time, the hydration time t is used as the abscissa, and P1, P2, and P3 correspond to the weight loss TG as the ordinate, as shown in Figure 2: It can be seen from Figure 2 that the amount of hydrated products bound to water P1 and calcium hydroxide P2 increases rapidly with the increase of hydration time.The growth rate gradually slows down when the hydration time reaches 3 d (72 h).
Among them, the hydration product P1 contains calcium hydroxide hydrate, calcium alum hydrate, C-S-H gel hydrate, and other lost bound water.The mechanism is more complex.In the thermogravimetric analysis, the cement slurry hydration process will generally be combined with SEM, EDS, XRF, and other instrumental analyses together.This is not specifically carried out in this project.
P2 is mainly the decomposition peak of calcium hydroxide.Its change law with the increase of hydration time is basically consistent with the trend of cement hydration heat release, which can be used as a basis for characterizing the hydration process of cement.
P3 is mainly a C-S-H gel crystal water dehydration peak.From Figure 2, it can be seen that the crystal water of C-S-H gel in the cement hydration process does not always increase the process.In the early stage of hydration, C-S-H gel with cement hydration will increase rapidly.This stage of the generation of C-S-H contains more crystal water.To hydration 3 d, C-S-H gel crystal water weight loss appears to decrease greatly, which is mainly cement slurry solidification, slurry free water becomes less, and C-S-H gel crystal water is further consumed by hydration.

Slurry of reference cement doped with ZQ-1
According to 2.3.1, samples of the hydration of the benchmark cement with ZQ-1 slump protector for 3 h, 6 h, 12 h, 24 h, 3 d (72 h), 7 d (168 h), 28 d (682 h) were prepared.The TG-DTG diagram of the hydration products of the benchmark cement slurry mixed with ZQ-1 slump protector at different ages was obtained by thermogravimetric test, as shown in Figure 3: From the TG diagram of the thermal decomposition of the benchmark cement hydration product doped with ZQ-1 in Figure 3, it can be seen that the total weight loss of thermal decomposition of the benchmark cement hydration product doped with ZQ-1 increases with the increase of hydration time, which is mainly the reason why the cement hydration product increases with the hydration time of cement.The main products are basically unchanged from the DTG diagram combined with Figure 1, mainly including interlayered water and free water of hydration products, Ca(OH)2 decomposition peak, and C-S-H combined water dehydration peak.Comparing Figure 3 and Figure 1, it can also be found that the free water and interlayer water volume and Ca(OH)2 formation in the hydration product of the benchmark cement slurry doped with ZQ-1 increase with the increase of hydration time.Compared with the early and late stages of the benchmark cement slurry, the increase in the middle period is faster, which is mainly the reason why the hydration rate in the early stage is reduced by the addition of ZQ-1.
Figure 4 shows the plotting with time t as abscissa and weight loss TG as ordinate.From Figure 4, it can be seen that the thermal decomposition P1 and P2 of the cement slurry hydration products mixed with ZQ-1 with the hydration time curve of the relative base cement slurry (Blank) both exceeded the standard cement in 24 h.This was related to the acceleration of cement hydration caused by the incorporation of ZQ-1 early strength agent into the cement slurry.

Characterization of cement slurry hydration progress
After synchronous thermal analysis of the hydration products of the base cement slurry and the base cement slurry doped with ZQ-1 for 3 h, 6 h, 12 h, 24 h, 3 d (72 h), 7 d (168 h) and 28 d (672 h), it can be seen from Figures 2 and 4 and their analysis results that the weight loss of Ca(OH)2 corresponding to the P2 peak on the DTG spectrum can better characterize the progress of cement slurry hydration.
The weight loss of calcium hydroxide of the reference cement slurry and the benchmark cement slurry of ZQ-1 for 3 h~672 h was plotted to obtain Figure 5.

Conclusion
By simultaneous thermal analysis of the hydration products of ZQ-1-doped reference cement and blank reference cement slurry without admixtures, it was obtained that: (1).Through synchronous thermal analysis test, the hydration products of the benchmark cement slurry can be accurately quantified, and the hydration products of the cement slurry mainly include free water, calcium hydroxide, C-S-H gel, etc.
(2).By comparing the peak occurrence of the three DTG spectra of cement slurry doped with early strength agent and the maximum thermal weight loss on thermal decomposition of the hydration product of blank benchmark cement slurry, calcium hydroxide, a hydration product with peak temperature between 400°C~450°C, can better characterize the effect of different admixtures on the hydration process of cement slurry.
(3).Using the method determined in this project to characterize the progress of cement hydration by the amount of calcium hydroxide generated by hydration, the hydration change rule of the reference cement mixed with early strength agent ZQ-1 can be well characterized.It is consistent with the theory, indicating that this method has good accuracy and practicability.

Figure 1 .
Figure 1.TG-DTG diagram of hydration products at different ages of the benchmark cement slurry.

Figure 2 .
Figure 2. Diagram of three peak thermal loss weight TG vs. hydration time.

Figure 3 .
Figure 3. TG-DTG diagram of hydration products of different ages of reference cement slurry doped with ZQ-1.

Figure 4 .
Figure 4. Three peak thermal loss weight TG vs. hydration time.

Figure 5 .
Figure 5. Relationship between calcium hydroxide thermal weight loss and hydration time of four cement slurry hydration products.

Table 1 .
: Main experimental instrument and equipment.