By Kayla Hanson
When you see the word hydration you might think of water, a sports drink commercial, or maybe you think of a hot, sunny summer day. Perhaps you even think of cement.
When it comes to concrete, hydration is just as critical as it is for humans. It is a series of chemical reactions that occurs when water and hydraulic cement come in contact. When water and cement are combined into cement paste, most of the cement grains immediately begin to dissolve, which initiates the hydration process. The reactions produce numerous new compounds, and as more cement hydrates, more water and cement is consumed and more compounds are produced. The compounds developing in the paste grow, spread and also begin to accumulate and interconnect. Eventually, the buildup of compounds results in stiffening, hardening and strength development, transforming plastic concrete into the strong, durable product we depend on every day. And it’s all thanks to hydration.
But hydration isn’t just about combining cement and water. Successful hydration, and the rate at which cement hydrates, depends on a variety of factors.
Understanding portland cement’s four main phases and the importance of sulfates
The four main mineral components formed during Portland cement clinker production are referred to as phases. They are similar to traditional compounds but contain traces of other elements and oxides. The four primary mineral phases in cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite, which are often abbreviated as C3S, C2S, C3A, and C4AF respectively.
In addition to the four phases, calcium sulfate dihydrate, or gypsum, is also a crucial component in any cement. Gypsum is blended with clinker during grinding to help regulate cement’s setting time. Without gypsum, plastic concrete would flash-set. Other calcium sulfates can also be used as regulators.
Other materials, often in the form of industrial byproducts, are added during clinker production to supplement the actions of the sulfates and the four main mineral phases. Adding these raw materials provides additional sources of calcium, iron, silica, alumina and sulfate to create a variety of cement types.
Cement types and their behaviors
Each cement type has a different chemical makeup that enables it to produce specific desired results when used correctly. The amounts and compositions of sulfates, main mineral phases and other materials used in manufacturing cement dictate cement’s type and properties, and determine its behavior.
Cements with relatively low to very low C3A contents offer the most resistance to sulfates. Cements with low C3A contents, low C3S contents and higher C2S contents offer lower heat of hydration. Cements with high to very high C3S contents are capable of developing early strength at faster rates.
ASTM C150 designates 10 types of cement:
Type I: Normal
Type IA: Normal, air entraining
Type II: Moderate-sulfate resistance
Type IIA: Moderate-sulfate resistance, air entraining
Type II (MH): Moderate heat of hydration and moderate-sulfate resistance
Type II (MH)A: Moderate heat of hydration, and moderate-sulfate resistance, air entraining
Type III: High early strength
Type IIIA: High early strength, air entraining
Type IV: Low heat of hydration
Type V: High sulfate resistance
ASTM C1157 describes six types of cement:
Type GU: General use
Type HE: High early strength
Type MS: Moderate-sulfate resistance
Type HS: High sulfate resistance
Type MH: Moderate heat of hydration
Type LH: Low heat of hydration
ASTM C595 identifies three main types of blended cement:
Type IS: Portland blast-furnace slag cement
Type IP: Portland-pozzolan cement
Type IT: Ternary-blended cement
Cement hydration products
The main products of cement hydration reactions are calcium silicate hydrate (CSH), calcium hydroxide (CH), and the AFt and AFm phases. The AFt and AFm phases found in hydrated cement are compounds of C3A, anhydrite and water. The most common AFt phase is ettringite and the most prevalent AFm phase is monosulfate.
Hydrated Portland cement paste usually consists of about 50% CSH and about 15-to-25% CH by mass. The majority of the strength exhibited by hydrated cement paste – specifically strength – can be attributed to CSH.
C3A is the most reactive of the four main cement mineral phases, but it only contributes slightly to early strength gain. C3A readily reacts with water in the cement paste to produce a gel rich with aluminate, a process that releases significant amounts of heat. The heat generated reduces quickly, typically only lasting a few minutes. The resulting gel, however, reacts with the various sulfates in cement, including gypsum, anhydrite, and hemihydrate, and produces ettringite. Ettringite development in early hydration stages helps control stiffening in plastic concrete. Days into hydration, ettringite is gradually consumed through reactions with C3A and is replaced with monosulfate.
C3S and water react to produce CSH and CH. C3S, also called alite, hydrates, reacts and hardens quickly, and is the largest contributor to concrete’s initial set and early strength development. C2S also reacts with water to create CSH and CH. However, C2S, or belite, reacts slowly relative to alite, and in turn is a large contributor to concrete strength gain beyond one week of age. C4AF is the least prevalent of the main four mineral phases and contributes little to strength development.
Mix designs’ roles in cement hydration kinetics
Set-accelerating admixtures offer options for increasing hydration rates, increasing early strength gain and decreasing the length of time to initial set and final set, often without affecting durability. Accelerators work by weakening the barrier around cement particles to allow mix water easier access to C3S and C2S phases, in turn increasing the minerals’ hydration rates. Accelerators are often used to offset the retarding effects of cold weather.
Conversely, set-retarding admixtures decrease hydration rates, decrease early strength gain and increase the length of time to initial and final set. Retarders slow hydration by inhibiting the formation and growth of certain hydration products. Retarders are often used to counteract the expedited curing rate caused by hot weather or to delay set to allow for special finishing techniques or difficult placing situations.
Set-accelerating admixtures and set-retarding admixtures both come in a variety of forms, but most often appear as liquid chemical additives. How these admixtures function depends on the composition, dose, time and sequence of their addition to the mix as well as ambient temperature and concrete temperature.
Concrete’s water-cementitious material ratio affects nearly every property of both plastic and cured concrete. For complete cement hydration, typically a 0.40 w/cm is required. The degree to which constituent materials hydrate depends on a variety of factors; however, if the w/cm is too high, excess water will remain in the concrete matrix. The additional water will remain until it evaporates, leaving void spaces that don’t contribute to compressive strength and greatly increase concrete’s susceptibility to a myriad of issues. Conversely, if the w/cm is too low, the mix water will be consumed or evaporated while unhydrated cement remains in the matrix. This offers no benefits to the concrete’s strength or durability, and adds cost to the precaster.
Supplementary cementitious materials (SCMs) are often added as a substitute for a portion of Portland cement in a concrete mix. SCMs exhibit behaviors similar to traditional cement. However, different types of SCMs enhance or inhibit certain hydration actions. Commonly used SCMs include fly ash, silica fume and ground granulated blast-furnace slag. SCMs like Class F fly ash and slag cement decrease concrete’s heat of hydration and increase setting time, while some natural pozzolans like calcined shale or clay and metakaolin decrease concrete’s heat of hydration but have no impact on setting time.
Temperature’s role in cement hydration kinetics
Ambient temperature during mixing, placing and curing plays a role in cement hydration kinetics. Although not all cements react the same way, typically as temperature increases, setting time decreases. In general, a fluctuation of 10 degrees Fahrenheit could change setting time by about 33%. Ideal curing temperatures typically range from 50-to-70 degrees. Temperatures below 50 degrees cause hydration to progress at a much slower rate. When temperatures fall below 40 degrees, early strength development is significantly hindered. However, when ambient temperatures exceed 70 degrees, hydration accelerates beyond a favorable rate and can lead to detrimental outcomes, including plastic shrinkage cracking, lower 28-day strengths and decreased durability.
Mix water temperature also plays a role in hydration, as it alters the temperature of the concrete. Mix water temperature can be adjusted, often through heating the water or adding ice to the mix water.
Curing techniques’ roles in cement hydration kinetics
Supplemental moisture added during curing replaces water lost through hydration and evaporation. Hydration rates remain largely unaffected when supplemental moisture is added. However, it does help ensure adequate moisture is available throughout hydration and curing. Inadequate moisture could result in cement remaining unhydrated, providing no beneficial properties to the concrete or concrete prematurely drying leading to small surface cracks. Additional moisture can be applied through spraying, saturated burlap or other coverings, fogging or immersion.
Moisture retention procedures rely on sheeting, coverings or membrane-forming compounds applied to products’ outer surfaces to trap moisture. Similar to supplemental moisture application, moisture retention has little impact on hydration rates. Instead, retention procedures help improve the curing environment by ensuring adequate moisture is available to sufficiently hydrate the cement.
Accelerated curing through heat and steam application increases hydration rates and strength development rates. Accelerated curing is especially beneficial in attaining early strength gain. These procedures are often used in cold weather concreting to create a more suitable environment for cement hydration.
In conjunction with external curing methods, internal curing involves the use of fully saturated lightweight aggregate, creating an internal water supply to help maintain sufficient moisture throughout hydration and curing. Internal curing has little impact on hydration rates. Rather, it helps ensure a favorable environment for optimum hydration to occur. Internal moist curing is often used with concretes containing high levels of cementitious materials.
As with any concrete mix design component, care needs to be taken to address the impacts different cement types, admixtures, SCMs, w/cm, curing techniques, and any other design and curing factors have on both concrete’s plastic characteristics and cured properties.
Kayla Hanson is a technical services engineer with NPCA.
Portland Cement Association, Design and Control Mixes, 15th Edition
Understanding Cement, Nicholas B. Winter
1. The abbreviations are not the substances’ true chemical formulas: tricalcium silicate (C3S) = 3CaO • SiO2; dicalcium silicate (C2S) = 2CaO•SiO2; tricalcium aluminate (C4AF) = 3CaO • Al2O3; tetracalcium aluminoferrite (C3A) = 4CaO • Al2O3•Fe2O3