Chemistry of cement

CHEMISTRY of CEMENTfor THIRD YEAR STUDENTS

MAJOR CHEMISTRY

Dr . Safaa El-gamal

introduction

Cement: given to a substances which can be used to bind together sand and any form of aggregate. (eg. broken stone).

Primitive cement is Ca(OH)2 (hydrated lime).

Hydraulic cement: is the cement that not only react with water ,it also form water resistant products.

Cement can beused as

Paste Mortar Concrete

Some important Terms:

1-Setting 2-Hardening

3-Hydration reaction. 4-Curing condition.

Oxide Abbreviation Compound Abbreviation

CaO C 3 CaO.SiO2 C3S

SiO2 S 2 CaO.SiO2 C2S

Al2O3 A 3 CaO.Al2O3 C3A

Fe2O3 F 4 CaO.Al2O3.Fe2O3 C4AF

MgO M 4 CaO.2Al2O3.SO3 C4A2S

SO3 S 3 CaO.2 SiO2.3 H2O C3S2H3

H2O H CaSO4.2 H2O CSH2

C3S = Tri Calcium Silicate, C2S = Di Calcium Silicate

Cement chemical nomenclature

Manufacture of Portland Cement

Lime stone (CaCO3) +Clay (or other compounds with similar bulk composition)heating to about 1450 oC → partial fusion →clinker(67%C, 22%S, 5%A, 3% F+ 3% other component) → grinding and mixed with few %gypsum (Ca SO4.2 H2O) →Portland cement

Main phases of PC clinker:1) C3S 2) C2S 3)C3A 4) C4AF

Rule of each phase in the Hydration characteristics of Portland Cement

Percentage and the rule of each phase in PCCI) Compressive strength:C3A,C4AF give little strength compared to C3 S, β-C2S.

II)Evolution of heat:hydration of PC is an exothermic process(C3S and C3A) make large contribution to heat of hydration.III) Sulphate resistant: some ground water and sea

water contains enough sulphate to attack PC and causes its expansion and disintegration.

Sulphate attack depends on C3A content, and formation of ettringite (C3A.3 CaSO4.32H2O).

Types of Portland Cement according to ASTM Classification

ASTM type and name

Composition ( % ) CharacteristicsC3S C2S C3A C4AF

I (Ordinary Portland cement)

42-65

10-30

0-17 6-18 no special requirements.

II ( Modified Portland cement)

35-60

15-35

0-8 6-18 moderate sulfate resistance,moderate heat of hydration.

III ( High - early strength Portland cement)

45-70 10-30

0-15 6-18 higher strengthsoon after pouring.

IV ( Low - heatPortland cement)

20-30 50-55

3-6 8-15 low heat of hydration.

V ( Sulfate -resistant Portland cement)

40-60 15-40

0-5 10-18

high sulfate resistance.

Specification of PC is based on

1)Test on unhydrated cement.2)Test on behaviour of cement during hydration.3)Test on the hardened pastes.White cement (other Type of Portland Cement):

made from iron free materials (e.g lime stone and china clay), they contain C3A,C3S and C2S.

non Portland cement:I) Aluminous cement:Lime stone +bauxite (heating) →complete fusion →

aluminous cement.

Properties of Aluminous cement1)set and harden quickly compared to PC2)High early strength.3)High resistant to sulphate attack.II) Slag cement:a) blastfurnace slag b)supersulphated cement.III- Pozzolanic Cement:Pozzalanas: are substances which are not in themselves cementitious, but reacts with Ca(OH)2 in presence of water and act as cement.Expansive cement: cement that give paste which expand slightly on hydration.

Mechanism of cementing action1)Reactivity of anhydrous compounds

1)C3S ,CA react rapidly with water and strongly hydraulic.2)β-C2S reacts slowly with water and strongly

hydraulic, but γ-C2S is inert.3)C3A,C4AF reacts rapidly with water, but they are

weakly hydraulic. b)β-C2S reacts slowly with water and strongly

hydraulic, but γ-C2S is inert.c)C3A,C4AF reacts rapidly with water, but they are

weakly hydraulic.

Explanation

i) many compounds are stable at room temperature when (Ca) atoms coordinated to six oxygen. In compounds formed at high temperature the coordination number of (Ca) is less than (6) and the reactivity is attributed to tendency of (Ca) to attain (6) CO.NO.

ii) Also irregularity in arrangement of coordinated oxygen was an important factor

iii) Finally presence of minor impurities may be a reason for reactivity.

2-product formed on hydrationi) Chemical nature of it.ii) Particle size and particle shape.iii)Surface structure: hydraulic reactivity is high for

compounds have surfaces in which coordination of ions or atoms are unsatisfactory.

iv-Condition of formation of hydration productsa) If the hydration product occurs under conditions

which do not causes disruption of the pre-existing structure it give strength.

b) If the reverse of (a) occurs it will causes low strength.

v-Condition of formation of hydration productsa) If the hydration product occurs under conditions

which do not causes disruption of the pre-existing structure it give strength.

b) If the reverse of (a) occurs it will causes low strength.

Theories of setting and hardening1) Crystallization hypothesis (by Lechatelier):Cement+H2O→solution(supersaturated relative to

hydration products) →precipitation of hydration products as crystals, interlocked with each other → hardened mass .

2) Gel hypothesis (by Michael’s):He attributed the hardening of hydraulic cement

due to formation of gel.a) Gel is soft and contains much water.b) Unreacted cement particles which still present

absorbs this water from this gel and use it for hydration

c) The gel becomes hard and impermeable

3-Modern Viewsa) Cement +H2O → dispersion system (cement grains not

close packed)b) Large amount of hydration products (of colloidal

dimension) were formed on the surface of grains.c) After few hours amount of hydration increase and

extended to meet each other, so that give gel in the space between the grains (setting stage).

d)(after few days), the colloidal particles between the cement grains becomes densely packed, give hardened materials, also the gel becomes closely packed in some regions and less dense in other, which give pores (hardening stage)

a) Through solution mechanism:-Reactants → dissolves gives ions in solution→ ions combined giving products → precipitate from

solution.b) Topochemical mechanism:- reaction occurs directly on

surface of solid. 1)hydration of C3S2 C3S +6H→ C3S2H3 +3 Ca(OH)2

2)hydration of C2S2 C2S +4H→ C3S2H3 + Ca(OH)2

Hydration of Portland Cement

3)Hydration of C3Aa)C3A+excessH2O→(C4AH19+C4AH8 ) these are metastable

hydrates by time → C3 AH6(more stable and less soluble) b) C3A+less H2O and room temp.→C3 AH6

3)Hydration of C4AF:C4AF +CaSO4.2H2O +Ca(OH)2 →3CaO(Al2O3.Fe2

O3).3CaSO4(aq) Role of gypsum in setting and hardeningIt added to cement to delay its setting.

Mechanism of hydration1)pre-induction period (first minutes) (fast hydration).induction period : the rate is slow down. ’’dormant

period’’ (period of relative inactivity) lasting 1-2 hours.3)acceleration period: (3-12 hours after mixing) due to

break up CSH coating and continuation of hydration process).

4)post acceleration: in this period the rate of hydration is slow down gradually as the amount of unhydrated materials decreases.

Blast-Furnace Slag Cement (BFC)BFS is by-product obtained in manufacture of pig-iron in the blast –furnace as molten at 1400-1500 oC.then cooling may be:a) Slow cooling which give grey crystalline ,stony material known as dense slag.

b) Rapid cooling using limited amount of water , giving light weight materials called foamed slagand after cursing and grinding used as light weight aggregates.

a) very rapid cooling of the slag by pouring into excess water

b)or by very rapid cooling using jets of water or of air and water.

The product known as granulated slag.Reaction of blast-furnace slag with waterIt react very slowly with water but without Setting. But when some Ca(OH)2 is added to it ,setting and hardening reactions occurs, similar to PC but with slower rate.The products formed are: CSH (tobermorite), C4AH13

Reaction of blast-furnace slag with water

It react very slowly with water but without setting. But when some Ca(OH)2 is added to it ,setting and hardening reactions occurs, similar to PC but with slower rate.The products formed are: CSH (tobermorite), C4AH13What is the rule of activator (e.g. Ca(OH)2 , NaOH or KOH)?

Mechanism of hydration of slag in slag –cement mixture

1)Fast hydration reaction from time of mixing with water(0.5 h) (pre-dormant period).

2)first dormant period, this period( ended at 0.5 day of hydration). Very slow –stage of hydration (due to initially formed hydrates with high amorphous character)

3)first acceleration period ends at 3 days of hydration.4) Second dormant period (after 3-days) the hydration is

slow down. 5)Second acceleration period (after 28-day) . In this

period a marked decrease in free lime and free slag were observed up to 180 days

The beginning and duration of each stage :depends ona) Fineness b) Composition of the original slag –cement mixture.Super sulphate Cement It made from (80-85%)of granulated blast furnace

slag + CaSO4 (as activator)+5% PC

Super sulphate Cement

It made from (80-85%)of granulated blast furnace slag + CaSO4 (as activator)+5%

PC