Introduction to Heat of Hydration
The heat of hydration refers to the heat energy released during the chemical reaction between water and cement compounds, a crucial process in concrete formation. As water interacts with cement, various clinker compounds undergo hydration, generating heat as a byproduct. This exothermic reaction plays a pivotal role in the development of concrete’s structural integrity and properties.
Understanding the heat of hydration is essential in the realm of concrete construction. The quantity of heat produced is dependent on the composition of clinker compounds present in the cement mixture. This heat evolution significantly impacts the temperature of the concrete mass during and after placement
Importance of Heat of Hydration
The evolution of heat during cement hydration bears significant importance, particularly in large-scale construction projects. This article aims to explore not only the nature of heat of hydration but also its testing methodology. By comprehending the heat of hydration and its implications, construction practices can be optimized to ensure the durability and stability of concrete structures
Heat of Hydration Test of Cement
The objective of conducting the heat of hydration test for cement paste is to quantify and analyze the heat released during the hydration process of the cement paste itself. This test aims to determine the specific heat generated as the cement paste undergoes hydration, providing crucial insights into its chemical reactions and thermal properties.
Equipment Required for Cement Heat of Hydration Test
Calorimeter:
- Conforming to the specifications of IS 11262 – 1985 standard.
Mortar and Pestle:
- Used for grinding partially hydrated samples.
- Diameter: 200 mm.
Glass or Plastic Vials:
- Vials equipped with tight-fitting stoppers or caps.
- Size: 80 × 20 mm.
Stopwatch or Timer:
- Capable of measuring to the nearest 0.5 seconds.
Sieve:
- IS sieves complying with IS 460 (Part I) – 1985.
- Dimensions: 150 µm and 850 µm.
Muffle Furnace:
- Capacity to maintain temperatures up to 900-950 °C.
Analytical Balance:
- Least count: 0.0002 g.
- Accuracy: ± 0.0002 g.
Standard Weights.
Weighing Bottles.
Camel Hair Brush.”
Required Materials for Cement Heat of Hydration Test
- Cement.
- Standard sand meeting IS 650 – 1991 standards.
- Potable or Distilled water complying with IS 1070 – 1977 specifications.
- Nitric acid of 200 ± 0.05 N concentration.
- Hydrofluoric acid of 40% (w/w) in analytical reagent quality.
- Zinc oxide with analytical reagent grade.
- Paraffin wax.
Precautions for Cement Heat of Hydration Test
- Use only potable or distilled water for the mixing process.
- Ensure the ambient temperature of the room and the temperature of both the cement and water utilized for the test are maintained at 27 ± 2 °C.
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Procedure for Heat of Hydration Test on Cement
A. Determination of Heat Capacity:
Preparation:
- Measure 9.6 ± 0.1 ml of hydrofluoric acid and 388 ± 0.1 ml of nitric acid at 27 ± 2 °C.
- Heat zinc oxide at 900-950 °C for an hour, then cool it in a desiccator with anhydrous calcium chloride.
- Grind the cooled zinc oxide to pass through a 150 µ IS sieve.
Calorimeter Setup:
- Heat 7 g of ZnO to 900-950 °C for 5 minutes, then cool for 2.5 hours (max 5 hours).
- Set up the calorimeter, run the stirrer for 5 minutes for temperature uniformity.
Calorimeter Readings:
- Determine initial heating/cooling corrections by taking temperature readings every minute for 5 minutes.
- Pour zinc oxide into the calorimeter, record readings until a constant rate of cooling/heating is achieved (within 20 minutes).
- Take additional readings for 5 minutes to obtain final heating/cooling correction.
Graph Plotting and Corrections:
- Plot a graph between initial/final heating/cooling rates and calorimeter temperature (Beckmann readings).
- Note the corrections for each temperature reading from the graph.
Heat Capacity Calculation:
- Calculate heat capacity using the formula:
Heat Capacity Calculation
\(Heat \, Capacity \, (J/ ^{\circ}C) = \frac{mass \, of \, ZnO \, (g)}{corrected \, temperature \, rise} \times [1.072 + 0.4 \times (30 - \phi) + 0.5 \times (\phi_{0} - \phi)]\)
Components involve mass, temperature rise, heat of solution, temperature coefficients, final calorimeter temperature (\(\phi\)), and room temperature (\(\phi_{0}\)).
6. Equation for Final Heat Capacity Correction
Equation for Final Heat Capacity Correction
\(Final \, Heat \, Capacity \, (J/ ^{\circ}C) = Total \, Corrections \, + Initial \, Heat \, Capacity\)
B. Preparation of Cement Sample
Mixing:
- Mix 60 g of cement with 24 ml of distilled water at 15-25 °C for 4 minutes.
- Fill glass/plastic vials with this mixture, cork, seal with wax, and store vertically at 27 ± 2 °C.
C. Determination of Heat of Solution (Unhydrated Cement)
1.Sample Preparation:
- Weigh 3 g of cement for heat of solution and another 7 g for ignition loss.
- Follow a similar procedure as with zinc oxide to determine the heat of solution for unhydrated cement.
2.Equation for Heat of Solution (Unhydrated Cement):
Equation for Heat of Solution (Unhydrated Cement)
\(Heat \, of \, Solution \, (kJ/kg) = \frac{heat \, capacity \times corrected \, temperature \, rise}{mass \, of \, sample \, corrected \, for \, ignition \, loss} - 0.8 \times (\phi_{0} - \phi)\)
Here, 0.8 = specific gravity of unhydrated cement
D. Determination of Heat of Solution (Hydrated Cement):
1.Sample Preparation:
- Open a sealed vial, discard wax and glass, grind cement passing through an 850 µ IS sieve.
- Weigh 4.2 g of cement and another 7 g for ignition loss.
- Conduct the same procedure as before to determine heat of solution for hydrated cement.
2.Equation for Heat of Solution (Hydrated Cement):
Equation for Heat of Solution (Hydrated Cement)
\(Heat \, of \, Solution \, (kJ/kg) = \frac{heat \, capacity \times corrected \, temperature \, rise}{mass \, of \, sample \, corrected \, for \, ignition \, loss} - 1.7 \times (\phi_{0} - \phi)\)
Here, 1.7 = specific gravity of hydrated cement
E. Calculation of Ignition Loss:
Sample Heating:
- Heat the sample gradually to 900 °C, maintain at 900-950 °C for 3-4 hours, then cool in a desiccator with anhydrous calcium chloride.
Observation of Heat of Hydration of Cement Test
| Heat of Solution | Sample 1 | Sample 2 | Sample 3 |
|---|---|---|---|
| Zinc oxide | |||
| Unhydrated cement | |||
| Hydrated cement |
Calculation of Heat of Hydration of Cement Test
Heat of Hydration of Cement Equation
\(Heat \; of\; hydration\; of \; cement = \\\\heat \; of\; solution\; of\; unhydrated\; cement\; - \; heat\; of \; solution \; of\; hydrated\; cement\)
Conclusion of Heat of Hydration of Cement Test
Heat of hydration test gives the heat of hydration of the cement sample which is crucial for quality control in some of the construction applications like mass concreting and cold weather concreting.
Hydration Test Procedure
| Step | Description |
|---|---|
| 1 | Prepare cement-water mix |
| 2 | Fill vials & seal with cork and wax |
| 3 | Measure unhydrated cement |
| 4 | Measure hydrated cement |
| 5 | Calculate heat of solution for both cements |
| 6 | Conduct loss on ignition test |
| 7 | Ignite ZnO & assemble calorimeter |
| 8 | Record temperature readings |
| 9 | Plot graph for heating/cooling rates |
| 10 | Calculate heat capacity |
F.A.Q.
Heat of Hydration Test of Cement
The test determines the heat evolved during the hydration process of cement, crucial for understanding temperature changes in concrete and its impact on strength and durability.
The evolution of heat increases the concrete temperature, impacting mass concreting where the interior may experience higher temperatures than the surface, affecting strength and potentially causing shrinkage cracks.
Yes, in cold weather conditions, the heat generated can be advantageous, provided the concrete is warm during placement and excessive heat loss is prevented.
The test involves multiple steps: preparation of cement samples, determination of heat of solution for unhydrated and hydrated cement, and ignition loss calculation, among other procedures, carried out meticulously in a controlled environment.
The early strength development is primarily due to the hydration of C3S (tricalcium silicate) in cement.
