TGA Cement Analysis: Quantifying Hydration and Carbonation in Silicate Cement

Q: 1. What does thermogravimetric analysis (TGA) reveal about silicate cement?

Thermogravimetric analysis (TGA) reveals the composition and thermal stability of hydration and carbonation products in silicate cement by measuring mass loss as temperature increases. It allows quantitative identification of phases such as C-S-H gel, ettringite, calcium hydroxide (Ca(OH)₂), and calcium carbonate (CaCO₃), which are critical for evaluating cement hydration progress and concrete durability.

Q: 2. How can TGA determine the degree of cement hydration?

TGA determines the degree of cement hydration by measuring chemically bound water released during heating. The degree of hydration (α) is calculated using the ratio of measured bound water to the theoretical maximum bound water (approximately 0.24 g per gram of fully hydrated Portland cement). Higher bound water content indicates more advanced hydration and strength development.

Q: 3. Why is calcium hydroxide (Ca(OH)₂) important in TGA analysis of cement?

Calcium hydroxide (Ca(OH)₂) is a key indicator of cement hydration progress and durability. In TGA, it decomposes between 400–480°C, releasing water that can be quantified. Its content reflects hydration completeness and available alkalinity for carbonation buffering. A decrease in Ca(OH)₂ often signals carbonation or durability reduction in concrete structures.

Q: 4. How does TGA help assess concrete carbonation?

TGA quantifies carbonation by measuring the decomposition of calcium carbonate (CaCO₃) between 500–1000°C. By comparing Ca(OH)₂ depletion and CaCO₃ formation, researchers can determine carbonation depth and progression. TGA-based measurements are significantly more accurate than traditional phenolphthalein testing, making them ideal for durability and service life prediction.

Cement is one of the most widely used construction materials across infrastructure, transportation, agriculture, and marine engineering. Among cement types, silicate cement holds a dominant position due to its versatility, durability, and long-standing industrial adoption. Understanding the composition and hydration state of cement is critical for predicting concrete durability and service life — and TGA cement analysis is one of the most direct and quantitative methods available for doing so.

Background: Why TGA Is Ideal for Cement Characterization

Thermogravimetric analysis (TGA) measures the mass of a sample as a function of temperature. For cement systems, this measurement is uniquely informative: each cement hydration and carbonation product decomposes at a characteristic temperature, releasing measurable amounts of water, carbon dioxide, or sulfate. A single TGA experiment therefore identifies and quantifies multiple cement phases simultaneously, without requiring separate chemical extractions or dissolution steps. This article demonstrates the four-stage TGA decomposition analysis of two silicate cement samples, compares their hydration and carbonation state, and shows how the results directly inform durability assessment and quality control. All measurements were performed using the AMI TGA 1000, part of AMI’s range of thermal analysis instruments. For a broader overview of AMI’s thermal characterization capabilities, see our thermal properties analysis overview.

The primary phase of silicate cement clinker comprises silicate minerals, with alite (C₃S) and belite (C₂S) accounting for over 75% of its composition. Upon exposure to atmospheric conditions, these components react with moisture and carbon dioxide to form secondary hydration and carbonation products:

Ettringite: an early hydration product forming rapidly after water contact, decomposing below 200°C
C-S-H gel (calcium silicate hydrate): the primary binding phase in hardened concrete, contributing most of the mechanical strength, decomposing between 200–400°C
Calcium hydroxide Ca(OH)₂ (portlandite): a key hydration product decomposing between 400–480°C, serving as an indicator of hydration progress and providing alkalinity that protects embedded steel reinforcement
Calcium carbonate CaCO₃: formed by carbonation of Ca(OH)₂, decomposing in two distinct stages (500–800°C for poorly crystalline, >800°C for highly crystalline) reflecting different carbonation origins and histories

Each phase decomposes at distinct temperatures, making TGA a powerful technique to identify and quantify cement hydration and carbonation products based on characteristic mass loss behavior. Unlike qualitative identification methods, TGA quantifies the mass fractions of each phase — providing the numerical data needed for degree-of-hydration calculations, carbonation depth assessment, and concrete service-life prediction.

For a related discussion of TGA applied to quantifying water of crystallization in inorganic crystalline hydrate compounds, including kinetic modeling of dehydration behavior, see our article on TGA crystalline hydrates dehydration kinetics.

Principles: TGA Decomposition Temperature Ranges for Cement Phases

The four decomposition temperature windows that define TGA cement analysis are (Figure 1; alt text: diagram of TGA decomposition temperature windows for cement phases):

Stage	Temperature Range	Phase Decomposing	What It Indicates
Stage 1	<200°C	Free moisture, C-S-H gel, ettringite	Early hydration products and free water; magnitude reflects water-to-cement ratio and early curing
Stage 2	400–480°C	Calcium hydroxide Ca(OH)₂	Hydration progress — higher Ca(OH)₂ indicates more complete hydration; key alkalinity reserve
Stage 3	500–800°C	Poorly crystalline CaCO₃	Environmental carbonation during curing — surface exposure to atmospheric CO₂
Stage 4	>800°C	Highly crystalline CaCO₃	Long-term carbonation or residual clinker phases — more stable, internally formed carbonate

Why heating rate matters: The heating rate directly influences decomposition temperature and peak resolution. Slower rates (5°C/min) provide better resolution of closely spaced decomposition events but increase analysis time. Faster rates (20°C/min) reduce time but can cause peak broadening and overlap between adjacent stages. The 10°C/min rate used in this study represents an optimal balance between resolution and efficiency for routine cement analysis.

Experimental Method

Thermogravimetric analysis was performed using the AMI TGA 1000 under the following controlled conditions:

Parameter	Condition	Rationale
Sample mass	~20 mg	Minimizes temperature gradients within the sample while providing sufficient signal
Sample holder	Platinum crucible	Chemically inert and thermally stable to 1000°C; prevents reaction between sample and container
Atmosphere	Nitrogen, 50 mL/min	Inert atmosphere prevents oxidation artifacts; consistent flow ensures reproducible evolved gas removal
Heating program	30°C to 1000°C at 10°C/min	Covers all four decomposition stages with adequate resolution

The analysis focused on the thermal decomposition behaviors of Ca(OH)₂ (Stage 2) and CaCO₃ (Stages 3 and 4), as these phases are the most informative indicators of hydration progress and carbonation extent. For discussions of how nitrogen purge atmosphere quality affects TGA measurement integrity — particularly relevant for long-duration experiments — see our article on oxygen-free TGA analysis and purge gas testing.

Results: Four-Stage Mass Loss Analysis

Figures 2 and 3 show the TGA profiles for two silicate cement samples (Sample A and Sample B), and the detailed quantitative results are reported in Table 1.

Stage 1 — Free Moisture and Low-Temperature Hydrates (<200°C)

Both samples show comparable initial mass loss in Stage 1: 5.253% for Sample A and 5.562% for Sample B. This stage represents evaporation of free capillary moisture and decomposition of low-temperature hydration products including C-S-H gel and ettringite. The similar values suggest comparable free moisture retention and early-stage hydration products between the two samples.

Stage 2 — Calcium Hydroxide Dehydration (400–480°C)

The dehydration of Ca(OH)₂ is the most significant difference between the two samples:

Sample A: 0.980% mass loss at 476.4°C — indicating relatively low Ca(OH)₂ content
Sample B: 2.422% mass loss at 437.0°C — significantly higher Ca(OH)₂ content

This difference indicates that Sample B has either undergone a more advanced hydration stage or was prepared with a higher water-to-cement ratio, generating more Ca(OH)₂. Sample A’s lower Ca(OH)₂ content may suggest partial consumption of portlandite by carbonation reactions — converting Ca(OH)₂ to CaCO₃, which then appears in Stages 3 and 4.

Degree of hydration calculation: The chemically bound water content from TGA can quantify degree of hydration (α) directly: α = measured bound water / maximum bound water. For Portland cement, complete hydration generates approximately 0.23–0.25 g of non-evaporable water per gram of cement (typically taken as 0.24). This relationship allows prediction of strength development, supplementary cementing material reactivity, and early-age concrete behavior.

Stage 3 — Poorly Crystalline CaCO₃ (500–800°C)

This stage shows the most dramatic difference between the two samples:

Sample A: 12.698% mass loss at 761.0°C — very high poorly crystalline CaCO₃ content
Sample B: 4.767% mass loss at 587.14–791.0°C — substantially lower poorly crystalline CaCO₃

Poorly crystalline CaCO₃ forms from environmental carbonation during curing — when atmospheric CO₂ reacts with Ca(OH)₂ near the concrete surface. Sample A’s high Stage 3 content (12.698%) indicates significant surface carbonation or prolonged atmospheric exposure during the curing period. This is consistent with its low Ca(OH)₂ content in Stage 2: the Ca(OH)₂ has been consumed by carbonation and converted to CaCO₃.

Stage 4 — Highly Crystalline CaCO₃ (>800°C)

Both samples show comparable highly crystalline CaCO₃ content:

Sample A: 1.073% mass loss above 800°C
Sample B: 1.168% mass loss above 800°C

Highly crystalline CaCO₃ originates from original clinker phases or long-term deep carbonation. The comparable values between samples suggest similar long-term carbonation history or similar original clinker composition — the difference between the two samples is primarily in the surface/environmental carbonation captured in Stage 3, not in deep or clinker-derived carbonate.

Mass Loss Stage	Sample A	Sample B	Key Interpretation
Stage 1 (<200°C) — Free moisture + C-S-H	5.253%	5.562%	Comparable early hydration and free moisture
Stage 2 (400–480°C) — Ca(OH)₂	0.980% at 476.4°C	2.422% at 437.0°C	Sample B shows more advanced hydration or higher w/c ratio
Stage 3 (500–800°C) — Poorly crystalline CaCO₃	12.698% at 761.0°C	4.767% at 587–791°C	Sample A has significantly more surface carbonation
Stage 4 (>800°C) — Highly crystalline CaCO₃	1.073%	1.168%	Comparable long-term carbonation / clinker phases
Residue	77.13%	82.93%	Sample B retains more non-volatile material

Advanced Applications: TGA Data for Durability Assessment

Carbonation Depth and Concrete Durability

Thermogravimetric analysis combined with spatial chemical analysis (TGA-CA) provides superior quantification of concrete carbonation depth compared to qualitative phenolphthalein indicator testing. TGA simultaneously measures both Ca(OH)₂ depletion and CaCO₃ formation, directly revealing:

Carbonation advancement: the transition zone where Ca(OH)₂ converts to CaCO₃ — the carbonation front
Durability reserves: remaining Ca(OH)₂ available for pH buffering of the pore solution, which protects embedded steel reinforcement from corrosion
Service life predictions: carbonation rate extrapolation from TGA data provides quantitative inputs to reinforcement durability models

Why carbonation matters for structural durability: Concrete’s protection of steel reinforcement relies on the high-pH environment (pH >12.5) maintained by dissolved Ca(OH)₂ in the pore solution. When carbonation converts Ca(OH)₂ to CaCO₃, the pH drops below 9 — depassivating the steel surface and enabling corrosion. TGA-based measurement of Ca(OH)₂ depletion and CaCO₃ formation provides a direct, quantitative measure of this critical durability mechanism, significantly more accurate than phenolphthalein colorimetric testing for structural assessment and service life prediction.

Limitations and Best Practices

Reliable TGA cement analysis requires attention to several potential sources of error:

Carbonation during sample preparation: exposure to atmospheric CO₂ between sample grinding and TGA analysis can artificially increase Stage 3 CaCO₃ content — samples should be ground and transferred to sealed containers rapidly, ideally under nitrogen
Overlapping decomposition ranges: calcareous aggregates in concrete samples produce CaCO₃ decomposition that overlaps with cement carbonation peaks — TGA-mass spectrometry coupling can resolve overlapping CO₂ signals when aggregate contributions must be separated
Moisture variability: different drying procedures (air dry, vacuum dry, acetone dry) produce different Stage 1 mass loss values — standardized pre-conditioning is essential for inter-laboratory comparability
Sample size control: samples of approximately 20 mg minimize temperature gradients within the specimen while providing sufficient mass for accurate quantification of minor phases such as Stage 2 Ca(OH)₂ in low-hydration samples

The AMI TGA 1000 for Cement Phase Quantification

Accurate TGA cement analysis requires precise temperature control, stable baselines, and high balance sensitivity — particularly for quantifying closely spaced decomposition stages (Stage 2 Ca(OH)₂ at 400–480°C and Stage 3 CaCO₃ beginning at 500°C are separated by only ~20°C). The AMI TGA 1000 (Figure 4; alt text: AMI TGA 1000 thermogravimetric analyzer for cement phase quantification) provides the measurement performance required for reliable cement characterization:

High-sensitivity microbalance with 0.1 μg resolution: resolves small mass loss events such as Stage 2 Ca(OH)₂ dehydration at <1% in low-hydration or highly carbonated samples
Stable baseline and low drift: essential for accurate integration of closely adjacent decomposition stages and reliable quantification of multiple phases in a single run
Precise temperature control: ensures consistent, reproducible decomposition temperature identification across samples and operators — critical when comparing Ca(OH)₂ peak temperatures between samples (437°C vs 476°C in this study)
Full temperature range to 1000°C: covers all four cement decomposition stages in a single experiment without instrument reconfiguration
Controlled nitrogen atmosphere: prevents oxidation artifacts and ensures evolved gas composition reflects cement phase chemistry rather than instrument atmosphere effects

Conclusion

TGA cement analysis provides a comprehensive, quantitative picture of silicate cement hydration and carbonation state from a single measurement. The four-stage decomposition profile — free moisture and C-S-H below 200°C, Ca(OH)₂ at 400–480°C, poorly crystalline CaCO₃ at 500–800°C, and highly crystalline CaCO₃ above 800°C — allows simultaneous quantification of phases that reflect both hydration progress and environmental carbonation history.

In this study, the contrast between Sample A (high Stage 3 CaCO₃, low Ca(OH)₂) and Sample B (high Ca(OH)₂, low Stage 3 CaCO₃) directly demonstrates how TGA cement analysis distinguishes cement samples with different curing histories and carbonation exposure — information essential for quality control in cement manufacturing and durability assessment of concrete structures. The AMI TGA 1000 provides the sensitivity, baseline stability, and temperature precision needed for reliable quantification of all four decomposition stages. Explore AMI’s full range of thermal analysis instruments, or visit the AMI Technical Library for further application notes on TGA, DSC, and thermal characterization methodology.

References

(1) Lavagna, L.; Nisticò, R. An insight into the chemistry of cement – A review. Appl. Sci. 2023, 13, 203.

(2) Choudhary, H. K.; A. V., A.; Kumar, R.; Panzi, M. E.; Matteppanavar, S.; Sherikar, B. N.; Sahoo, B. Observation of phase transformations in cement during hydration. Constr. Build. Mater. 2015, 101, 122-129.

(3) Alarcon-Ruiz, L.; Platret, G.; Massieu, E.; Ehrlacher, A. The use of thermal analysis in assessing the effect of temperature on a cement paste. Cem. Concr. Res. 2005, 35, 609-613.

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Frequently Asked Questions

1. What does thermogravimetric analysis (TGA) reveal about silicate cement?

Thermogravimetric analysis (TGA) reveals the composition and thermal stability of hydration and carbonation products in silicate cement by measuring mass loss as temperature increases. It allows quantitative identification of phases such as C-S-H gel, ettringite, calcium hydroxide (Ca(OH)₂), and calcium carbonate (CaCO₃), which are critical for evaluating cement hydration progress and concrete durability.

2. How can TGA determine the degree of cement hydration?

TGA determines the degree of cement hydration by measuring chemically bound water released during heating. The degree of hydration (α) is calculated using the ratio of measured bound water to the theoretical maximum bound water (approximately 0.24 g per gram of fully hydrated Portland cement). Higher bound water content indicates more advanced hydration and strength development.

3. Why is calcium hydroxide (Ca(OH)₂) important in TGA analysis of cement?

Calcium hydroxide (Ca(OH)₂) is a key indicator of cement hydration progress and durability. In TGA, it decomposes between 400–480°C, releasing water that can be quantified. Its content reflects hydration completeness and available alkalinity for carbonation buffering. A decrease in Ca(OH)₂ often signals carbonation or durability reduction in concrete structures.

4. How does TGA help assess concrete carbonation?

TGA quantifies carbonation by measuring the decomposition of calcium carbonate (CaCO₃) between 500–1000°C. By comparing Ca(OH)₂ depletion and CaCO₃ formation, researchers can determine carbonation depth and progression. TGA-based measurements are significantly more accurate than traditional phenolphthalein testing, making them ideal for durability and service life prediction.

5. What are the optimal TGA conditions for analyzing cement samples?

For reliable cement characterization, recommended TGA conditions include a heating rate of 10°C/min, a sample mass of approximately 20 mg, nitrogen atmosphere at 50 mL/min, and use of inert platinum crucibles. These parameters ensure accurate phase separation, minimal peak overlap, and reproducible quantification of hydration and carbonation products.