Chapter III Concrete Making Materials Cementitious Binders”myyardimci.weebly.com/uploads/1/6/3/4/16347790/civ415_ch... · 2020. 3. 12. · CIV415 CONCRETE TECHNOLOGY Chapter III

CIV415 CONCRETE TECHNOLOGY

Chapter III

Concrete Making Materials “Cementitious Binders”

Assist.Prof.Dr. Mert Yücel YARDIMCI Spring, 2014/2015

Advanced Concrete Technology - Zongjun Li 1

Classification of binders


Organic binders (Can easily burned and can not stand with fire) Polymer and asphalt

Inorganic binders Different natural mineral

Non-hydraulic cement It does not mean that it does not need water! Means only that such cement cannot harden and thus gain strength in water. Typical examples are gypsum and lime! They have been used since 6000 BC.

Hydraulic cement Hydraulic cement can harden and gain strength in water.

The main difference in composition between two types of inorganic cements is that the hydraulic cement contains some amounts of clayey impurities (silicate composition). Examples of hydraulic cement :

• Hydraulic lime, • Pozzolan cement, • Portland cement.

Portland Cement (PC)


Portland cement (PC) concrete is the most popular and widely used building material, due to the availability of the basic raw materials all over the world, and its ease of use in preparing and fabricating all sorts of shapes.

There are two major drawbacks with respect to sustainability:

• About 1.5 tons of raw materials is needed in the production of every ton of PC, while, at the same time, about 1 ton of carbon dioxide (CO2) is released into the environment during the production.

• Concrete made of PC deteriorates when exposed to harsh environments, under either normal or severe conditions. Cracking and corrosion have significant influence on service behavior, design life, and safety.

To overcome these problems, other different cementitious materials have been developed recently. Two of them are geopolymer and magnesium phosphate cement (MPC).

Portland Cement (PC)


Summary of development of PC

1796 James Parker, England, Patent on a natural hydraulic cement. 1813 Vicat, France, Artificial hydraulic lime. 1824 Joseph Aspdin, England, Portland cement.

Portland cement was developed by Joseph Aspdin in 1824, so named because its color and quality are similar to a kind of limestone, Portland stone (Portland, England).

Manufacture of Portland cement


Portland cement is made by blending an appropriate mixture of limestone and clay or shale together, and by heating them to 1450◦C in a rotary kiln.

The capability of a modern rotary kiln can reach 10,000 metric tons daily.

Manufacture of Portland cement


The raw feed must have a uniform composition and be of fine enough size that reactions among the components can be completed in the kiln.

The burned clinker is ground with gypsum to form the familiar gray powder known as Portland cement.

The basic raw materials used for manufacturing Portland cement are limestone, clay, and iron ore.

Reactions in calcination process


Clay is mainly providing silicates (SiO2) together with small amount of Al2O3 and Fe2O3. The decomposition of clay happens at a temperature around 600◦C

Limestone (CaCO3) is mainly providing calcium (CaO) and is decomposed at 1000◦C

Iron ore and bauxite provide additional aluminum and iron oxide (Fe2O3), which help the formation of calcium silicates at low temperature. They are incorporated into row mix.

There are different temperature zones in a rotary kiln. At various temperatures between 1000 and 1450◦C, different chemical compounds are formed. The initial formation of C2S occurs at a temperature of around 1200◦C. C3S is formed around 1400◦C.

The final product from the rotary kiln is called clinker. Pulverizing the clinker into small sizes (

How fine the cement is…


Major Compounds of Portland Cement


Major Compounds of Portland Cement


It should be indicated that C3S and C2S occupy 68 to 75% of Portland cement.

Since the primary constituents of Portland cement are calcium silicates, we can define Portland cement as a material that combines CaO and SiO2 in such a proportion that the resulting calcium silicate will react with water at room temperature and normal pressure.


Developed by Bogue and adopted by ASTM C150

C, S, A, F, and S are weight percentage of corresponding oxide in a Portland cement such as what listed in Table 2-5.

valid only when A/F ≥ 0.64 valid only when A/F ≥ 0.64

Minor Components of PC


• Gypsum, • MgO • Alkali sulfates

Gypsum (2CaSO4 2H2O) is added by 4-5 % by mass in the last procedure of grinding the clinker to produce Portland cement. The reason for adding gypsum cement is to avoid the flash setting caused by fast reaction of C3A, because it can react with C3A and form a hydration product called ettringite on the surface of C3A to prevent the further reaction of C3A

Alkalies (MgO, Na2O, and K2O) can increase the pH value of concrete up to 13.5, which is good for reinforcing steel protection.

Hydration of Cement


Hydration of cement is the reaction between cement particles and water, including chemical and physical processes. The properties of fresh concrete, such as setting and hardening, are the direct results of hydration.

The rate of hydration during the first few days is in the order of C3A > C3S > C4AF > C2S

Hydration of Cement


The two peaks in the curve represent the dominant effect of C3S or C3A correspondingly and their order of occurrence can be reversed. The two peaks in the curve represent the dominant effect of C3S or C3A correspondingly and their order of occurrence can be reversed.

Stage I: Dissolution Stage II: Dormant Stage III: Acceleration Stage IV: Deceleration Stage V: Steady state


Traditionally, it is believed that the initial set of cement corresponds closely to the end of the induction (dormant) period, 2–4 h after mixing.

Traditionally, it is believed that the initial set of cement corresponds closely to the end of the induction (dormant) period, 2–4 h after mixing.


On first contact with water, calcium ions and hydroxide ions are rapidly released from the surface of each C3S grain the pH values rises to over 12 within a few minutes. The induction (dormant) period is caused by the need to achieve a certain concentration of ions in solution, before crystal nuclei form, from which the hydration products grow. At the end of the dormant period, CH starts to crystallize from the solution with the concomitant formation of C–S–H, and the reaction of C3S again proceeds rapidly (the third stage, acceleration, begins). CH crystallizes from the solution, while C–S–H develops on the surface of C3S and forms a coating covering the grain. As hydration continues, the thickness of the hydrate layer increases and forms a barrier through which water must flow to reach the unhydrated C3S and through which ions must diffuse to reach the growing crystals. Eventually, movement through the C–S–H layer determines the rate of reaction, and hydration becomes diffusion controlled and moves into the 5th stage, the steady-state stage.


The initial set of cement corresponds closely to the end of the induction period, 2–4 h after mixing. The initial set indicates the beginning of gel formation. It is controlled primarily by the rate of hydration of C3S. The final set occurs 5–10 h after mixing, which indicates that sufficient hydration products are formed and the cement paste is ready to carry some external load. It should be noted that the initial and the final set have a physical importance. However, there is no fundamental change in the hydration process for these two different sets. The rate of early hardening, which means a gain in strength, is primarily determined by the hydration of C3S, and the strength gain is roughly proportional to the area under the heat peak in the calorimetric curves of Portland cement. The strength development is mainly derived from the hydration of silicates.

Types of PC


According to ASTM standards

According to EN standards


From the information provided in Table 2-8, we can evaluate the behavior of each type of cement. The various behaviors provide the basic justification in selecting cement for engineering practice. For instance, for massive concrete structures, hydration heat is a big consideration because too much heat will cause a larger temperature gradient, thermal stress, and cracking. Hence, type IV cement should be the first candidate and type III should not be used. For a marine structure, high sulfate resistance and lower ettringite are needed; thus, type V should be selected. If high early strength is needed, type III will be the best choice. Generally, type I is the most popular cement used in civil engineering.

The role of water on hydration


The water is necessary for the hydration of cement. The water added in the mix is usually much higher than what the chemical reaction needs due to the fluidity requirement of concrete for placing. Thus, we can distinguish the three kinds of water in cement paste according to their roles :

• Chemically reacted water • Absorbed water • Free water



The chemically reacted water or chemically bonded water is the water that reacts with C, S, A, F, and S to form a hydration products such as C–S–H, CH, and AFt. This type of water is difficult to remove from cement paste and a complete decomposition happens at a temperature about 900◦C. Absorbed water is the water molecules inside the layers of C–S–H gel. The loss of absorbed water causes shrinkage, and the movement or migration of absorbed water under a constant load affects the creep. Free water is the water outside the C–S–H gel. It behaves as bulk water and creates capillary pores when evaporated, and can influence the strength and permeability of concrete.



Porosity is a major component of the microstructure that is mainly caused by loss of water. The size of the capillary pores formed due to the loss of free water is in the range of 10 nm to 10 μm. The size of the gel pores involved in absorbed water is in the range of 0.5 to 10 nm. A knowledge of porosity is very useful since porosity has such a strong influence on strength and durability. According to experiments, the gel porosity for all normally hydrated cements is a constant, with a value of 0.26.



The total volume of the hydration products (cement gel) is given by

where α represents the degree of hydration.

The capillary porosity can then be calculated by;

where w is the original weight of water, c is the weight of cement, and w/c is the water to cement ratio.

With an increase of w/c, the capillary pores increase! With an increase of w/c, the capillary pores increase!



The gel/space ratio (X) is defined as

The gel/space ratio reflects the percentage of solid materials in a cement paste.

The higher the ratio, the more solid the materials and hence the higher the compressive strength. The higher the ratio, the more solid the materials and hence the higher the compressive strength.

The gel/space ratio is inversely proportional to the w/c. It can be deduced that a higher w/c leads to a low compressive strength of cement paste or concrete.

The minimum w/c ratio for complete hydration is assumed to be 0.36 to 0.42. Complete hydration never happens and that residual anhydrate cement is beneficial for attaining a high ultimate strength.

Basic tests of Portland cement


• Fineness (surface area / weight) • Normal consistency • Time of setting • Soundness • Strength • Heat of hydration test • Other experiments, including sulfate expansion

and mortar air content

The detailed test method can be found in the textbook and the laboratory manuals which were provided the students in lecture CIV204 .

Fineness


It is an important quality index and represents the average size of the cement grains. It controls the rate and completeness of hydration due to the exposure surface of cement particles to the water. The finer the cement particles, the more rapid the reaction, the higher the rate of heat evolution, and the higher the early strength. However, finer cement particles can lead to high hydration heat, high possibility of early age cracking, and possible reduced durability.

Fineness


The fineness of Portland cement can be measured by different methods. Two common methods are the Blaine air permeation method defined in ASTM C204 and sieving (ASTM C194-94). In Blaine method, cement particles are placed on a porous bed and then a given volume of fluid (air) is passed through the bed at a steady diminishing rate. After all the air is passed, the time (t ) for the process is recorded. The specific surface of the cement can be calculated using

where K is a constant. In practice, S (or K) can be determined by comparing the sample to the known surface area issued by the U.S. National Institute of Standards and Technology.


Blaine apparatus

Time of setting


Time of setting: This test is undertaken to determine the time required for the cement paste to harden. The initial set cannot be too early due to the requirements of mixing, conveying, placing, and casting. Final setting cannot be too late owing to the requirement of strength development. Time of setting is measured by the Vicat apparatus with a 1-mm-diameter needle. The initial setting time is defined as the time at which the needle penetrates 25mm into the cement paste. The final setting time is the time at which the needle does not sink visibly into the cement paste.

Normal consistency test : This test is undertaken to determine the water requirement for the desired cement paste plasticity state required by the setting and soundness test for Portland cement. The normal consistency test is regulated in ASTM C187. The w/c for a normal consistency of Portland cement is 0.24 to 0.33

Soundness


Unsoundness in cement paste results from excessive volume change after setting. Unsoundness in cement is caused by the slow hydration of MgO or free lime. Another factor that can cause unsoundness is the later formation of ettringite. Since these reactions are very slow processes, taking several months and even years to finish, and their hydration products are very aggressive, their crystal growth pressure will crack and damage the already hardened cement paste and concrete. The soundness of the cement must be tested by an accelerated method due to the slow process. One test is called the Le Chatelier test (BS 4550), and is used to measure the potential for the volumetric change of the cement paste. The Le Chatelier test is used mainly for free lime detection.

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Chapter III Concrete Making Materials Cementitious Binders”myyardimci.weebly.com/uploads/1/6/3/4/16347790/civ415_ch... · 2020. 3. 12. · CIV415 CONCRETE TECHNOLOGY Chapter III