Cement and binding minerals, Chemistry tutorial

Introduction:

Cement is the general word provided to the powdered materials that initially encompass plastic flow whenever mixed by water or other liquid, however consists of the property of setting to a hard solid structure in several hours having varying degree of strength and bonding properties. The use of cement can be traced to the Greeks, Romans and Egyptians who utilized volcanic stuff like pozzolanic cement, a volcanic tufa obtained close to Pozzuoli in Italy. This is a kind of natural cement which might be regarded as a mixture of burnt silicates and lime. James Parker in the year 1780 developed natural cement made up by burning septaria (that is, nodules which are found in certain clay deposits and that have both clay minerals and calcium carbonate). The burnt nodules were ground to a fine powder. This product known as the 'Roman cement' whenever made to a mortar with sand, sets in 5 to 15 minutes. In spite of the popularity of this cement, it was largely substituted by Portland cement in the year 1850. Portland cement was introduced by Joseph Aspidin, a bricklayer and later the production was refined via William Aspidin and Isaac Charles Johnson.

Mineral composition of Cement:

Cement is a binder, a substance which sets and hardens and can bind other materials altogether. On the basis of the utilization of cement in construction, it can be characterized as being either hydraulic or non-hydraulic based on the capability of the cement to be employed in the presence of water.

Non- hydraulic cement will not set in the wet conditions or underwater, it sets as the cement dries and reacts by carbon-dioxide in the air. This is prone to chemical attack after setting.  Hydraulic cement lets setting in wet condition or underwater and further protects the hardened material from chemical attract. Considering the durability, quality and setting time, we will agree that hydraulic cements are more viable. Therefore, we shall base our consideration mostly on hydraulic cements. Now what mineral contributes to the binding capability of cements? The minerals that constitute hydraulic cements like Portland cement are mostly Belite (2CaO. SiO2); Alite (3CaO.SiO2); Celite (3CaO.Al2O3) and Brownmillerite (4CaO, Al2O3  Fe2O3). The silicates are mainly responsible for the mechanical properties of the cement whereas the Celite (that is, tricalcium aluminate) and the brownmillerite (tetracalciumaluminoferite) are necessary to allow the formation of the liquid phase throughout the kilnsintering .

Classes of Cements:

Over the years, three groups or classes of cement have been developed commercially:  natural cements (that is, non-hydraulic), aluminous and Portland cements (both groups are hydraulic). This is true that we have established the fact that our illustration will be mainly based on the hydraulic cements, though, the chemistry comprised in the production of natural cements is worthy of note.

Natural Cements:

Hydraulic lines and slaked line are the illustrations of natural cements how are natural cements made up of? What is the chemistry ruling the action of such non-hydraulic cements?

=> Manufacture:

At first carbon-dioxide is eliminated from pure limestone (that is, calcium carbonate) by heating for around 10 hours, at atmospheric pressure and temperatures above 825oC (1,517oF). This method is termed as calcination and generates quick lime (that is, calcium oxide) which slakes fast in water by the evolution of significant amount of heat. Calcium hydroxide (that is, slaked lime) is therefore formed and it doesn't set under water. Once the water in surplus from the slaked lime is fully evaporated, carbonation begins (that is, hardening of slaked lime in the presence of carbon-dioxide which is naturally present in the air). Carbonation yields in the manufacture of calcium carbonate. The calcined product of a limestone which consists of high calcium content is known as fat lime. Calcined products having comparatively low content of silica (that is, 10 to 20%) and alumina are generally known as hydraulic limes. The methods illustrated above can be summed up as represented in the equations below:

CaCo3 → CaO + CO2 (Calcination)

CaO + H2O → Ca(OH)2 (Slaking)

Ca(OH)2 + CO2 → CaCO3 + H2O (Carbonation)

This entire method is termed as the lime cycle.

Aluminous Cement:

The Aluminous cement is necessarily calcium aluminates cement and can be made up by heating a mixture of limestone and bauxite at around 1550 to 1600oC. This cement is made up from alumina rich rocks like bauxite and limestone or lime. Aluminous cement such as Portland cement is rapid hardening hydraulic cement. The burning of raw mix is completed till sintering or fusion sets in at 1450 to 1650oC. The constituents of this cement are tricalcium aluminates (3CaO. Al2O3, abbreviated C3A), dicalcium silicate (2CaO. SiO2, C2S) and dicalcium silicate aluminates (2CaO. Al2O3. SiO2, C2AS). The major mineral ingredient of aluminous cements is monocalcium aluminate, CaO. Al2O3. This predetermines the fast hardening capacity of such cements.

The Dicalcium silicate is a slowly hardening mineral, helenite and magnesium spinel are inert admixtures deteriorating the properties of high alumina cement. Thus, this is of great significance to prepare aluminous cement from the available raw materials free of MgO and SiO2 for aluminum oxide (Al2O3) not to combine to inert materials.

There are fundamentally two ways of manufacturing the aluminous cement namely: (a) By burning the raw mix till sintering sets in or (b) By burning it to fusion.

Alumina cement releases a huge amount of heat and whenever the temperature of hardening cement increases above 20oC, the monocalcium aluminate and water combine to dicalciumheptahydratehydroaluminate by scaly crystals.

2(CaO.Al2O3) + 10H2O → 2CaO. Al2O3. 7H2O + 2Al(OH)3 

Such crystals then recrystallized to tricalciumhexahydratehydroaluminate by cubic crystals since in such conditions 2(CaO.Al2O3).7H2O is not stable. Alumina cements are chemically comprised of aluminium oxide, CaO, SiO2 and Fe2O3 with percentages of 35 to 55%, 35 to 45%, 5 to 10% and 1 to 15% correspondingly.

Portland cement:

Portland cement is by far the most common kind of cement in general use around the world. This cement is termed to be chemically comprised of Al2O3 (4 to 7%), CaO (64 to 67%), SiO2 (19 to 24%) and Fe2O3 (2 to 6%). The chemistry ruling the action of Portland cement similar to its hydraulic counterpart is hydration.

Portland cement is prepared by heating limestone (that is, calcium carbonate) having small quantities of other materials like clay to 1450oC in a kiln, in a procedure termed as calcination whereby a molecule of carbon-dioxide is released from the calcium carbonate to form calcium oxide or quicklime, which is then blended by the other materials that have been comprised in the mix. The resultant hard substance, termed as 'clinker', is then ground by a small amount of gypsum to a powder to form the cement.    

This cement is a fundamental ingredient of concrete and mortar.  Its most general use is in the production of concrete. Concrete is a composite material comprising of aggregate (that is, gravel and sand), cement and water. There are different kinds of Portland cement based on the rate of setting, heat evolution and strength characteristics. Five kinds of Portland cement are recognized that have varying amounts of the clinker compounds C2S, C2S, C4AF and MgO. These are: 

A) Regular Portland cements:

They are common products for general concrete construction and harden to full strength in around 28 to 30 days. They have 40 to 60% C2S, 10 to 30% C2AS and 7 to 13% C3A. White cements, oil wall cement and rapid setting cements belong to this group or class.

B) Modified Portland cements:

These are sulphate resisting cements that are employed where moderate heat of hydration is needed. The heat evolved from such cements in expected not to surpass 70 and 80 cal/gm after one week and four weeks correspondingly. Such cements are characterized through higher C2S/C3S ratio.  

C) High early strength (HES) Portland cements:

Such cements have higher percentage of C3S and C3A having finer grinding to raise hydration rate. This high proportion of C3S causes a quicker hardening as compare to regular Portland cement and therefore acquires strength of regular Portland cement in just three days. Roads constructed from the HES cement can be put to service much sooner as compare to those constructed from the regular cements.

D) Low heat Portland cement:

Such cements have a lower percentage of C3S and C3A and therefore reduce the heat evolution. It is expected that the heat evolved must not surpass 60 and 70 cal/gm after 7 and 28 days correspondingly. Such cements are designed for the massive structure work.

E) Sulphate resisting Portland cement:

These are good for the sea water contact and resist sulphates better than the other four kinds. They are lower in C2A (? 4%) and higher in C4AF.

Note:

Dicalcium silicate is abbreviated as C2S

Tricalcium silicate is abbreviated as C3S

Tricalcium aluminate is abbreviated as C3A

Dicalcium silicate aluminate is abbreviated as C2A  

Tetracalciumaluminoferrite is abbreviated as C4AF

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