Advanced thermal analysis for quantifying cement hydration phases, carbonation products, and durability assessment in construction materials.
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⁽¹˒²⁾:
EttringiteC-S-H gelCalcium hydroxide (Ca(OH)₂)Calcium carbonate (CaCO₃)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⁽³⁾. 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.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 contentCalcium hydroxide concentration - Related to hydration progressCarbonation extent - From poorly crystalline and highly crystalline calcium carbonate contentPhase identification - Specific decomposition temperatures for each hydrate phaseDurability assessment - Correlation between hydration/carbonation products and concrete performanceThermal 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]
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 lossesThe 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.
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 gelDecomposition 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 hydrationIndicates availability for carbonation reactions - Ca(OH)₂ is the primary reactant consumed during atmospheric carbonationCorrelates with concrete durability - Ca(OH)₂ provides chemical protection; its depletion via carbonation reduces durabilityThe 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 CO₂Curing conditions impact - Environmental humidity and CO₂ concentration affect this product distributionProgressive carbonation advancement - As concrete ages, carbonation moves inward, converting Ca(OH)₂ to CaCO₃Stage 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 productionLong-term carbonation - Fully carbonated zones where Ca(OH)₂ has been completely convertedCalcareous aggregates - If present in concrete mixturesThis 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]
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.
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 strengthWhen supplementary cementing materials (fly ash, silica fume) have consumed available calcium hydroxideThe rate of strength development in early-age concreteAssessing 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 CaCO₃Durability reserves - Remaining calcium hydroxide available for pH bufferingService life predictions - Carbonation rate extrapolation for reinforcement durabilityResearch shows that TGA-based carbonation measurements are 2-3 times more accurate than traditional phenolphthalein testing, making it essential for high-reliability durability assessments.Modern thermogravimetric analyzers offer significant improvements in sensitivity, temperature control, and data resolution compared to older generation instruments: Key Performance Features ✓ Ultra-sensitive balances - Detect mass losses <1 μg for accurate phase quantification ✓ Precise temperature control - ±1°C accuracy ensures consistent decomposition temperature detection ✓ Low baseline drift - Stable measurement baselines eliminate signal noise from long-duration experiments ✓ Fast heating/cooling rates - Improve sample throughput for high-volume characterization ✓ Evolved gas analysis coupling - Hyphenated techniques (TGA-MS, TGA-FTIR) for phase identification These technological advances make comprehensive thermal analysis more accessible for academic research and industrial quality control of cementitious materials.
Common Challenges in TGA Interpretation Carbonation during sample preparation: Exposure to atmospheric CO₂ between cement testing and TGA analysis can artificially increase calcium carbonate peaks. Mitigation strategies include rapid sample grinding and storage in sealed containers under nitrogen. Overlapping decomposition ranges: When cement contains calcareous aggregates, calcium carbonate decomposition may overlap with cement phase decomposition (500-1000°C). Coupled TGA-mass spectrometry can resolve these overlapping signals by identifying evolved CO₂ composition. Moisture variability: Different drying procedures (air dry, vacuum dry, acetone dry) produce different free moisture content measurements. Standardized drying protocols are essential for inter-laboratory result comparison. Recommended Experimental Practices For reliable cement TGA characterization:
Standardize heating rates - Use 10°C/min for routine analysis; slower rates (5°C/min) for detailed peak resolutionControl sample size - Keep samples around 20 mg to minimize temperature gradients within the sampleUse inert crucibles - Platinum crucibles prevent reactions between sample and containerVerify temperature calibration - Use standard materials (indium, lead) to confirm furnace temperature accuracyDocument atmospheric conditions - Control nitrogen flow rate (50 mL/min standard) for reproducibilityThermogravimetric analysis has become an indispensable characterization technique in cement science and concrete durability research. The ability to simultaneously quantify multiple hydration and carbonation products—from early-stage hydrates like ettringite to long-term carbonation products like crystalline calcium carbonate—makes TGA invaluable for:Quality control in cement and concrete manufacturingDurability assessment of concrete structuresResearch applications investigating cement chemistry and hydration mechanismsService life prediction based on quantified phase compositionThe relative amounts of Ca(OH)₂ and CaCO₃ determined by TGA directly reflect hydration progress and carbonation extent, both of which critically impact concrete durability and performance in real-world environments. By understanding TGA decomposition profiles, engineers and researchers can make informed decisions about concrete mix design, curing conditions, and service life expectations. For laboratories seeking comprehensive materials characterization solutions, advanced thermal analysis instrumentation provides precise, quantitative data essential for modern cement and concrete science.
(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.
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.
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.
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.
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.
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.
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