Cement Hydration: Products and Reactions

Cement hydration is the chemical process that turns cement powder and water into the hard stone of concrete: the water reacts with the clinker minerals, new compounds grow and interlock, the paste sets and gains strength, and the result carries the loads of buildings, bridges and dams: this article explains the products of hydration in complete technical detail: the clinker phases, the reactions, the kinetics, the heat, the setting and the practical consequences that every concrete engineer must understand.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes the Products.Hydration course module, dated October 2002, among its training files: the module teaches the chemistry of the cement products from the clinker composition to the hardened concrete: this article presents the same content in written form, so the reader gets the full map of the module: the phases, the numbers and the practical rules of the industry.

Why hydration matters to everyone in the industry: concrete is the most used man-made material on Earth, and its properties come directly from the hydration of the cement: the strength development, the heat of hydration, the setting time, the durability, the sulfate resistance and the winter concreting problems are all manifestations of the same chemistry: this article, like the module, explains the subject so that the engineer, the technician and the student all profit: hydration is the heart of cement science.

1. The Portland Cement Clinker: The Raw Material of the Reaction

Hydration begins with the clinker minerals, and the module opens the route from the clinker composition to the hydrated phases:

• The four major phases of the clinker: alite (C3S, 3CaO.SiO2), belite (C2S, 2CaO.SiO2), aluminate (C3A, 3CaO.Al2O3) and ferrite (C4AF, 4CaO.Al2O3.Fe2O3), plus the gypsum added at the grinding stage: the typical phase shares in Portland clinker are 50 to 70 percent alite, 15 to 30 percent belite, 5 to 12 percent aluminate and 5 to 15 percent ferrite: these proportions decide the properties of the product cement;
• The role of gypsum: pure Portland clinker would flash-set when mixed with water: gypsum (CaSO4.2H2O) is added at the cement mill, typically to reach 2 to 3.5 percent SO3 in the cement, to regulate the hydration of the aluminate: the gypsum is not a spectator: it is the speed regulator of the setting;
• The fineness and the surface: hydration happens on the surface of the cement particles: the finer the cement, the larger the specific surface (Blaine values of 280 to 480 square metres per kilogram for normal cements), and the faster and more complete the reaction: the module shows the relation between the Blaine surface and the rate of hydration and strength gain;

The module anchors every hydration concept to the concrete clinker composition: the reader who knows the alite from the belite can follow the whole chemical chapter, and the link back to the cement mill and the raw meal chemistry is always kept visible.

2. The Hydration of the Individual Clinker Phases

Each clinker mineral reacts with water in its own way, at its own speed and with its own products: the module teaches the four reactions separately, equation by equation:

• The hydration of alite (C3S): C3S plus water gives calcium silicate hydrate (C-S-H) and calcium hydroxide (CH, portlandite): the reaction is fast compared to the others, delivers the main share of the strength in the first 28 days, and produces the gel that binds the concrete: the typical C-S-H of a mature paste carries a lime-to-silica ratio near 1.7;
• The hydration of belite (C2S): the chemistry is similar, C2S plus water gives C-S-H and CH, but the reaction is much slower: alite delivers the early strength and belite the long-term strength: after months and years the belite contributes as much as the alite: the module compares the 1-day strength where alite wins and the 1-year strength where the two meet;
• The hydration of aluminate (C3A): the fastest and most exothermic of the four: without gypsum the aluminate reacts with water in minutes and causes the flash set: with gypsum it forms ettringite (AFt), and when the gypsum is consumed it continues to the monosulfate (AFm): the module covers the aluminate chemistry in detail because the sulfate resistance and the false set both live here;
• The hydration of ferrite (C4AF): the ferrite hydrates similarly to the aluminate but more slowly, forming iron-bearing analogues of ettringite and monosulfate: the ferrite contributes early heat and plays a role in the color and the hydration rate of the cement;

The per-phase teaching of the module builds the total hydration picture: the engineer who knows which phase gives the early strength and which gives the long-term strength understands the design of the cement product, and the sulfate resistance becomes understandable only through these equations.

3. The Cement-Water System: The Paste as a Whole

Real concrete never has a single phase reacting alone: the four minerals react together, their products interlock, and the paste is the result of the simultaneous reactions: the key concepts of the module:

• The hydration products in the paste: the C-S-H gel, the portlandite crystals, the ettringite, the monosulfate and the water-filled voids: in a mature paste of typical composition the C-S-H occupies roughly 50 to 60 percent of the volume, the portlandite 15 to 25 percent, and the remaining volume belongs to the AFm phases and the pores;
• The water-to-cement ratio: the chemically required water is about 0.22 to 0.25 parts per part of cement: any water above this remains in the paste as capillary pores: the strength of the concrete falls as the w/c ratio rises: a concrete at 0.4 w/c develops roughly double the strength of the same concrete at 0.6 w/c: the water is the quality control lever of the concrete;
• The degree of hydration: the fraction of the cement that has reacted: in a typical structure at 0.5 w/c about 60 to 80 percent of the cement hydrates within the first 28 days: the remaining particles react over the years: the degree of hydration is the link between the chemistry and the strength;
• The time scale of hardening: a few hours for the setting, 28 days for the standard strength, and years for the slow continuation of the reaction: the long-term gain of strength in old concrete is the slow tail of the belite hydration;

The module teaches the paste as a whole system because no single phase story explains the concrete: the reader of this chapter understands the interior of the paste, the porosity, the internal humidity and the degree of hydration, and that understanding is the base of the durable concrete design.

4. The Setting of the Cement Paste

The first visible event of hydration is the setting: the paste loses its fluidity and becomes a solid: the module explains the setting in physical and chemical terms:

• The stages of the setting: the initial set, when the paste can no longer be worked, and the final set, when the paste has acquired a measurable strength: the standard Vicat test values: initial set in the range of 45 to 200 minutes and final set within about 10 hours for normal cements: the European standard EN 197 demands an initial set above 45 minutes for the common strength classes;
• The chemical stages: the hydration begins with the rapid dissolution of the ions, follows with the induction (dormant) period of slow reaction while the pore water reaches saturation, then the acceleration period when the main C-S-H formation releases heat and the paste sets, and finally the deceleration period when the diffusion through the growing gel controls the reaction;
• The factors of the setting: the temperature, the fineness, the SO3 content and its balance with the aluminate: hot weather doubles the rates, cold weather slows them dramatically: the module presents the time-temperature curves and the influence of the admixtures;
• The false set and the flash set: the two pathological cases: the false set comes from the dehydrated gypsum in the mill and can be broken by continued mixing, while the flash set comes from the uncontrolled aluminate reaction and ruins the concrete: the module teaches the diagnosis that separates the two;

The setting is the first practical contract of the construction: the concrete must be placed before the initial set, and the joints and the forms must respect the setting schedule: the module trains the reader to follow the setting physically and chemically and to diagnose the abnormal cases with the correct tests.

5. The Heat of Hydration and Its Consequences

The binding reaction is strongly exothermic: every kilogram of cement releases about 200 to 400 kilojoules of heat during hardening, and in mass concrete that heat becomes a design problem:

• The level of the heat: typical heat release values are 120 to 200 joules per gram at 3 days and 230 to 330 joules per gram at 7 days for ordinary Portland cement at 20 degrees: the phases contribute unevenly: the aluminate releases roughly 1,000 to 1,500 joules per gram, the alite 500 to 700, the ferrite 400 to 500 and the belite 250 to 300;
• The temperature rise of the pour: in a large mass the core can reach 60 to 80 degrees Celsius: the temperature difference between the core and the surface creates thermal stresses, and the early-age contraction cracks follow: the module documents the classic cases of the large dams and the heavy foundations;
• The control of the thermal cracking: the low-heat cements, the substitution of the clinker by supplementary materials, the limits on the placing temperature, the insulation of the surface, the cooling pipes and the slow stripping of the forms: the module gives the mass concrete control plan with the core temperature measurement and the gradient limits;
• The cold and the hot casting: at low temperatures the hydration slows and the concrete can freeze before it gains strength: at high temperatures the hydration accelerates and the final strength of the hot-cast concrete is often lower than the same mix cast at normal temperature: the module explains the summer and the winter practices of the industry;

The heat of hydration is the welcome energy of the setting and the enemy of the mass pour: the module teaches both faces with the numbers and the temperature control practice of the largest constructions: the casting season and the thermocouples of the building are part of the same story.

6. The Kinetics of Hydration: The Rate of Hardening

Hydration is a process in time, and the rate of the reaction controls the development of the strength, the heat and all the properties: the module teaches the kinetics:

• The rate laws: the early reaction is controlled by the chemistry at the particle surface, and the later reaction by the diffusion of water and ions through the growing gel layer: the temperature sensitivity follows the Arrhenius law: in the normal range the rate roughly doubles with every 10 degrees of temperature rise;
• The strength development: the standard curve of the industry: about 50 percent of the 28-day strength is reached at 3 days and 75 to 80 percent at 7 days, and the remaining 20 to 25 percent appears between 7 and 28 days: the module gives the development curves for the common strength classes;
• The maturity concept: the strength of a concrete cast at a certain temperature history can be predicted by the maturity method: the product of the time and the temperature above a reference point: the module teaches the equivalent age calculation that the precast industry uses every day;
• The measurement methods: the isothermal calorimetry with its characteristic heat flow curve, the measurement of the non-evaporable water, the chemical shrinkage and the quantitative X-ray analysis: the module presents the laboratory methods that the industry uses to understand the rate of hydration;

The kinetics is the frame of the practical: the age of the form stripping, the precast cycle, the 1-day and the 28-day strengths are all rates: the module teaches the engineer to predict the strength by time and temperature and to know when the hydration is delayed or stopped.

7. The Products of Hydration and the Quality of the Concrete

The hydrates decide the quality of the concrete, and the module connects the chemistry directly to the quality parameters that the constructor controls:

• The C-S-H and the strength: the C-S-H is the glue of the concrete: its quantity, density and distribution are the source of the mechanical strength: the high-performance concretes of low w/c ratio owe their performance to the denser C-S-H and the finer pores;
• The calcium hydroxide and the durability: the portlandite is the soluble component: it reacts in carbonation, in acid attack and in leaching water: but it is also the buffer that keeps the pore solution at pH 12 to 13, and that alkalinity protects the steel reinforcement from corrosion: the module explains the double role of the portlandite;
• The porosity and the transport: the hydration fills the pores over time, and the transport of water, chlorides and carbon dioxide into the concrete depends on the porosity: the module teaches the relation between the w/c ratio, the pore structure and the durability: the skin of the concrete is where the durability is won or lost;
• The bleeding and the surface quality: the water that rises to the surface during the setting, the finish of the slabs and the bond between the paste and the aggregate: the module covers the practical quality issues that the site engineer sees every day;

The visible performance of the concrete and the invisible chemistry of the hydration are the same story: the module ties the strength cube to the C-S-H, the reinforcement protection to the portlandite, and the durability to the pore structure: this micro-to-macro view is exactly what the Products.Hydration chapter delivers.

8. The Special Cements and the Hydration Variants

The module also covers the hydration of the non-Portland and the special-purpose cements:

• The high alumina cement: the hydraulic phase is the monocalcium aluminate: it reacts fast, delivers rapid strength and high heat, and then suffers the conversion of the hydrates in warm moist conditions with a loss of strength: the module explains why the use of the CAC is today limited to specific applications;
• The sulfate-resisting cements: the low aluminate cement, typically below 3 percent C3A: the reduction of the aluminate reduces the formation of the expansive ettringite in sulfate-bearing environments: the module covers the hydration mechanism and the application in soils and groundwater;
• The white cement: the clinker is burned from the low-iron raw materials, and the hydration chemistry is the same while the color of the product is white: the module explains the finish applications and the color stability;
• The blended cements and the supplementary materials: the fly ash, the slag and the natural pozzolans react with the portlandite to form additional C-S-H: the modern cements carry 20 to 50 percent of these substitutions: the module teaches the synergy of the blends and their hydration at the late ages;

The diversity of the cement products flows from the hydration chemistry: the module closes the tour with these families, and the reader recognizes that the modern cement is not a single chemical species but a family of hydration options tailored to the service of the concrete.

9. The Hydration in the Practice of the Concrete Works

The final chapter of the module returns the chemistry to the job site:

• The standards and the acceptance: the European and the national standards set the requirements for the setting time, the heat of hydration and the composition: the module is written in the frame of the EN 197 cement standard and the concrete test standards, so the reader works with the same numbers of the specification;
• The destructive testing: the cubes and the cylinders of the concrete are tested at 1, 3, 7 and 28 days: the interpretation of the results, the maturation curves and the decision rules for the acceptance: the module teaches the reading of the compression test reports;
• The admixtures: the superplasticizers, the retarders and the accelerators change the hydration kinetics with a few grams per kilogram of cement: the module covers the interaction of the admixtures with the hydration and their effect on the workability and the strength;
• The curing: the fresh concrete must be kept moist and at a reasonable temperature: the wet curing of the slabs, the membrane curing compounds and the minimum curing periods of the standards: the module states honestly that the curing is the cheapest and most effective quality control of the concrete;

The engineer closes the module with the complete picture: the knowledge of the hydration turns the mix design from a recipe into a controllable chain: the module delivers the practical field experience of the construction industry together with the chemistry of the products.

10. The Frequently Asked Questions

How much water does cement need to hydrate completely?

The chemically bound water of the Portland cement hydration is approximately 0.22 to 0.25 parts per part of cement: in practice the mixing water is higher, 0.4 to 0.6 parts, to provide the workability and to fill the space: the excess water remains in the paste as pores and reduces the strength: the lower the water-to-cement ratio, the stronger the concrete, within the limits of the workability and the complete hydration.

Why does cement set faster in summer?

Hydration is a chemical reaction and its rate increases with the temperature: the rate roughly doubles for every 10 degrees in the normal range: the hot summer accelerates the setting and the hardening, so the concrete must be placed faster and the retarding admixtures are often used: the heat of the mass pours is also higher in summer, and the cooling of the mix components becomes part of the practice.

What happens if fresh concrete freezes?

Below zero the pore water freezes and the hydration practically stops: the expanding ice damages the young paste and the concrete may never reach its design strength: the winter practice protects the concrete with insulation, heated enclosures and accelerating admixtures, and no fresh concrete should be left exposed to the frost: even at 5 degrees the hydration becomes very slow.

Why is gypsum added to the cement?

To regulate the fast hydration of the aluminate: the gypsum dissolves in the mixing water and the aluminate reacts to form ettringite instead of flash-setting: the typical SO3 content of the cement is 2 to 3.5 percent, and the correct balance with the aluminate content is one of the fine arts of the cement mill.

How much heat does the concrete release?

For a typical concrete with 350 to 400 kilograms of cement per cubic metre, the hydration heat amounts to roughly 200 to 350 megajoules per cubic metre over the first days: the core of a large mass can rise 40 to 60 degrees above the placing temperature: the mass concrete design controls the rise with the low-heat cements, the cooling pipes and the insulation.

Does hydration continue after 28 days?

Yes: the standard tests stop at 28 days, but the hydration continues for months and years at a decreasing rate: the belite in particular keeps reacting and old concrete keeps gaining strength: the practical rule of the industry is that concrete at 90 days reaches about 110 to 120 percent of its 28-day strength, and the durability of the structure improves over time if the water and the environment are not aggressive.

11. Conclusion

Cement hydration is the transformation chain of the industry: from the clinker minerals through the C-S-H and the portlandite, through the setting and the heat, from the first hours to the years of hardening: the Products.Hydration module of the Complete Cement Technical Package follows that chain with the phases, the reactions, the water, the kinetics and the concrete practice: this article has presented the full skeleton of that course, and the reader now owns the map of the hydration: the heart of the cement science.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the presentations: $249.99 one-time: instant download, lifetime access) holds the Products.Hydration module of October 2002 and the connected chapters of the same course: the composition, the hydration, the testing and the properties of the cement products: the full chain of the cement chemistry under one purchase: the engineer who makes the hydration his own has the superior understanding of the field: the click of the PayPal button puts the century of the cement chemistry on the desk.

Qualitative and Quantitative Control of the Hydration

The hydration of the cement is the chemistry of the setting: the control of the plant and the laboratory watch the same events from the two sides:

• The calorimetry of the laboratory: the heat flow profile of the paste against the hours: the induction period, the acceleration peak, the deceleration: the profile speaks about the fineness and the sulfate of the clinker: the laboratory reference for the plant batches;
• The physical evidence: the setting time, the soundness, the strength development of the mortar: the data of the days 2, 7, 28: the control of the production is the control of the hydration rate of the cement sold;
• The chemistry tools: the XRD for the clinker phases of the paste, the TGA for the calcium hydroxide and the carbonate read, the ESEM for the microstructure: the file shows which tools answer which question of the hydration;
• The practice of the quality: the sampling at the silos, the frequency of the mortar tests, the action limits for the setting and strength: the practical protocol of the hydration control in the plant;

The grand message of the chapter is that the hydration is not an abstract chemistry: the strength the contractor buys is the product of the hydration of the clinker phases with exactly the water and time of the specification: the file enables the quality chemist to judge the cement by its chemistry and the laboratory data, the two eyes of the cement quality control.

The Hydration in the Field: The Practical Diary

The diary of a concrete job is the diary of the hydration: the file gives the connection of the everyday practice to the chemistry:

• At the batch plant: the temperature of the delivered paste, the setting check at the site, the addition of the water at the second stage the Snowball; the restraint: the crew watches the temperature rise of the mix unlocking the early hydration and the slump control:
• The 28-day law: the standard practice cures the cubes to 28: the curve of the strength vs the days of the file (1, 2, 3, 7, 28, 90) shows the doubling rules: the strength doubles from 7 to 28 and the 50% of the 28-day strength appears: the crews decide the formwork on these laws:
• The admixtures: the plasticizers and the retarders change the hydration pace without the chemistry violation: the dosage windows versus the clinker and the climate; the compatibility cells of the file help the mixes at trial:
• The durability link: the hydration completeness gates the pores and the durability: the properly hydrated cement is the dense web: the curing days, the water-cement ratio of the hydration quality: the curing practice part of the hydration discipline;

The Hydration Stop: A Case Note of the Overheated Delivery

A practical case closes the hydration: a summer batch arrives at 33°C, sets in 2 h 20 instead of the 3 h 30 designed, and the crew loses the finishing window: the cause chain: the hot clinker at the mill, the water addition limited, the gypsum dihydrate partly dehydrated to the hemihydrate in the mill heat: the earlier setting of the paste: the remedy set in the file:

• The clinker cooled before the mill (the yard residence above the 72 h),
• The mill ventilation increased and the spray cooling applied to cap the outlet at 105°C,
• The SO3 blend adjusted for the summer stock,
• The delivery cooling and the retarder addition at the batch plant,

The file case shows the hydration event is never one cause: the temperature, the sulfate form and the admixture together set the clock of the setting: the knowledge of the file prevents the summer surprises of the job.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.