What is meant by reactions with materials?

Apart from knowing the material properties, civil engineers need to equip themselves with the basic concepts of chemistry. It will aid in understanding the reaction behavior of different materials used with other materials. Knowing the reaction mechanism of materials will help the compatibility of that particular material with other materials. Knowing the reaction products will also aid engineers to develop better materials so that they can withstand corrosive environments, high temperatures, low temperatures, and so on. There are certain areas in civil engineering, where the knowledge of chemistry and reactions are important. Some of the areas are outlined below:

• Chemistry is involved in the water treatment process to remove pollutants and harmful chemicals.
• In-depth chemistry is involved in sewage water treatment. It makes water useful for horticulture, agriculture, and other various purposes.
• Designing marine structures and machines such as bridges and ships makes use of chemistry to know the reaction mechanism of marine water salts with metals. It will ensure safeguarding the material from corrosion damages.
• Suitable selection of secondary water resources and their associated filtration process makes it suitable for drinking purposes, makes extensive use of chemistry and reaction knowledge.
• The manufacturing of cement from its raw ingredients for construction involves concepts of chemistry.
• Like cement, wood is the second building material. Wood is mixed with many other materials to enhance its properties. Hence, chemistry plays a major role in this area.
• Alteration of properties of steels and other metals to make them suitable to perform specific tasks requires the knowledge of material science and chemistry.

Chemical reactions during cement manufacturing

Cement is the only primary building construction material used commonly in construction works all over the globe. Cement is in the powdery form which when mixed with water turns into a hard mass due to hydration. This is also known as the setting of cement. The most common cement used is Portland cement. The chemical formula of Portland cement is $CaO.A{l}_2{O}_3.F{e}_2{O}_3$. A variety of raw materials are required in making Portland cement, some of them are:

Limestone

Limestone is also known as calcium carbonate, bearing the chemical formula $CaC{O}_3$. It is used in the manufacturing of cement to make use of calcium oxide $CaO$ from $CaC{O}_3$.

Clay

Clay is a substance that consists of aluminum oxide $\left(A{l}_2{O}_3\right)$, silicon dioxide $\left(Si{O}_2\right)$, water $\left(H_{2}O\right)$, and iron oxide $\left(F{e}_2{O}_3\right)$. Clay is especially hydrated aluminum silicate and iron oxide. It bears the chemical formula $A{l}_2{O}_{3}Si{O}_{2}F{e}_2{O}_3.2H_{2}O$.

Cement chemical composition

The following are the chemical composition (in percentage) of cement.

• Calcium oxide, $CaO$ - 60% to 70%
• Silicon dioxide, $Si{O}_2$ - 20% to 25%
• Aluminum oxide, $A{l}_2{O}_3$ - 5% to 10%
• Iron oxide, $F{e}_2{O}_3$ - 2% to 3%

Chemical reactions involved during cement manufacturing

The following are the chemical reactions involved during the cement manufacturing process.

• Conversion of limestone into lime during the high-temperature heating process. The chemical reaction is given as,

$CaC{O}_3\stackrel{Heat}{\to }CaO+C{O}_2$

• Calcium oxide reacts with silicon oxide to produce dicalcium silicate. The reaction is given by,

$2CaO+Si{O}_2\to 2CaO.Si{O}_2$

• The lime, that is, calcium oxide further reacts with silicon dioxide to aid tricalcium silicate, the chemical reaction is given by,

$3CaO+Si{O}_2\to 3CaO.Si{O}_2$

• During the process, lime reacts with aluminum oxide to aid tricalcium aluminate. The reaction is given by,

$3CaO+A{l}_2{O}_3\to 3CaO.A{l}_2{O}_3$

• During the last phase of the reaction, calcium oxide, aluminum oxide, and iron oxide together react to aid cement. The reaction is given as,

$4CaO+A{l}_2{O}_3+F{e}_2{O}_3\to 4CaO.A{l}_2{O}_3.F{e}_2{O}_3$

Reactions involved during sewage water treatment

Wastewater treatment is the process of removing harmful substances from water discharged from residential areas, industries, and other places so that, they can be discharged safely into the environment. The treatment is undertaken in waster water treatment plants and there are multiple chemical processes associated. In this section, some of the common processes of wastewater treatment and their reaction processes are outlined.

Precipitation reactions in wastewater treatment

The basic reactions involved in the wastewater treatment, in this case, are outlined below:

$C{a}^{2+}$and $M{g}^{2+}$ removal

These are generally known as alkaline earth elements or ions. Removal of these ions is known as water softening. To remove these ions, two chemicals are generally used and they are calcium hydroxide $Ca{\left(OH\right)}_2$ and soda ash $N{a}_{2}C{O}_3$. The chemical reaction involved in the process is given by,

$M{g}^{2+}\left(aq\right)+2O{H}^{-}\left(aq\right)\to Mg{\left(OH\right)}_2\left(s\right)$ This reaction aids in the removal of $M{g}^{2+}$ions.

$C{a}^{2+}\left(aq\right)+C{O}_3^{2-}\left(aq\right)\to CaC{O}_3\left(s\right)$ This reaction aids in the removal of $C{a}^{2+}$ions.

Removal of $F{e}^{2+}$and $M{n}^{2+}$

These two elements are generally known as transition elements. They promote bacterial growth and lead to staining. To remove them, they are first converted to solids by oxidation and then removed by the filtration process. Different oxidation agents such as chlorine $\left(C{l}_2\right)$ and potassium permanganate $\left(KMn{O}_4\right)$are used. 

The following reaction is used in the removal of $F{e}^{2+}$ elements,

$3F{e}^{2+}\left(aq\right)+Mn{O}_4^{-}\left(aq\right)+2H_{2}O\left(l\right)\to 3F{e}^{3+}\left(aq\right)+Mn{O}_2\left(s\right)+4O{H}^{-}\left(aq\right)$

$F{e}^{3+}$ and $O{H}^{-}$ further reacts at high PH values to yield, $Fe{\left(OH\right)}_3\left(aq\right)$.

The element $M{n}^{2+}$ is removed by the following chemical reaction,

$M{n}^{2+}\left(aq\right)+Cl\left(a{q}_2\right)\to M{n}^{4+}\left(aq\right)+2C{l}^{-}\left(aq\right)$

They can be easily removed by the addition of oxygen $O_2$, i.e.,

$M{n}^{4+}\left(aq\right)+O_2\left(g\right)\to Mn{O}_2\left(s\right)$ which is a salt.

Phosphate removal

Phosphates are primarily removed from wastewater by reacting with lime. The reaction is given below,

$10C{a}^{2+}\left(aq\right)+6P{O}_4^{3-}\left(aq\right)+2O{H}^{-}\left(aq\right)\to C{a}_{10}{\left(P{O}_4\right)}_6{\left(OH\right)}_2\left(s\right)$

The resulting reaction product is easily removed by filtration.

Hydration process of cement

In chemistry, the term hydration means the reaction of a substance with water. In construction works, which extensively make use of cement, the process of hydration is very important to stimulate the setting of cement. When cement reacts with water to turn into a solid mass, it is usually known as hydration. 

When water is added to cement, each of the ingredients of cement undergoes hydration. The calcium silicate is responsible for only strength, while most of the early strength is due to hydration tricalcium silicate. Dicalcium silicate contributes to strength at the later stage. The hydration equation with tricalcium silicate is given below,

$2C{a}_{3}Si{O}_5+7H_{2}O\to 3CaO.2Si{O}_2.4H_{2}O+3Ca{\left(OH\right)}_2+Energy$

The hydration equation of dicalcium silicate with water is given below,

$2C{a}_{2}Si{O}_4+5H_{2}O\to 3CaO.2Si{O}_2.4H_{2}O+Ca{\left(OH\right)}_2+Energy$

Context and Applications

The topic is extensively taught in many undergraduate and postgraduate degree courses of:

• Bachelors of Technology (Civil Engineering)
• Masters of Technology (Building Materials)

Practice Problems

Q1. What is the chemical formula of Portland cement?

a. $CaC{o}_3.A{l}_2{O}_2.Si{O}_2$

b. $Ca{\left(OH\right)}_2.Si{O}_2.A{l}_2{O}_3$

c. $CaO.A{l}_2{O}_3.F{e}_2{O}_3$

d. none of these

Answer: Option c

Explanation: Portland cement is the most common type of cement constituting clay and limestone. The chemical formula is $CaO.A{l}_2{O}_3.F{e}_2{O}_3$.

Q2. Which of the following are the ions of alkaline earth metals?

a. $C{a}^{2+}$ and $M{n}^{2+}$

b. $C{a}_3^{2+}$ and $M{g}^{2+}$

c. $O{H}^{-}$ and $M{g}^{2+}$

d. $C{a}^{2+}$and $M{g}^{2+}$

Answer: Option d

Explanation: $C{a}^{2+}$ and $M{g}^{2+}$ are generally the ions of alkaline earth metals as they are constituent parts of calcium and magnesium. The removal of these ions is known as water softening.

Q 3. What is the meaning of hydration?

a. The reaction of substances with water

b. The reaction of substances with cement

c. The removal of water during a reaction

d. The formation of water during a reaction

Answer: Option a

Explanation: Hydration is the reaction of substances with water. In civil construction, hydration of cement, that is, the reaction of cement with water leads to the formation of hard solid mass.

Q 4. Which of the following element is/are known as transition elements?

a. $M{n}^{2+}$

b. $M{g}^{2+}$

c. $F{e}^{2+}$and $M{n}^{2+}$

d. $M{n}^{2+}$and $M{g}^{2+}$

Answer: Option c

Explanation: $F{e}^{2+}$and $M{n}^{2+}$ are known as transition elements, they lead to staining and promote bacterial growth in wastewater.

Q 5. Which of the following chemical compound reacts with calcium oxide to form tricalcium silicate?

a. silica

b. silicon trioxide

c. silicon pentachloride

d. silicon dioxide

Answer: Option d

Explanation: When silicon dioxide reacts with calcium oxide, it aids in the formation of tricalcium silicate. This reaction can be summarized as $2CaO+Si{O}_2\to 2CaO.Si{O}_2$.