CONCRETE MATERIALS

2.2.1 Cement

The raw materials from which cement is made are lime, silica, alumina and iron oxide. These constituents are crushed and blended in the correct proportions and burnt in a rotary kiln. The resulting product is called clinker. The cooled clinker can be mixed with gypsum and various additional constituents and ground to a fine powder in order to produce different types of cements. The main chemical compounds in cement are calcium silicates and aluminates.

The Euro standard for cements is BS EN 197-1:2011 Cement –Part 1:

Composition, specifications and conformity criteria for common cements.

When water is added to cement and the constituents are mixed to form cement paste, chemical reactions occur and the mix becomes stiffer with time and sets.

The addition of gypsum mentioned above retards and controls the setting time.

This ensures that the concrete does not set too quickly before it can be placed in its final position or too slowly so as to hold up construction.

2.2.1.1 Types of Cements

The code gives five groups of cements, all of which are mixtures of different proportions of clinker and another major constituent. The five groups are:

1. CEM I Portland cement: This comprises mainly ground clinker and up to 5% of minor additional constituents.

2. CEM II Portland composite cement: This comprises of seven types which contain clinker and up to 35% of another single constituent.

i. Portland slag cement (CEM II/A-S and CEM II/B-S). This comprises of clinker and blast furnace slag which originates from the rapid cooling of slag obtained by smelting iron ore in a blast furnace. The percentage of the slag varies between 6 and 35%.

ii. Portland silica fume cement (CEM II/A-D). This comprises of clinker and silica fume which originates from the reduction of high purity quartz with coal in an electric arc furnace in the production of silicon and ferrosilicon alloys.

iii. Portland-Pozzolana cement (CEM II/A-P, CEM II/B-P, CEM II/A-Q, CEM II/B-Q). This comprises clinker and natural pozzolana such as volcanic ashes or sedimentary rocks with suitable chemical and mineralogical composition or Natural calcined pozzolana such as materials of volcanic origin, clays, shales or sedimentary rocks activated by thermal treatment.

iv. Portland-fly ash cement (CEM II/A-V, CEM II/B-V, CEM II/A-W, CEM II/B-W). This mixture of clinker and fly ash dust-like particles precipitated from the flue gases from furnaces fired with pulverised coal.

v. Portland burnt shale cement (CEM II/A-T, CEM II/B-T). This consists of clinker and burnt shale, specifically oil shale burnt in a special kiln at 800^OC.

vi. Portland-limestone cement (CEM II/A-L, CEM II/B-L, CEM II/A-LL, CEM II/B-LL).

vii. Portland-composite cement (CEM II/A-M, CEM II/B-M).

3. CEM III blast furnace cement (CEM III/A, CEM III/B, CEM III/C): This comprises clinker and a higher percentage (36-95%) of blast furnace slag than that in CEM II/A-S and CEM II/B-S.

4. CEM IV pozzolanic cement (CEM IV/A, CEM IV/B): This comprises of clinker and a mixture of silica fume, pozzolanas and fly ash.

5. CEM V composite cement (CEM V/A, CEM V/B): This comprises clinker and a higher percentage of blast furnace slag and pozzolana or fly ash.

Table 2.1 Clinker content in cements Cement type Clinker content

A B C

CEM II 80-94% 65-79%

CEM III 35-64% 20-34% 5-19%

CEM IV 65-89% 45-64% -

CEM V 40-64% 20-38% -

The letters A, B and C designate respectively higher, medium and lower proportion of clinker in the final mixture. However the percentage of clinker with the designations A, B, C can be different in different types of cement as shown in Table 2.1.

The second constituent in cement in addition to clinker is designated by the second letter as follows:

S = blast furnace slag D = silica fume P = natural pozzolana

Q = natural calcined pozzolana V = siliceous fly ash

W = calcareous fly ash (e.g., high lime content fly ash) L or LL = limestone

T = burnt shale

M = combination of two or more of the above components

2.2.1.2 Strength Class

There are three classes of strength as shown in Table 2.2. The strength class of cement classifies its compressive strength at 28 days.

Table 2.2 Strength class Strength

class Compressive strength, MPa Initial

setting time Early strength Standard strength

2 day 7 day 28 day Minutes

32.5 N - ≥ 16.0 ≥ 32.5 ≤ 52.5 ≥ 75

32.5 R ≥ 10.0 - ≥ 32.5 ≤ 52.5

42.5 N ≥ 10.0 - ≥ 42.5 ≤ 62.5 ≥ 60

42.5 R ≥ 20.0 - ≥ 42.5 ≤ 62.5

52.5 N ≥ 20.0 - ≥ 52.5 - ≥ 45

52.5 R ≥ 30.0 - ≥ 52.5 -

2.2.1.3 Sulfate-Resisting Cement

Sulfate resisting cements are used particularly in foundations where the presence of sulfates in the soil which can attack ordinary cements. The sulfate resisting cements have the designation SR and they are produced by controlling the amount of tricalcium aluminate (C3A) in the clinker. The available types are:

i. Sulfate resisting Portland cements CEM I-SR0, CEM I-SR3, CEM I-SR5 which have the percentage of tricalcium aluminate in the clinker less than or equal to 0, 3 and 5% respectively.

ii. Sulfate resisting blast furnace cements CEM III/B-SR, CEM III/C-SR (no need for control of C3A content in the clinker).

iii. Sulphate resisting pozzolanic cements CEM IV/A-SR, CEM IV/B-SR (C3A content in the clinker should be less than 9%).

2.2.1.4 Low Early Strength Cement

These are CEM III blast furnace cements. Three classes of early strength are available with the designations N, R and L respectively signifying normal, ordinary, high and low early strength as shown in Table 2.2.

2.2.1.5 Standard Designation of Cements

CEM cement designation includes the following information:

i. Cement type (CEM I-CEM V) ii. Strength class (32.5-52.5) iii. Indication of early strength

iv. Additional designation SR for sulfate resisting cement v. Additional designation LH for low heat cement Examples:

1. CEM II/A-S 42.5 N

This indicates Portland composite cement (indicated by CEM II), with high proportion of clinker (indicated by letter A) and the second constituent is slag (indicated by letter S) and the strength class is 42.5 MPa (indicating that the characteristic strength at 28 days is a minimum of 42.5 MPa) and it gains normal early strength (indicated by letter N).