Thermogravimetric Analysis of Silicate Cement: Identifying Hydration and Carbonation Products

Advanced thermal analysis for quantifying cement hydration phases, carbonation products, and durability assessment in construction materials.

TGA Analysis for Cement Characterization

Cement is one of the most widely used construction materials across industries including infrastructure, transportation, agriculture, and marine engineering. Among cement types, silicate cement holds a dominant role 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.

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 products such as:

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Ettringite
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C-S-H gel
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Calcium hydroxide (Ca(OH)₂)
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Calcium carbonate (CaCO₃)
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Each of these phases decomposes at distinct temperatures, making thermogravimetric analysis (TGA) a powerful technique to identify and quantify cement hydration and carbonation products based on characteristic mass loss behavior. This thermal analysis approach provides quantitative data on cement hydration progress and environmental carbonation extent—both critical factors affecting concrete durability and performance in real-world applications.

Principles of Thermogravimetric Analysis for Cement Systems

Thermogravimetric analysis is a thermal analysis technique that measures the mass change of a material as temperature increases at a controlled rate. For cement science applications, TGA offers several advantages over alternative characterization methods:

Why TGA is Ideal for Cement Characterization

Unlike qualitative identification techniques, thermogravimetric analysis quantifies specific mineralogical phases in hydrated cementitious materials. The technique measures water (H₂O), hydroxide groups (OH⁻), carbon dioxide (CO₂), and sulfate (SO₄²⁻) released during heating under a nitrogen atmosphere. This quantitative approach allows researchers to determine:

Degree of cement hydration - Based on chemically bound water content

Calcium hydroxide concentration - Related to hydration progress

Carbonation extent - From poorly crystalline and highly crystalline calcium carbonate content

Phase identification - Specific decomposition temperatures for each hydrate phase

Durability assessment - Correlation between hydration/carbonation products and concrete performance

Thermal Decomposition Ranges for Cement Phases

Different cement hydration products decompose within specific temperature windows, allowing for phase quantification:

✓ Free and adsorbed water: Below 200°C ✓ C-S-H gel and Ettringite: 200-400°C ✓ Calcium hydroxide Ca(OH)₂: 400-480°C ✓ Poorly crystalline CaCO₃: 500-800°C ✓ Highly crystalline CaCO₃: Above 800°C

Figure 1: TGA Temperature Decomposition Windows for Cement Phases]

Experimental Design for TGA Analysis of Silicate Cement

Proper experimental methodology is essential for obtaining accurate, reproducible TGA results. The thermal analysis conditions must be carefully controlled to ensure reliable quantification of cement phases.

Sample Preparation and Analysis Conditions

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

Sample mass: ~20 mg (representative of hydrated cement systems)

Sample holder: Platinum crucible (inert, high-temperature resistant)

Atmosphere: Nitrogen (50 mL/min - inert gas prevents oxidation)

Heating program: 30°C to 1000°C at 10°C/min (controlled heating rate)

Balance type: Ultra-sensitive microbalance for detecting small mass losses

The analysis focused on the thermal decomposition behaviors of Ca(OH)₂ and CaCO₃, allowing their relative quantities to be determined based on water and CO₂ release, respectively. These two phases are particularly important indicators of cement hydration progress and environmental carbonation.

Why Heating Rate Matters in TGA Results

The heating rate (β) directly influences desorption temperature (Tdes) and overall thermal decomposition profiles. Slower heating rates (e.g., 5°C/min) provide better resolution of closely-spaced decomposition peaks but increase analysis time. Faster rates (e.g., 20°C/min) reduce analysis time but may cause peak broadening and overlap. The 10°C/min rate selected represents an optimal balance for cement analysis, providing both resolution and efficiency.

Interpreting TGA Profiles: Four-Stage Mass Loss Analysis

The TGA thermogram of silicate cement can be interpreted in four distinct mass loss stages, each corresponding to decomposition of specific cement phases. This systematic interpretation allows for quantitative assessment of hydration and carbonation products.

Figure 2: Complete TGA Thermogram with Four Decomposition Stages Labeled]

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

This initial mass loss represents:

Evaporation of free moisture (capillary water)

Decomposition of low-temperature hydration products including C-S-H gel

Decomposition of ettringite (an early-age hydration product)

The magnitude of this peak indicates early-stage hydration and residual free water content. Higher values suggest incomplete drying or high water-to-cement ratios. This stage is critical for understanding moisture retention in concrete pores.

Stage 2: Calcium Hydroxide Dehydration (400-480°C)

The second stage corresponds to dehydration of calcium hydroxide:

Ca(OH)₂ → CaO + H₂O

This peak is particularly important because:

Ca(OH)₂ content directly reflects cement hydration progress - Higher Ca(OH)₂ indicates more complete hydration

Indicates availability for carbonation reactions - Ca(OH)₂ is the primary reactant consumed during atmospheric carbonation

Correlates with concrete durability - Ca(OH)₂ provides chemical protection; its depletion via carbonation reduces durability

The relative peak area allows quantification of calcium hydroxide content based on the known mass loss from water release (H₂O = 0.241 × mass of Ca(OH)₂).

Stage 3: Poorly Crystalline Calcium Carbonate (500-800°C)

The third decomposition stage represents amorphous and poorly crystalline calcium carbonate:

CaCO₃ → CaO + CO₂

Poorly crystalline CaCO₃ forms from environmental carbonation during curing, typically near concrete surfaces. This stage indicates:

Surface carbonation extent - Higher peak suggests greater surface exposure to atmospheric CO₂

Curing conditions impact - Environmental humidity and CO₂ concentration affect this product distribution

Progressive carbonation advancement - As concrete ages, carbonation moves inward, converting Ca(OH)₂ to CaCO₃

Stage 4: Highly Crystalline Calcium Carbonate (>800°C)

The final stage represents thermally stable, highly crystalline calcium carbonate:

CaCO₃ → CaO + CO₂ (high-temperature form)

Highly crystalline CaCO₃ originates from:

Original clinker phases - Residual carbonated phases from cement production

Long-term carbonation - Fully carbonated zones where Ca(OH)₂ has been completely converted

Calcareous aggregates - If present in concrete mixtures

This peak decomposition at higher temperature indicates more stable carbonate phases, typically from deep internal carbonation rather than surface exposure.

Figure 3: Derivative TGA (DTG) Curve Showing Individual Peak Positions]

Case Study: Comparative TGA Analysis of Cement Samples

To demonstrate the practical application of TGA for cement characterization, two silicate cement samples were analyzed under identical conditions, providing insights into different hydration and carbonation states.

Sample A vs. Sample B: Key Findings

Decomposition Phase	Sample A	Sample B	Interpretation
Ca(OH)₂ content	0.980%	2.422%	Sample B shows more advanced hydration stage or higher water-to-cement ratio
Poorly crystalline CaCO₃	12.698%	4.767%	Sample A experienced more surface carbonation during curing
Highly crystalline CaCO₃	1.073%	1.168%	Comparable long-term carbonation or residual clinker phases

What These Results Reveal

Sample A Profile: Higher poorly crystalline CaCO₃ content (12.698%) indicates this sample experienced significant surface carbonation or environmental exposure during curing. The lower Ca(OH)₂ content (0.980%) suggests carbonation has consumed available portlandite, converting it to calcium carbonate. This pattern is typical of concrete cured in outdoor or high-CO₂ environments.

Sample B Profile: The elevated Ca(OH)₂ content (2.422%) indicates a more advanced hydration stage with less carbonation conversion. This sample likely experienced either protected curing conditions or shorter atmospheric exposure. The lower poorly crystalline CaCO₃ (4.767%) confirms less surface carbonation relative to Sample A. Sample B shows better preservation of calcium hydroxide, a key indicator of durability protection.

Advanced Applications: Using TGA Data for Durability Assessment

Quantifying Cement Hydration Progress

The chemically bound water (Wb) content calculated from TGA data provides a quantitative measure of cement hydration degree (α). Using established relationships:

Degree of Hydration (α) = Chemically Bound Water / Maximum Bound Water

For Portland cement, complete hydration generates approximately 0.23-0.25 grams of non-evaporable water per gram of cement (typically taken as 0.24). This relationship allows researchers to predict:

Whether concrete has sufficient time to develop full strength

When supplementary cementing materials (fly ash, silica fume) have consumed available calcium hydroxide

The rate of strength development in early-age concrete

Assessing Concrete Carbonation Depth

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

Carbonation advancement - The transition zone where Ca(OH)₂ converts to CaCO₃

Durability reserves - Remaining calcium hydroxide available for pH buffering

Service life predictions - Carbonation rate extrapolation for reinforcement durability

Research shows that TGA-based carbonation measurements are 2-3 times more accurate than traditional phenolphthalein testing, making it essential for high-reliability durability assessments.

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.