Introduction:
Thermogravimetry (TG) is widely used to evaluate the hydration process and the degree of reaction of cementitious materials. However, it is still challenging to accurately quantify the overlaps of hydration products using TG alone because cement hydration produce complex and multiphase composites. Mass spectrometry (MS) with its high specificity, sensitivity and fast testing speed, can promote TG quantification by eliminating overlapping decomposition peaks, further achieving the accurate quantification of cementitious hydration products. Therefore, this study designed the exposure conditions with different temperatures (5, 20, and 40 ℃) and humidity levels (RH ≈ 33%, RH ≈ 59%, and RH ≈ 95%) to study a dissolution process of the hydrated granulated blast furnace slag (GGBS). Hydration products at 6 different periods of dissolution (5, 10, 20, 30, 60 min, and 120 min) were analyzed to assess the applicability and feasibility of the TG–MS coupling technique for quantifying the early reaction products of GGBS.
Methods:
Granulated blast furnace slag (GGBS) was used in dissolution-carbonation experiments, using deionized water and the 4 mol/L NaOH solution as solvents. The liquid-solid ratio was 50:1, and the dissolution temperatures were set at 5, 20 ℃, and 40 ℃. Three ambient relative humidity (RH) (33%, 59% and 95%) were controlled using saturated MgCl2, NaBr, and KNO3 solutions. Mechanical stirring was performed at 750 r/min, and the vessel was covered with parafilm throughout the dissolution process. The samples were taken at 5, 10, 20, 30, 60 min, and 120 min. The precipitated products were hydration-stopped using the solvent-exchange method, followed by carbonation in a CO2 concentration of 600 × 10–6 for 28 d. Water and carbon dioxide were detected using a TG–MS system.
Results and discussion:
The C-(A)-S-H thermal decomposition mass loss in TG correlated well with the H2O signal in the MS system under tested conditions, whereas the relation depends on degree of hydration. This phenomenon may be attributable to the following factors: on the one hand, it is due to the overlapping peaks observed in the thermogravimetric loss curve affected by water vapor mass loss released by C-(A)-S-H decomposition and heat absorption peaks of the dehydration of loss-combined water in the hydrotalcite-like phases (Mg-Al-LDH). On the other hand, it is related to the partial-pressure system in the MS system that was constantly changing during the gas release. In the quantitative analysis of CaCO3, the samples with different degrees of hydration exhibited disparate degrees of carbonation, indicating that the degree of hydration influenced the degree of carbonation. Notably, CaCO3 quantification (expressed as the amount of CO2 detected) presents different correlations, and there is also a correlation in the quantitative analysis of C-(A)-S-H gel. With the increased hydration (temperature and time), the amount of C-(A)-S-H gel increased, resulting in more CaCO3 generated by carbonation. Since the decomposition of the C-(A)-S-H gel occurred throughout the entire heating process, an overlap between the decomposition curves of C-(A)-S-H and CaCO3 should be taken into account. However, the current study did not fully account for gas partial pressures, which led to multiple linear relationships in the correlations within the TG–MS system. This phenomenon can be quantified and calibrated using equivalent characteristic spectrum analysis (ECSA).
Conclusions:
TG–MS was used to analyze the dissolution-precipitation-natural carbonation products of GGBS under several storage humidity and temperature conditions. This method enabled the quantitative description of the amorphous hydration product C-(A)-S-H gel and the crystalline product CaCO3. The degree of GGBS hydration and natural carbonation varies depending on the dissolution environment (storage humidity, dissociation temperature, and dissolution time), and these processes exhibit a positive correlation between mass loss and MS quantification. Using MS helps to separate overlapping peaks in the TG quantitative process, serving as a complementary and validation method for thermogravimetric analysis. However, when using the TG–MS system for correlation analysis, experimental errors may occasionally arise due to the inherent inaccuracies of TG, the amount of gas within the TG–MS system, and the presence of multiple gases.