CMA

Since the prehistoric times the use of a diverse range of materials has been one of the characteristic features of humankind. Human civilisation found its identity in many materials as evident from the Stone Age, the Bronze Age, and then the Iron Age; each era is defined by new material. In the present world, it would not be wrong to say that modern civilisation is characterised by Cement.⁴Notably, concrete is the most consumed resource on earth after water. The earliest evidence of cementing materials dates to the construction marvels created by Egyptians and to the architecture of the Roman Empire. The Industrial Revolution marked an exponential rise in the use of cement and iron/steel for modern architecture, which continues to grow to date. Cement is a fine powdered substance used as an intermediate binding agent in construction activities. It is a manmade material comprising of limestone, clays, shells, silica sand, etc processed together in a controlled environment (around 1500 °C). Cement is a manmade wonder, which sets, hardens and brings strength to the constructed structures over time through its peculiar adhesive and tensile properties. This makes it an indispensable component of building materials, such as – Mortar and Concrete. The former is a mixture of cement and other fine aggregates. It is mainly used for masonry activities. The latter is a composite material made up of cement mixed with aggregates (sand, gravel, other additives) and water; it is the most extensively used construction and building material for a wide range of structures.⁵

Composition of Cement

A typical composition of Ordinary Portland Cement combines calcareous materials (or calcium carbonate) and argillaceous substances (silicates of alumina) crushed and ground together above 1500º C to form a uniform powder (in the dry process) or a homogenous paste (through the wet process).⁶ Figure illustrates the principle ingredients of cement whose characteristic properties are described as follows:

Lime (as Calcium Oxide)

Lime or Calcium oxide (CaO) is one of the most essential ingredients of cement. The proportion of lime may vary between 60% to 67% depending upon the desired property of cement for construction activity. Naturally occurring minerals such as limestone, chalk, shale, etc. are the principal sources of lime. Limestone (calcium carbonate) with other materials (such as clay) releases carbon dioxide at high temperatures in cement kilns to form calcium oxide, also known as quicklime or lime. Inside kilns, lime reacts chemically with added argillaceous substances to produce forms of silicates and aluminates of calcium. The percentage of lime needs to be controlled with high precision; its deficiency could result in poor strength in the final cement product whereas an excess of lime leads to unsound cement, ie, cement losing its volume after setting due to delayed expansion and which could also lead to cracks in the structure.⁷

Silica (as Silicon dioxide)

Silica is the most abundant naturally occurring mineral on earth’s crust. Chemically speaking, it is an oxide of silicon (chemical formula: SiO₂), commonly found as quartz, and is a major constituent of sand and argillaceous rocks. It is the second major ingredient of cement with a share ranging between 17% to 25%. Silica assists in the formation of calcium silicates, which brings strength to the final cement product. Excess of Silica enhances the strength of cement but at the cost of delayed settling properties. Differing proportions of silica can be tested for desired properties with engineered structures.

Alumina (as aluminium oxide)

Alumina (Al₂O₃) is mainly derived from Bauxite, which is a naturally occurring ore of metal Aluminum. The other sources could be certain forms of clays, and industrial by-products or reject(s) having recoverable quantities of alumina. It is an important additive in cement for quick setting properties. Most compositions of cement find alumina ranging between 3% to 8%; an excess of alumina could weaken the final composition. Another trade off with its use is a drop in clinkerisation temperature inside the kilns as it acts as a flux material. This directly impacts the strength of cement for which reason a suitable proportion needs to be carefully maintained.

Iron Oxide

Iron Oxide (Fe₂O₃) imparts a grey colour to cement and it acts as a flux agent assisting the chemical reactions inside cement kilns. It is required in the range of 0.5 % to 6%. At high temperatures it forms tricalcium aluminoferrite by reacting with aluminium and calcium, which aids in bringing strength to cement. It is generally sourced from shale materials, clays, and blast furnace rejects from the steel industry.

Magnesia (as Magnesium Oxide)

Magnesia or Magnesium Oxide (MgO) is desired in small quantities (0.1% to 3%) to bring hardness and colour to cement. An increase in quantity can disturb the soundness of cement directly impacting strengthening properties.

Sulphur (as Sulphur trioxide)

Sulphur trioxide (SOS₃) checks undue expansion of cement after it sets once bringing soundness to the final composition. It is a very critical function to bring stability to structures and to avoid undue cracks after construction. A typical share of Sulphur trioxide ranges between 1% to 3%.

Alkali materials

Soda (Na₂O) or Potash (K₂O) are the most common alkali materials present in very small quantities (0.1% to 1%) in cement. An excess of alkali causes efflorescence, ie, appearance of while salty deposits over the surface of concrete structures.

Gypsum (as Calcium Sulphate)

Gypsum, chemically known as Calcium Sulphate (CaSO₄) naturally occurs with Limestone. It has a unique purpose of increasing the settling time of cement for specific requirements with construction structures.Most constituents discussed here find an inverse relationship between the strength and settling properties. Controlled chemistry between materials holds high importance in the manufacturing of a fit-for-purpose cement product.⁸

Chemistry of Cement

Each constituent of the cement material undergoes chemical reactions at high temperatures inside cement kilns during the manufacturing process. The resultant compounds are popularly known as Bogue compounds. There are mainly four types of compounds:

Tricalcium silicate (3CaO.SiO₂) or C₃ S
Dicalcium silicate (2CaO.SiO₂) or C₂S
Tricalcium aluminate (3CaO.Al2O₃) or C₃A
Tetracalcium aluminoferrite (4CaO.Al₂O₃.Fe2O₃) or C₄AF

Their proportion controls the chemical properties of cement in terms of strength and settling properties. Most cements are hydraulic in nature, ie, they set in the presence of water and release heat due to reactions between these components. A certain category of cement is non-hydraulic where they set in dry form and reaction occurs in the presence of carbondioxide in the air. The latter category is meant for specialised applications such as resistance to chemicals, etc. This section highlights the chemical process associated with commonly used Portland cement, which is a form of a hydraulic type of cement. The following steps illustrate chemical reactions, which govern the settling and hardening properties of the cement when applied to a construction structure.⁹

Hydration of cement is an exothermic reaction (releases heat) that enables initial set (also known as flash set) of the material within a day. The paste becomes rigid due to the hydration of C₃A into a crystalline form. Gypsum avoids the early setting of C3A by acting as a retarding agent.
The initial strength of cement is delivered by hydration of C₃S, which takes almost a week to complete the chemical process. It forms a new compound called Tobermonite gel having high surface area and tremendous adhesive properties.
C₂S slowly reacts with water and takes up to a month bringing a gradual increase in overall strength to the constructed structure. It also results in forming Tobermonite gel structure and crystallising of calcium hydroxide which enhance the overall strength with time.
C₄AF after initial hydration results in hardening of the structure due to crystallisation process along with C₂S.
Both hydration¹⁰ and hydrolysis¹¹ impart compressive properties to the cement material.

Supplementary Cementing Materials (SCM)

These materials can be added to the concrete as they contribute to the hardening process due to their hydraulic and pozzolanic properties. The most commonly used SCMs are:

Fly ash – obtained as a by product from coal fired power plants
Slag – obtained from blast furnace of the steel industry. Molten slag is rapidly chilled and ground to make it usable as cementing material
Silica fumes – It is a residue material from the ferro-silicon alloy industry

The Cement industry is the only industry that demonstrates a great example of converting ‘waste to wealth’ by making use of rejects from other industries. Over time, the Cement Industry has created several opportunities around the optimisation of resources and energy inputs. The Indian Cement sector is at the forefront of energy and fuel efficiency related innovation with a performance at par with the best available technologies globally. Moving forward, supporting policy frameworks will enable more advances in the sector.