A cement hardening-reaction experiment is an experiment that investigates the hydration reactions, heat generation, setting, hardening, and strength development that occur when water is added to cement.
Cement does not harden simply by drying; instead, cement minerals chemically react with water and generate hydrates, causing the material to harden.
Therefore, the progress of hardening is greatly affected by the amount of water, temperature, curing conditions, mixing condition, age, and other factors.
In a discussion of cement hardening reactions, it is not sufficient simply to write that “it hardened,” “the temperature increased,” or “the strength increased.”
It is necessary to explain why heat is generated, how setting differs from hardening, how C-S-H gel and calcium hydroxide are related to strength, and why a high water-cement ratio tends to reduce strength.
This article clearly explains, as examples of discussions that can be used in laboratory reports on cement hardening-reaction experiments, hydration reactions, heat generation, setting, hardening, C-S-H gel, ettringite, calcium hydroxide, water-cement ratio, curing, age, compressive strength, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of cement hardening-reaction results obtained in inorganic materials chemistry experiments, materials chemistry experiments, construction materials experiments, and ceramics experiments at universities and similar institutions.
For the actual type of cement, water-cement ratio, mixing conditions, curing conditions, strength-test method, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is the Hardening Reaction of Cement?
- Main Items to Include in the Results
- What Is a Hydration Reaction?
- Why Cement Generates Heat
- Difference Between Setting and Hardening
- C-S-H Gel and Strength Development
- Discussion of Ca(OH)2 Formation
- Discussion of Ettringite Formation
- Effect of the Water-Cement Ratio
- Discussion When the Amount of Water Is Too Small
- Effect of Curing Conditions
- Age and Strength Development
- Discussion of the Heat-Generation Curve
- Effect of Temperature
- Effects of Supplementary Materials and Admixtures
- Relationship Between Voids and Strength
- Drying Shrinkage and Cracking
- Discussion of Strength Testing
- Causes of Error in Cement Hardening-Reaction Experiments
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Cement Hardening Reactions
- Summary
What Is the Hardening Reaction of Cement?
The hardening reaction of cement is the process in which mineral components in cement react with water to generate hydrates, causing the material to lose fluidity and become hard.
Cement does not harden simply by losing water and drying.
New solid products are formed through hydration reactions, and these products bind the particles together, causing hardening.
Cement hardening includes setting, in which fluidity is lost in the early stage, and hardening, in which mechanical strength subsequently increases.
Setting is related to workability, while hardening is related to strength development.
In experiments, the progress of hydration reactions can be discussed from temperature changes, setting time, mass changes, strength, appearance changes, and other observations.
Example Discussion:
When water is added to cement, hydrates are generated through hydration reactions and the material gradually loses fluidity and hardens.
This is not simply drying, but is caused by chemical reactions between cement minerals and water.
Therefore, the progress of hardening is strongly affected by the amount of water, temperature, curing conditions, age, and other factors.
Main Items to Include in the Results
In a cement hardening-reaction experiment, organize the type of cement used, water-cement ratio, mixing time, specimen shape, temperature changes, setting time, curing conditions, age, strength-test results, appearance, cracking, mass changes, and other information.
Particularly when dealing with heat generation or strength development, it is important to organize the results with a clear time axis.
Main Items to Include in the Results
- Type of cement used
- Water-cement ratio
- Amount of water and cement
- Mixing time
- Dimensions of the specimen
- Fluidity or workability
- Initial setting time
- Final setting time
- Temperature change
- Presence or absence of a heat-generation peak
- Curing temperature
- Curing humidity
- Age
- Compressive strength or flexural strength
- Presence or absence of cracks or defects
- Mass change
- Causes of error and points for improvement
Example of How to Write the Results:
When water was added to the cement and the mixture was mixed, its fluidity decreased over time and the sample set.
In addition, the sample temperature increased some time after mixing, confirming heat generation associated with hydration reactions.
In the specimens after curing, strength increased with increasing age, suggesting that the hardened structure became denser as the hydration reactions progressed.
What Is a Hydration Reaction?
A hydration reaction is a reaction in which cement minerals react with water and generate hydrates.
Portland cement mainly contains minerals such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite.
These react with water to produce C-S-H gel, calcium hydroxide, ettringite, and other products.
C-S-H gel generated through hydration reactions contributes greatly to strength development in hardened cement.
Calcium hydroxide forms as crystals and exists within the hardened body.
Ettringite is also related to initial setting and volume changes.
These products fill the spaces between cement particles and form the hardened body.
Example Discussion:
When water is added to cement, cement minerals undergo hydration reactions and form hydration products such as C-S-H gel and Ca(OH)2.
C-S-H gel fills the spaces between cement particles and forms the framework of the hardened body, thereby contributing greatly to strength development.
Therefore, the increase in strength with age was considered to result from the progress of hydration reactions and an increase in the amount of hydrates.
Why Cement Generates Heat
Cement hydration reactions are exothermic reactions.
When cement minerals react with water and generate hydrates, heat is released as a result of the chemical reactions.
Therefore, when the temperature of cement paste or mortar is measured, a temperature increase may be observed after mixing.
The amount of heat generated and the timing of the heat-generation peak change depending on the type of cement, water-cement ratio, temperature, admixtures, fineness of the particles, and other factors.
Cement with fast initial reactions tends to show a temperature increase at an earlier stage.
In large specimens or under insulated conditions, heat escapes less readily and the temperature increase may appear larger.
Example Discussion:
The temperature of the cement paste increased over time because the hydration reaction is exothermic.
Heat was generated during the process in which cement minerals reacted with water and formed hydrates, causing the sample temperature to increase.
The degree of heat generation is considered to be affected by reaction rate, water-cement ratio, sample amount, and ease of heat dissipation.
Difference Between Setting and Hardening
Setting is the process in which cement paste loses fluidity and begins to retain its shape.
Initial setting is an important point at which workability begins to be lost, although the material has not yet developed full strength.
Final setting is the state in which the paste has hardened further and begins to show a certain level of resistance.
Hardening is the process in which hydration reactions continue after setting and mechanical strength increases.
In other words, setting is mainly related to changes in fluidity, whereas hardening is related to strength development.
| Item | Meaning | Point for Discussion |
|---|---|---|
| Setting | Process in which fluidity begins to be lost | Related to workability and initial reactions |
| Hardening | Process in which strength develops | Related to hydrate formation and densification of the structure |
Example Discussion:
The stage at which the cement paste lost fluidity was setting, and the subsequent process in which strength increased was hardening.
The sample became harder over time because hydrates were generated through hydration reactions and the bonds between particles became stronger.
Setting and hardening are continuous phenomena, but they must be distinguished in terms of loss of workability and strength development.
C-S-H Gel and Strength Development
C-S-H gel is an abbreviation for calcium silicate hydrate and is one of the most important hydration products for strength development in hardened cement.
C-S-H gel is a finely divided product with low crystallinity and forms so as to fill the spaces between cement particles.
This gel forms the framework of the hardened body and supports its strength.
As hydration reactions proceed, the amount of C-S-H gel increases, voids are filled, and the structure of the hardened body becomes denser.
Therefore, strength tends to increase as age increases.
However, if there is insufficient water or drying occurs, the hydration reactions do not readily proceed, and insufficient formation of C-S-H gel may result in lower strength.
Example Discussion:
The increase in compressive strength with age was considered to result from an increase in C-S-H gel produced by hydration reactions, which bonded the cement particles together.
C-S-H gel forms the framework of the hardened body and fills the voids, making the structure denser.
Therefore, the amount and distribution of C-S-H gel are strongly related to strength development.
Discussion of Ca(OH)2 Formation
In cement hydration reactions, calcium hydroxide Ca(OH)2 is generated together with C-S-H gel.
Ca(OH)2 may exist as relatively large crystals within the hardened body.
Although its contribution to strength is not as large as that of C-S-H gel, it contributes to maintaining the high alkalinity of the hardened cement.
High alkalinity may play a role in passivating reinforcing steel in reinforced concrete and suppressing corrosion.
On the other hand, Ca(OH)2 may also be involved in leaching and carbonation.
When discussing hydration products, it is useful to distinguish the roles of C-S-H gel and Ca(OH)2.
Example Discussion:
In cement hydration reactions, Ca(OH)2 is generated together with C-S-H gel.
Ca(OH)2 is related to the high alkalinity of the hardened body, but C-S-H gel makes the greater contribution to strength development.
Therefore, when discussing increases in strength, densification of the structure by C-S-H gel should be considered as the main factor.
Discussion of Ettringite Formation
Ettringite is a needle-like hydrate formed through the involvement of aluminate components in cement, gypsum, and water.
It forms during the early stages of hydration and is related to regulation of setting and formation of the initial structure.
Gypsum is added to cement to suppress the rapid reaction of tricalcium aluminate and prevent abnormal setting.
Ettringite is a useful hydration product in the early stage, but under certain conditions it may also be related to expansion and cracking.
If the initial setting was too rapid or large volume changes were observed in the experiment, the effects of aluminate reactions and ettringite formation can be discussed.
Example Discussion:
In the initial reactions of cement, aluminate components react with gypsum and water to form ettringite.
Ettringite formation is related to initial structure formation and progression of setting.
Gypsum suppresses the rapid reaction of tricalcium aluminate and is considered to regulate the setting time.
Effect of the Water-Cement Ratio
The water-cement ratio is the ratio of the mass of water to the mass of cement.
The smaller the water-cement ratio, the fewer voids tend to remain after hardening and the higher the strength tends to become.
On the other hand, a larger water-cement ratio increases fluidity during mixing, but excess water more readily leaves voids after evaporation.
In a hardened body with many voids, the amount of actual solid material supporting the load decreases and the strength becomes lower.
Permeability also increases and durability tends to decrease.
Therefore, the water-cement ratio is a very important condition when considering strength development and durability.
Water-cement ratio = Mass of water / Mass of cement × 100
Example Discussion:
The lower strength of the sample with a high water-cement ratio was considered to result from excess water leaving voids after hardening.
A hardened body with many voids has a coarse structure and contains less solid material to support the load.
On the other hand, in the sample with a lower water-cement ratio, fewer voids remained and bonding by C-S-H gel became denser, resulting in higher strength.
Discussion When the Amount of Water Is Too Small
Although a lower water-cement ratio tends to produce higher strength, if the amount of water is too small, mixing may become insufficient and many unhydrated cement particles may remain.
Because water is required for hydration reactions, the reactions do not proceed sufficiently when the amount of water is extremely small.
In addition, poor workability makes it more likely that voids or unfilled regions will remain inside the specimen.
As a result, strength may decrease even though the water-cement ratio is low.
The amount of water must be set by considering the balance between strength and workability.
Example Discussion:
One possible reason why the strength was not as high as expected under a low water-cement-ratio condition is poor mixing caused by insufficient water and the presence of unhydrated cement particles.
Hydration reactions require water, and if the amount of water is too small, formation of C-S-H gel does not proceed sufficiently.
In addition, low fluidity may leave voids inside the specimen and lead to reduced strength.
Effect of Curing Conditions
Curing is the process of maintaining an appropriate temperature and humidity so that hydration reactions in the hardened cement can proceed sufficiently.
Because cement hydration reactions require water, drying during the early stages of hardening makes the reactions more likely to stop.
As a result, strength development becomes insufficient.
Under moist curing or water curing, moisture is retained, making hydration reactions easier to proceed and strength more likely to increase.
On the other hand, under dry conditions, moisture is lost from the surface and cracking or shrinkage may occur.
Curing conditions greatly affect strength development and durability.
Example Discussion:
The higher strength of the specimen subjected to moist curing was considered to result from sufficient retention of the moisture required for hydration reactions.
Cement hardening proceeds through hydration reactions rather than drying, so moisture retention during curing is important.
Under dry conditions, hydration reactions become insufficient and voids or shrinkage cracks may form, reducing strength.
Age and Strength Development
Age refers to the time or number of days elapsed since water was added to the cement.
The strength of hardened cement tends to increase with age.
This is because hydration reactions proceed over time, increasing hydration products such as C-S-H gel and making the structure denser.
At early ages, minerals with fast reaction rates contribute greatly to strength development.
At later ages, more slowly reacting minerals also hydrate and the strength increases further.
However, if the curing conditions are poor or moisture is insufficient, sufficient strength may not be obtained even at a long age.
Example Discussion:
The increase in compressive strength with increasing age was due to the progress of hydration reactions and an increase in hydration products such as C-S-H gel.
Hydration products fill the spaces between cement particles and make the structure of the hardened body denser, thereby increasing strength.
However, if hydration reactions are inhibited by drying or other factors, strength development may remain insufficient even at a long age.
Discussion of the Heat-Generation Curve
When the heat generated by cement hydration is recorded against time, changes in the heat-generation rate may be observed.
Immediately after mixing, heat is generated by initial reactions, followed by an induction period during which the reaction temporarily becomes slower, after which heat generation increases during the main reaction period.
This heat-generation behavior is related to setting and early strength development.
If the heat-generation peak appears early, the reaction may be progressing rapidly.
On the other hand, if the heat-generation peak is delayed, the effects of low temperature, admixtures, retarders, the water-cement ratio, or other factors may be involved.
Temperature-measurement results provide clues for estimating the progress of hydration reactions.
Example Discussion:
If the temperature increase became large not immediately after mixing but after a certain period, an induction period may have existed in the cement hydration reaction.
After the induction period, hydration of calcium-silicate minerals progressed and heat generation increased together with formation of C-S-H gel.
The timing and magnitude of the heat-generation peak are related to reaction rate, setting, and early strength development.
Effect of Temperature
In general, the higher the temperature, the faster the hydration reactions of cement proceed.
Therefore, under high-temperature conditions, setting may occur earlier and early strength may become higher.
On the other hand, under low-temperature conditions, the reaction rate becomes slower and setting and strength development may be delayed.
However, if hydration proceeds rapidly at high temperature, the distribution of products may become nonuniform or long-term strength may increase less readily.
In addition, in large concrete structures, heat of hydration may accumulate internally and cause thermal cracking.
Temperature affects both initial reactions and long-term performance.
Example Discussion:
Setting occurred earlier under high-temperature conditions because the hydration-reaction rate increased.
At higher temperatures, the reactions between cement minerals and water proceed more readily and initial hydration products form more quickly.
However, rapid hydration may lead to internal temperature rise or nonuniformity of the structure, so control of the curing temperature is important.
Effects of Supplementary Materials and Admixtures
Supplementary materials such as fly ash, ground granulated blast-furnace slag, and silica fume, as well as admixtures such as water-reducing agents, retarders, and accelerators, may be added to cement.
These affect hydration reactions, fluidity, heat generation, setting time, and strength development.
If additives were used in the experiment, their effects should be included in the discussion.
For example, a water-reducing agent makes it possible to reduce the amount of water while maintaining the same fluidity, making it easier to achieve high strength with a low water-cement ratio.
Retarders delay setting, while accelerators speed up initial reactions.
Supplementary materials may affect long-term strength and durability.
Example Discussion:
If the setting time or strength development changed in a sample containing an admixture, the additive was considered to have affected the dispersion state of cement particles or the hydration-reaction rate.
A water-reducing agent makes it easier to maintain fluidity with a smaller amount of water and reduce voids after hardening.
Therefore, strength may have increased under conditions where the water-cement ratio could be kept low.
Relationship Between Voids and Strength
The strength of hardened cement is greatly affected by the amount of internal voids.
When there are many voids, there is less solid material supporting the load and stress concentration becomes more likely.
Therefore, hardened bodies with high porosity tend to have lower strength.
Voids are generated by evaporation of excess water, entrainment of air during mixing, insufficient compaction, insufficient hydration, and other factors.
As hydration reactions proceed and C-S-H gel forms, some of the voids are filled and the structure becomes denser.
When discussing strength development, it is important to consider both hydrate formation and the void structure.
Example Discussion:
The sample with low strength may have contained many internal voids.
In a hardened body with many voids, there is less solid material supporting the load and stress concentration also occurs more easily, reducing compressive strength.
If the water-cement ratio was high or compaction was insufficient, voids may have increased because of excess water or entrained air.
Drying Shrinkage and Cracking
Hardened cement may shrink when it loses moisture through drying.
This is called drying shrinkage.
When drying shrinkage is large, tensile stress develops internally or on the surface and may lead to cracking.
Cracks cause reductions in strength and durability.
Particularly when rapid drying occurs during the early stages of hardening, not only does the moisture required for hydration reactions become insufficient, but surface-shrinkage cracking also becomes more likely.
Therefore, maintaining moist conditions during initial curing is important.
If cracks are observed in an experiment, drying, shrinkage, and insufficient curing can be discussed.
Example Discussion:
One possible cause of cracks on the specimen surface is shrinkage caused by drying during curing.
If moisture is rapidly lost during the early stage of hardening, the surface shrinks and tensile stress develops because of the difference in deformation between the surface and interior.
As a result, drying-shrinkage cracks may form and reduce strength and durability.
Discussion of Strength Testing
The strength of hardened cement is evaluated by compression tests or flexural tests.
Compressive strength is an indicator of how much compressive load a specimen can withstand.
Strength is affected by age, water-cement ratio, curing conditions, voids, specimen dimensions, loading conditions, and other factors.
If the end surfaces of the specimen are not smooth or the load is applied unevenly, the measured strength may become lower.
If bubbles or cracks exist inside the specimen, fracture occurs earlier.
When discussing the results of strength testing, not only the material conditions but also specimen preparation and loading conditions should be checked.
Example Discussion:
Possible causes of variation in compressive strength among the samples include differences in the water-cement ratio and curing conditions as well as internal voids and nonuniformity of the specimen end surfaces.
If bubbles or cracks are present in a specimen, stress concentration occurs when a load is applied and fracture occurs more readily.
Therefore, preparation accuracy and loading conditions are also important in strength testing.
Causes of Error in Cement Hardening-Reaction Experiments
Causes of error in cement hardening-reaction experiments include errors in measuring the amount of water, errors in the amount of cement, insufficient mixing, differences in mixing time, temperature changes, differences in curing humidity, variation in specimen dimensions, entrainment of air, insufficient compaction, and differences in the timing at which measurement begins.
Because cement reactions progress over time, the timing of operations is also important.
In heat-generation measurements, errors may occur because heat escapes to the surroundings, the thermometer position is different, or the sample amount varies.
In strength tests, specimen shape, end-surface treatment, loading rate, and curing conditions affect the results.
Causes of error are easier to organize when divided into preparation, curing, measurement, and strength testing.
Example Discussion:
Possible causes of variation in the experimental results include deviations in the water-cement ratio, insufficient mixing, entrainment of air, and differences in curing conditions.
Because cement hardening proceeds through hydration reactions, even small differences in the amount of water, temperature, or humidity affect setting time and strength.
In addition, specimen dimensions, smoothness of the end surfaces, and loading conditions may also be causes of error in strength testing.
When the Results Can Be Considered Good
A cement hardening-reaction experiment can be considered to have produced good results when fluidity decreases over time after mixing, a temperature increase is observed, and strength increases with age.
These results indicate that hydration reactions, heat generation, setting, hardening, and strength development progressed continuously.
In addition, if specimens with lower water-cement ratios show higher strength and specimens subjected to moist curing show good strength development, the results can be considered reasonable and consistent with void content and the progress of hydration reactions.
It is important to relate the trends in the results to hydrate formation, densification of the structure, void content, and curing conditions.
Example Discussion:
In this experiment, the sample temperature increased after mixing and setting and hardening progressed over time.
This was because the cement minerals underwent hydration reactions and formed hydration products such as C-S-H gel while generating heat.
In addition, because strength increased with age, the structure of the hardened body was considered to have become denser as the hydration reactions progressed.
Example Discussions When the Experiment Did Not Go Well
When a cement hardening-reaction experiment does not go well, possible causes should be considered from results such as delayed setting, low heat generation, low strength, cracking, variation among samples, or failure to harden.
Organizing the causes according to water-cement ratio, mixing condition, curing conditions, temperature, specimen preparation, and measurement method makes the discussion easier.
Example Discussion:
One possible reason why the strength was lower than expected is that the water-cement ratio was high and many voids caused by excess water remained after hardening.
In a hardened body with many voids, the structure becomes coarse and there is less solid material supporting the load, so compressive strength decreases.
In addition, if drying occurred during curing, hydration reactions may not have progressed sufficiently and C-S-H gel formation may have been insufficient.
Another Example Discussion:
Possible reasons why heat generation was not clearly observed include the small sample amount, which allowed generated heat to escape easily to the surroundings, and an inappropriate thermometer position.
In addition, under low-temperature conditions, the hydration-reaction rate becomes slower and the temperature increase may appear small.
Therefore, in heat-generation measurements, the sample amount, insulation conditions, thermometer position, and measurement interval must be standardized.
How to Write Points for Improvement
In a discussion of cement hardening reactions, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to material measurement, mixing, molding, curing, measurement, and strength testing.
Improvements to Material Measurement and Mixing
- Measure the masses of water and cement accurately
- Keep the water-cement ratio consistent
- Keep the mixing time constant
- Avoid insufficient mixing
- Keep the material temperature consistent
- Accurately measure the amounts of supplementary materials and admixtures
Improvements to Molding and Curing
- Keep specimen dimensions consistent
- Mold the specimens while avoiding air entrainment
- Compact sufficiently
- Keep the curing temperature constant
- Prevent drying
- Accurately standardize the age
Improvements to Measurement and Strength Testing
- Keep the thermometer position constant
- Keep the heat-generation measurement interval consistent
- Prepare conditions that reduce heat loss
- Make the specimen end surfaces smooth
- Keep the loading rate constant
- Use multiple specimens and calculate the mean value
Example of How to Write Points for Improvement:
To accurately compare cement hardening reactions, it is necessary to standardize the water-cement ratio, mixing time, specimen dimensions, curing temperature, and curing humidity.
In addition, because internal voids and nonuniform end surfaces affect the results of strength testing, sufficient compaction during molding and smoothing of the end surfaces are important.
In heat-generation measurements, the sample amount and thermometer position should be standardized and the conditions under which heat escapes should be made as consistent as possible.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of cement hardening reactions, simply writing that “it hardened,” “heat was generated,” or “it became stronger” results in a superficial discussion.
A good discussion relates hydration reactions, hydration products, heat generation, setting, hardening, water-cement ratio, voids, and curing conditions.
| Superficial Discussion | Good Discussion |
|---|---|
| The cement hardened. | Cement minerals underwent hydration reactions, and hydration products such as C-S-H gel bonded the particles together, causing setting and hardening to proceed. |
| The temperature increased. | The hydration reactions of cement are exothermic, and heat was generated during hydrate formation, causing the sample temperature to increase. |
| It became stronger over time. | As age increased, hydration reactions progressed and C-S-H gel increased, filling voids and making the structure of the hardened body denser, thereby increasing strength. |
| It was weaker when more water was used. | When the water-cement ratio was high, excess water remained as voids after hardening and the structure became coarser, reducing compressive strength. |
| The results varied. | Differences in the water-cement ratio, mixing time, entrained air, curing humidity, specimen dimensions, end-surface treatment, and loading conditions may have affected strength and setting time. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of cement hardening reactions.
Adjust the necessary parts according to your own experimental results.
- Cement hardening proceeds through hydration reactions rather than drying.
- Hydration reactions generate hydration products such as C-S-H gel and Ca(OH)2.
- C-S-H gel forms the framework of the hardened body and contributes greatly to strength development.
- Cement hydration reactions are exothermic, and a temperature increase is observed after mixing.
- Setting is the process of losing fluidity, while hardening is the process of increasing strength.
- A high water-cement ratio leaves many voids after hardening and tends to reduce strength.
- Drying during curing inhibits hydration reactions and reduces strength development.
- As age increases, hydration reactions progress and the structure of the hardened body becomes denser.
- The heat-generation peak is related to hydration-reaction rate, setting, and early strength development.
- Strength-test results are also affected by voids, end-surface condition, and loading conditions.
Points to Check When Discussing Cement Hardening Reactions
Checking the following points before writing the report makes the discussion easier to write.
- Is it explained that cement hardening occurs through hydration reactions?
- Is heat generation related to hydration reactions?
- Are setting and hardening distinguished?
- Are C-S-H gel and strength development related?
- Are the roles of Ca(OH)2 and ettringite explained?
- Are the water-cement ratio and void content discussed?
- Is insufficient hydration caused by insufficient water or drying considered?
- Are curing conditions and strength development related?
- Is the increase in strength with age explained?
- Is the effect of temperature on reaction rate considered?
- Are causes of error in strength testing considered?
- Do the points for improvement correspond to the causes of error?
Summary
The hardening reaction of cement proceeds through hydration reactions in which cement minerals react with water and generate hydrates.
Cement does not simply harden by drying; setting and hardening occur through formation of hydration products such as C-S-H gel, Ca(OH)2, and ettringite.
Because hydration reactions are exothermic, the sample temperature may increase after mixing.
Formation of C-S-H gel is particularly important for strength development.
As age increases, hydration reactions proceed and C-S-H gel bonds the particles together and fills voids, making the structure of the hardened body denser.
On the other hand, if the water-cement ratio is too high, many voids caused by excess water remain and strength tends to decrease.
In addition, if the amount of water is too small or dry curing is used, hydration reactions may become insufficient.
In a report, rather than simply writing that “it hardened” or “heat was generated,” organize and discuss hydration reactions, heat generation, setting, hardening, C-S-H gel, Ca(OH)2, ettringite, water-cement ratio, voids, curing conditions, age, strength testing, causes of error, and points for improvement.
Cement hardening-reaction experiments are important experiments for understanding the relationship between chemical reactions and mechanical properties of inorganic materials.
