9.5. Portland cement. Technology bases

Portland cement is a hydraulic binder, which hardens in water and in the air conditions. It is produced by compatible milling of clinker, necessary amount of gypsum and other additives. Portland clinker is a product of sintering of raw material mixture of certain chemical composition which provides after the burning prevailing content of minerals-silicates.

Cement industry produces a large amount of cements types based on Portland clinker, which can be conditionally divided into cements of the common and special designation.

Cements of the common designation are divided into five types according to composition and compressive strength at 28-days (EN 197-1:2000):

- Cem I – Portland cement (may contain 0 to 5% mineral additives),

- Cem II – Portland-composite cement ( 6 to 35% blast furnace, granular slag or 6 to 20% mineral additives);

- Cem III – Blast furnace cement (36 to 80% blast furnace granular slag);

- Cem IV - Pozzolanic cement (21 to 55% mineral additives);

- Cem V - Composite cement (36 to 80% mineral additives).

Typical compositions of the different types cement are given in the Table 9.2.

According to the early age strength(after two or seven days of hardening) cements are divided into two kinds: cement with normal strength in early age and high-early-strength cement.

The type, class, and special signs (high strength in the early age – R; rationed mineralogy - N) are indicated at the symbolic notation of cement.

The limestone and clay rocks or their natural mixtures – marls are the basic raw materials for obtaining Portland cement. The various waste products are also used: slags, ashes, nepheline sludge and etc.

The Portland cement production consists of two stages: production of clinker and its fine milling with additives.

The chemical composition of Portland clinker, as a rule, falls between the following percentage limits:

CaO – 63-66	MgO – 0.5-5
SiO2 – 21-24	SO3 – 0.3-1
Al2O3 – 4-8	Na2O+K2O – 0.4-1
Fe2O3 – 2-4	TiO2+Cr2O3 – 0.2-0.5

The basic technological operations of cement clinker production (Fig. 9.5) are:

- excavation and preparation of raw materials, which include mining, grinding and drying as a necessity;

- production of a homogeneous raw material mixture by milling and mixing of components;

- firing of raw material mixture, which provides passing of physical and chemical processes of clinker formation;

- cooling of the product.

There are two basic methods of clinker production - wet and dry. For the wet method, the raw material mixture is prepared and burned as a water suspension – sludge with humidity 35-42%. A dry method includes drying of materials to its compatible grinding. Raw material mixture at a dry method goes on burning in the powdery or granular state.

Таble 9.2

Typical composition of common cements (percentage by mass)

Type of cement	Clinker	Mineral additive	Type of cement	Clinker	Mineral additive
CEM I	95-100	-	CEM II/A-L	80-94	Limestone (L) 6-20
CEM II/A-S	80-94	Blast furnace slag (S) 6-20	CEM II/B-L	65-79	21-35
CEM II/B-S	65-79	21-35	CEM II/A-LL	80-94	Limestone (LL) 6-20
CEM II/A-D	90-94	Silica fume (D) 6-10	CEM II/B-LL	65-79	21-35
CEM II/A-P	80-94	Natural Pozzolan (P) 6-20	CEM II/A-M	80-94	Composition of additives (M) 6-20
CEM II/B-P	65-79	21-35	CEM II/B-M	65-79	21-35
CEM II/A-Q	80-94	Natural Pozzolan calcine (Q) 6-20	CEM III/A	35-64	Blast furnace slag (S) 36-65
CEM II/B-Q	65-79	21-35	CEM III/B	20-34	66-80
CEM II/A-V	80-94	Fly ash siliceous (V) 6-20	CEM III/C	5-19	81-95
CEM II/B-V	65-79	21-35	CEM IV/A	65-89	Pozzolan, silica fume, fly ash 11-35
CEM II/A-W	80-94	Fly ash calcareous (W) 6-20	CEM IV/B	45-64	36-55
CEM II/B-W	65-79	21-35	CEM V/A	40-64	Blast furnace slag (S) 18-30 Pozzolan additives (P,D,V,W) 18-30
CEM II/A-T	80-94	Burnt shale (T) 6-20	CEM V/B	20-38	Blast furnace slag (S) 31-50 Pozzolan additives (P,D,V,W) 31-50
CEM II/B-T	65-79	21-35

Notes:

1. Up to 0-5% minor constituents may be added to all types of cements.

2. In Portland-composite cements CEM II/A-M and CEM II/B-M, in Pozzolanic cements CEM IV/A and CEM IV/B and in composite cements CEM V/A and CEM V/B, all main constituents other than clinker shall be declared by designation of the cement.

I n many countries, a wet method of production of clinker has been the most widespread. This is mostly due to the need to achieve high homogeneity of raw material mixture, which favorably affects quality of clinker. However, the dry method of production involves 1.5 - 2 times less heat consumption than the wet method. Presently, due to creation of effective constructions of clinker burning furnaces the dry method has the wider distribution.

Basic raw materials, required for the production of cement, are mined in careers near by plants by the opened method. Grinding down of raw materials is a two-stage process involving (a): crushing in crushing machines (jaw, hammer crusher, roll crusher, gyratory breaker) and (b): milling i.e., fine grinding in ball or other mills.

Grinding of the raw materials to a high fineness is characteristic for modern technology of Portland cement, because increased fineness and homogeneity of mixture accelerates the physical and chemical processes.

Firing of the homogeneous raw material mixture is a main technological operation in the production of Portland cement. Firing is carried out, as a rule, in rotary furnaces. Modern furnaces used for the wet method are typically 185-230 m in length, and 5-7m in diameter. The productivity of a 5185 m furnace is 1800 t/day, and a 7230m rotary furnace produces about 3000 t/day. For the dry method, furnace sizes diminishes considerably without a corresponding decline in the productivity. So, rotary furnaces with cyclone heat-transfer apparatus have sizes of 575 and 795 m, and a daily productivity of 1600 and 3000 t respectively.

The material passes through the sloping drum of the furnace, and on coming into contact with the hot gases, experiences a series of complex physical and chemical transformations. Firstly, physically bound water is removed from the raw material mixture at about 110^oC, and then the chemically bound or constitution water is removed from the clay component at 600-700 °C. The dissociation of CaCO₃ occurs at 750-1000 °C. At the temperature about 1300 °C exothermal reactions results in the formation of basic compounds - clinker minerals: dicalcium silicate 2CaOSiO₂ or shortly C₂S, tricalcium aluminate 3CaOAl₂O₃ (C₃A), tetra-calcium aluminoferrite 4СаОА₂О₃Fе₂О₃ (C₄AF). At a temperature of about 1300°C partial melting of raw material begins. CaO and 2CaOSiO₂ dissolve in the melt and leads to the main reaction of clinker formation (- reaction of two-calcium silicate satiation CaO to the tricalcium silicate - 3CaOSiO₂ (C₃S)). The formation of C₃S takes place in the sintering zone of the furnace at a temperature range of 1300-1450°C. This ends the formation of conglomerates of clinker grains-granules, most of which have the sizes 10-60 mm depending on the construction and mode of operations of furnace.

The cement clinker is then cooled and stored. After cooling and storage comes the milling in the ball mills with a capacity of 50-100 t/hour. The required amount of gypsum stone is added into to the mills, which provides content in cement to 3.5% SO₃ to regulate the time of setting. Other active mineral additives can also be added into Portland composition at the milling, to improve water resistance and some other properties. The type and content of additives depends on the type of cement. Specific designation is given to identify Portland cements containing mineral additives. Portland cement (type II), is divided into cement with additive of granulated blast furnace slag, pozzolan, flue ash and limestone and other additives. Portland cement of group A includes 6-20% additives; group B - 21-35% (Table 9.2). The blast furnace cement (type III) group A includes 36-65% of granulated blast-furnace slag, group B - 66-80%, in pozzolanic cement (cement type IV) – respectively 11-35 and 36-55% of flue ash or its mix with the pozzolanic component. It is allowed to enter 18-30% blast-furnace granular slag and 18-30% of flue ash and pozzolans into composite cement group A, and group B - respectively 31-50 % blast-furnace slag and 31%-50% flue-ash and pozzolanic additives.

A clinker milling is the most power-consuming technological operation in cement production. More than 60% of total electric power, which is consumed on the production of cement, has been known to be spent on milling. At a milling plant, special surface-active substances can be entered for intensification of grinding down or providing cement with the special properties (technical lignosulphonate (LST), fatty acid and others), in an amount not more than 0.3% (by mass) of cement content.

After milling, the cement is transferred to the silo section, equipped with the pneumatic devices for aeration, homogenizing and unloading.

Portland cement is shipped in valvular paper bags or in bulk in special transport vehicles, cement trucks or cement carriers, covered carriages, etc. Cements should be saved separately according to the types and classes in the stationary and movable automated storages (silos) or in the special containers and bunkers which have a device for a reception and delivery of cement. The serial number, complete designation of cement and its type, kind and amount of additives, normal consistence of cement paste and other information are indicated in a document of cement quality.