The central constituent of all hydraulic cements is lime. It is the compound responsible for the setting and hardening of cements. The common types of this kind of cements are discussed below.
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i.Natural cements
They are made from impure limestone. Raw limestone isfirst calcined at moderate temperatures and subsequently cooled, ground and bagged for use as cement.
Calcination is the process whereby calcium carbonate (limestone) is heated to produce calcium oxide and carbon dioxide according to the reaction as follows:
CaCO3 !
M CaO þ CO2 ð5:1Þ
Natural cements require very stringent compositions, and their manufacture requires strict control measures.
ii.Hydraulic limes (Argillaceous limestone)
These are also made from impure limestone which has high clay matter. The clay content could be as high as 25%. Hydraulic lime is formed from the limestone by burning the limestone in an ordinary stack kiln, then cooling and hydrating or slaking it. The hydraulic properties of this cement are due to the silicates and aluminates which form when the lime reacts with the silica, alumina and iron oxide present in the clay.
iii.Portland cements
Portland cement is made by grinding together Portland clinker and gypsum. The percentage of gypsum is often about 5% (wt). Clinker itself is made by burning a mixture of limestone and clay matter at very high temperatures near 1450 °C, then cooling the product and grinding it into powder. The main constituents of Portland clinker are calcium silicates, calcium aluminates and calcium aluminoferrite. The name Portland cement is derived from a stone in Portland (Portland stone), an island on the coast of England, which has a color similar to Portland clinker. There are several types of Portland cement based on the composition of the main constituents.
iv.Granulated blast furnace slag cement
Blast furnace slag is a product of pig iron-making processes. The residue dumped from the furnace after extracting the pig iron when cooled quickly or quenched and then ground has cementitious properties. The quenching process is called granu- lationand the resultant product is calledgranulated blast furnace slag. This product when mixed with lime or Portland clinker forms cement calledslag cement. Slag from other industries can also be used to make Portland slag cement. Only gran- ulated slag is effective as a cementing agent. Two types of cement are commonly made from this slag, namely:
(a) Portland blast furnace slag cement (PBFSC): This contains up to 70% (wt) blast furnace slag, 5–6% (wt) gypsum and the remaining Portland cement clinker.
This kind of cement has all the physical properties of ordinary Portland cement (OPC). It is therefore used as an economic alternative to OPC. It is more sulfate-resistant and has a low heat of hydration. It develops strength slowly
5.2 Hydraulic Cements 137
and has very high ultimate strength. As the slag content is increased, early strength is reduced. However, all compositions produce high ultimate strength.
(b) Super-sulfated cement (SSC): This is more sulfate-resistant than PBFSC and SRC (sulfated slag cement). SSC is prepared by grinding together granulated slag, anhydrite (or gypsum) and clinker (or lime) in a proportion of approxi- mately 80:15:5. It exhibits good resistance to aggressive agents in general.
Again, they produce high strength by the formation of ettringite with strength growth similar to slow Portland cement.
v.Portland pozzolana cements (PPC)
Pozzolanas are natural or synthetic clay matter, which, when ground with lime or clinker and mixed with water, produce cementitious compounds. Portland poz- zolana cement is made by mixing and grinding highly reactive pozzolana orfly ash, Portland cement clinker and 5–6% (wt) gypsum. The Pozzolana orfly ash content should not be more than 25% (w). PPC has a much lower heat of hydration and is also fairly sulfate-resistant. It develops strength slowly and has very high ultimate strength. Its physical properties are the same as that of OPC, except that it has lower shrinkage. It can, therefore, be used for all construction work for which OPC is used. Fly ash is afine powder residue resulting from the burning of pulverized coal.
It is generallyfiner than cement and consists mainly of glassy-spherical particles as well as residues of hematite and magnetite, char, and some crystalline phases formed during cooling.
vi.Masonry cement (MC)
It is made by inter-grinding Portland cement clinker with limestone, sandstone or granulated slag in the proportion of 1:1. Hydrated lime and/or plasticizer are often added to give better plasticity. Masonry cement, unlike the other types of cement, gives a more plastic mortar. It is used mostly for masonry work such as laying bricks (blocks) and for plastering. The other cements when mixed with sand and water to form mortar give a somewhat harsh mortar that does not retain water very well.
vii.Calcium aluminate cements
These are made by grinding limestone and bauxite together. Monocalcium alumi- nate (CaAl2O4) and Mayenite (Ca12Al14O33) are the active ingredients of this type of cement. Strength is formed by hydration to calcium aluminate hydrates. They are well adapted for use in refractory concretes, e.g. furnace linings.
viii.Calcium sulfo-aluminate cements
These are produced from a mixture of clinkers that has Ye’elimite (Ca4(AlO2)6SO4) as the base material. During hydration, ettringite is formed and unique properties are developed by adjusting the concentration of calcium and sulfate ions in the medium. Energy requirements for its production are lower because of the lower kiln temperatures required for reaction, and the lower amount of limestone in the mix.
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They are used in expansive cements in ultra-high early strength cements and low-energy cements.
ix.Geopolymer cements
These are made from mixtures of water-soluble alkali metal silicates and alumi- nosilicate mineral powders such as fly ash and metakaolin. These are very low-energy materials; hence the production of geopolymer cement results in very large energy savings. The properties of geopolymer cement, when used to make concrete, are equivalent to other cements in terms of the structural qualities of the resulting concrete. The properties of geopolymers are significantly dependent on the silica/alumina ratio. For example, a geopolymer cement with Si/Al ratio of 1 is used in bricks and ceramics, while that with Si/Al ratio of 2 is used in low CO2concretes.
To obtain a geopolymer cement with acceptable mechanical properties, it is nec- essary to enhance the activity and solubility of Al-Si source materials in the alkali solution. One way of meeting this condition effects on thefinal properties is thermal activation of the source material.
5.2.1 Variations of Portland Cement
The main constituents of the different variations of cement are the same. They vary only in the composition of the constituents and in the end-use to which they are put.
However, the method of manufacture is the same for all of them. The range of composition of constituents of Portland cement types is shown in Table5.1.
In practice, it is the presence and relative amounts of the compounds tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C3AF) in cement that imparts the different properties to the different types of Portland cement. The probable composition of these compounds in Port- land cement has the ranges shown in Table5.2. The specific composition of the compounds and the unique properties they confer of the cement products are listed in Table5.3.
Table 5.1 Constituents of
Portland cement Compound Range of composition (%wt)
SiO2 19.0–25.0
Al2O3 2.0–8.0
Fe3O2 0.3–6.0
CaO 60.0–65.0
MgO 1.0–6.0
SO3 1.0–3.0
Alkalis 0.5–1.5
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5.2.2 Special Cements
Special cements have Portland as the base but are formulated to impart on them special properties for special use. Cements falling under this category are listed in Table5.4.
Table 5.2 Compounds in Portland cement that imparts its properties
Compound Composition (%wt)
C2S 20.0–60.0
C3S 20.0–60.0
C3A 0.0–16.0
C4AF 1.0–16.0
Table 5.3 Types of Portland cement
Type Composition Unique properties
Ordinary Portland cement (OPC)
55% C3S, 19% C2S, 10% C3A, 7% C4AF
Develops enough compressive strength in 3 days, 7 days and 28 days
Moderate heat Portland cement (MHPC)
51% C3S, 24% C2S, 6% C3A, 11% C4AF
They release less heat when hydrated than OPC
Rapid hardening cement (RHC)
56% C3S, 19% C2S, 10% C3A, 7% C4AF
These give higher strength in 24 h than OPC will give in 3 days
Low heat Portland cement (LHC)
28%C3S, 49% C2S, 4%
C3A, 12% C4AF
These give very low heats of hydration.
Used in large concrete works such as in dam construction when contraction cracks must be avoided
Sulfate-resisting cement (SRC)
38%C3S, 43% C2S, 4%
C3A, 9% C4AF
Concretes from such cements resist sulfate attacks. Used in areas where soils and water with high concentrations of sulfates Oil well cement
(OWC)
Portland cement with C3A content of less than 3%
Grout from such cement does not set for 90– 120 min at high well bottom temperatures.
Once the grout is in place it is expected to set and harden within 24 h. Used in lining the annulus of oil wells
White cement Proportion of iron oxide is reduced to below 0.4%
It does not burn well during clinker formation. It has all the physical properties of OPC
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