2) CEMENT

At the end of class students should understand……1) Raw materials in cement manufacturing process2) Chemical compositions of Portland cement and their functions3) Hydration process of cement4) Types of Portland cement and their applications

Ingredients Manufacturing process Chemical compositions Type of cements Hydration process Testing of cement

Is the product obtained by grinding clinker formed by burning raw-materials (argillaceous and calcareous) primarily consisting of lime (CaO), silicate (SiO2), alumina (Al2O3), and iron oxide (Fe2O3)

Characteristics of cement Setting and hardening when mix to a paste with water Cohesive and Adhesive

History of Portland Cement 1824 – patented by Joseph Aspdin Materials to produce cement were found on the English Isle of Portland Thus, called PORTLAND CEMENT

Cement Argillaceous + Calcareous

Silica (SiO2 -60%) Lime (CaO -20%)Alumina (Al2O3 -10%)Ferum oxide (Fe2O3)( clay, sandstone, shale) ( limestone, chalk)

Hydraulic cement – able to set and harden in the presence of water e.g. OPC Non-Hydraulic cement – requires no water to harden, only require air to harden e.g. lime

Manufacturing Process

Two types of process: Wet Process – softer materials (clay + chalk) Dry Process – harder materials (shale + limestone)

Steps in manufacturing process1) Grinding and mixing of raw materials2) Burning in a rotary kiln3) Cooling, grinding and sieving4) Storing, packing and distributing

Wet Process1) Materials being ground and mixed with the correct proportions2) The mixture in a slurry form is fed into a rotary kiln where it is heated/burnt to a temperature of 1500

°C to form clinker

Grinding Raw Materials

Mixing

Burning 100 °C – water is driven off 850 °C – limestone changes to calcium oxide and CO2 is liberated

1500 °C – CaO reacts with SiO2 to form Calcium Silicate (CS)

CaO reacts with other compounds to form Calcium Aluminate (CA) and Calcium Aluminoferite (CAF)

Grinding

3) Cooling and grinding – Gypsum [Ca(SO)4] is added during the grinding process to prevent flash setting

4) Storing – stored in silo5) Final Products – cement

Brands of Cement

Why clinker need to be ground?

Dry Process

The process is relatively the same as wet process except in dry process the raw materials (harder materials) are mixed in dry condition.

Small amount of water is added to form 12mm in diameter ‘balls’ The ‘balls’ are burnt in a rotary kiln and the chemical reactions are the same as wet process

CHEMICAL COMPOSITION OF PORTLAND CEMENT

Will affect: Strength of cement Rate of hydration Amount of heat of hydration Durability of cement

Chemical Composition of Portland CementAbb.Abb. CompoundCompound OxideOxide PercentPercent

CC33SS Tricalcium silicateTricalcium silicate 3CaO.SiO3CaO.SiO2254%54%

CC22SS Dicalcium silicateDicalcium silicate 2Cao.SiO2Cao.SiO2217%17%

CC33AA Tricalcium AluminateTricalcium Aluminate 3CaO.Al3CaO.Al22OO3311%11%

CC44AFAF Tetracalcium Tetracalcium AluminoferriteAluminoferrite

4CaO.Al4CaO.Al22OO33.Fe.Fe22OO339%9%

OthersOthers 9%9%

Alkalis in Cement Na2O and K2O May cause problem – Alkali Silica Reaction (ASR)

HYDRATION PROCESS OF PORTLAND CEMENT Is the chemical reaction between the compounds of cement and water that yields products that

achieve the binding property after hardening Two stages: 1) setting 2) hardening

Hydration Process of Cement

Cement + H2O → C-S-H gel + Ca(OH)2 It is an exothermic process where heat is liberated (heat of hydration) The silicates, C3S and C2S, are the most important compounds, which are responsible for the strength

of hydrated cement paste C3S provides the early strength and liberated higher heat of hydration

Set – reaches a state in which its form cannot be changed without rupture

Schematic view of setting

Setting Flash set – rapid development of permanent rigidity of the cement paste – along with high heat False set – rapid development of rigidity without the evolution of heat

Hydration of cement

C2S reacts slowly, provide later strength, highly chemical resistance (sulphate, chloride)

C3A is undesirable, contribute little or nothing to the strength of cement except at early ages, and when hardened cement paste is attacked by sulphates, the formation of sulphoaluminate may cause disruption

Hydration of C 3A

C-S-H gel

Ca(OH)2

C4AF does not affect the behaviour of cement hydration significantly. However, it reacts with gypsum to form calcium sulphoferrite and its presence may accelerate the

hydration of silicates

Rate of Hydration

Hydration Process

Effect on porosity

W/C effect on porosity

Strength vs porosity

TYPES OF PORTLAND CEMENT

Types of Portland Cement1) Ordinary Portland Cement (OPC) – Type I2) Rapid Hardening Portland Cement (RHPC) – Type III3) Sulphate Resisting Portland Cement (SRPC) – Type V4) Modified Portland Cement (MPC) – Type II5) Low Heat Portland Cement (LHPC) – Type IV6) Portland-Blastfurnace Cement (PBC) – Type IS7) Portland-Pozzolan Cement (PPC) – Type IP8) Others

Effect of chemical composition Cement differing in chemical composition may exhibit different properties when hydrated Strength, durability, rate of strength gain, heat of hydration

Ordinary Portland Cement (Type I) By far the most common cement used in general concrete construction when there is no exposure to

sulphates in the soil or groundwater Minimum fineness of 225 m2/kg Rate of hardening is moderate Standards: BS 12:1991, MS 522,ENV 197-1:1992

Rapid Hardening Portland Cement (RHPC – Type III ) Rapid strength gain due to higher C3S content (70%) Minimum fineness of 325 m2/kg Used when formwork is to be removed early for reuse or where sufficient strength fro further

construction is required quickly Should not be used in mass concrete construction or in large structural sections because of its higher

rate of heat development For construction at low temperatures, the use of RHPC may provide a satisfactory safeguard against

early frost damage The setting time and chemical composition of RHPC and OPC is relatively the same

Sulphate Resisting Portland Cement (SRPC – Type V ) This cement has low C3A (3.5%) content so as to avoid sulphate attack from outside the concrete Minimum fineness of 250 m2/kg Active salts are magnesium and sodium sulphates Sulphate attack is greatly accelerated if accompanied by wetting and drying (splash zone) The heat develop by SRPC is not much higher than the low-heat cement, which is an advantage

Sulphate attack

Portland-Blastfurnace Cement Made by inter-grinding or blending Portland cement clinker with granulated blast-furnace slag Known as slag cement Slag contains lime, silica and alumina, but not in the same proportions as in Portland cement Minimum fineness of 275 m2/kg Early strength are generally lower than OPC but later strength are similar or even higher Typical uses are in mass concrete because of low heat of hydration and in seawater construction due to

better sulphate resistance ( lower C3A content) than with the OPC The amount of slag replacement between 25 to 70% of the mass of the mixture

Portland-Pozzolan Cement Made by inter-grinding or blending pozzolans with Portland cement A pozzolan is a siliceous or siliceous and aluminous material which itself possesses little or no

cementitious value BUT in finely divided form and in the presence of moisture, chemically react with Ca(OH)2 liberated during the hydration of Portland cement to form compounds possessing cementitious properties

Portland-Pozzolan cement gain strength slowly and therefore require curing over a comparatively a long period, but the long-term strength is high

Amount replacement between 25 to 40% More durable than OPC

What to consider? Materials? Cement? Type of construction?

Discussion What are the main materials used for the manufacture of cement? What is the purpose of adding gypsum to cement? Why clinker (cement) needs to be ground? What are the chemical composition of Portland cement? Different types of OPC and applications.

Summary Cement ingredients Manufacturing process Chemical compositions Hydration process Types of cement and application

Concrete with silica fume

Normal plain concrete

DENSE CONCRETE