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Unit 5. MINERAL BINDERS. AIR-SETTING BINDING MATERIALS. GYPSUM BINDING MATERIALS.

Mineral binders are fine powders that are capable of producing a plastic pasty mass on mixing with water and passing into a stony state when exposed to physical and chemical action. This property makes the binders suitable for the preparation of mortars, concretes (mixtures of binders, water and aggregates such as sand, crushed stone or gravel), artificial cast stone materials and items.

Mineral binders are subdivided into air-setting and hydraulic setting varieties.

Air-setting binding materials are substances that pass into a stone state, gaining and retaining mechanical strength in the air only. Rep­resentatives of air-setting binding materials are gypsum, magnesian binding materials, air-hardening lime and acid-resistant cement.

Hydraulic-setting binding materials are substances that pass into a stone state, gaining and retaining strength not only in the air, but in water as well.

The group of hydraulic-setting binding materials includes portland cement and its varieties, puzzolana and slag binding materials, alu­mina and expanding cements, hydraulic lime and Roman cement. They are used in above- and below-ground and underwater construc­tions.

Distinction is made of autoclave-setting binding materials, which set only when treated in autoclaves with saturated steam at pressures from 0.8 to 1.2 MPa and temperatures between 170 and 200°C.

Falling into the autoclave-setting group are lime-silica and lime-nepheline binders, as well as sand portland cements which, although capable of setting under a variety of conditions, acquire maximum strength after autoclave curing only.

 

A. AIR-SETTING BINDING MATERIALS

1. Gypsum Binding Materials

Gypsum binders are subdivided into two groups: low- and high- burning varieties. Low-burning gypsum binders are obtained by hea­ting dihydrate gypsum (CaSO₄×2Н_аО) to a temperature of 150-160°C; dihydrate gypsum is partially dehydrated and converted to semihyd rate gypsum (CaSO₄•0.5H₂O).

High-burning (anhydrite) binding materials are obtained by burning dihydrate gypsum at a higher temperature (700-1000°C) to-a complete loss of chemically bound water and formation of anhydrous calcium sulphate, the anhydrite (CaSO₄). Building and extra strong gypsum fall into the low-burning category, and the anhydrite cement and the estrich gypsum, into the high-burning category.

The source materials for the manufacture of gypsum binding mate­rials are 'natural gypsum rock and natural anhydrite CaSO₄, as well as chemical industry waste containing dihydrate or anhydrous calcium sulphate, e.g., phosphorous gypsum.

 

Fig. 1. Flowsheet for the manufacture of construction gypsum with the use of curing kettles

1 - overhead grab crane; 2 - gypsum stone bin; 3 - pan feeder; 4 - jaw crusher; 5 - belt conveyors; 6 - crushed gypsum stone bin; 7 - rotary-table feeder; 8 - shaft mill; 9 - twin cyclone; 10 - multicyclones; 11 - fan; 12 - sleeve filter; 13 - dust settling chamber; N - screw conveyors; 15 - raw crushed gypsum bin; 16 - holding chamber; 17 - curing kettle; 18 - elevator; 19 - finished gypsum bin; 20 - rake conveyor

 

Building gypsum is an air-setting binder composed mainly of semi- hydrate gypsum and obtained by thermal processing of gypsum rock at temperatures of 150-I60°C. In the process, CaS0₄×2H₂O contained in gypsum rock dehydrates according to the reaction

CaSO₄ •2H₂O=CaSO₄×0.5H₂O+1.5H₂O

The manufacture of building gypsum involves crushing, grinding and thermal processing (dehydration) of gypsum rock.

There are several flowsheets for processing gypsum; in some of them, grinding precedes burning, in some, it follows burning, and in some others, grinding and burning are combined in a single apparatus. The latter method is called flash burning of gypsum.

Thermal processing of gypsum rock may be effected in curing pans, rotary roasters, shaft mills, etc.

 

 

Through the cylinder, the gases burn in gypsum rock, which is then ly ground in mills.

Flash burning of gypsum two operations are combined: grinding burning. Crushed gypsum is fed to a grinding mill (shaft, ball or er type) which is simultaneously supplied with hot combustion es. Fine gypsum particles (of commercially available size) are drawn of the mill by a stream of hot combustion gases and burned during process. The particle-laden gases enter cyclones and precipitators arе gypsum settles down.

If the three processes discussed above, the latter one has the grea- t capacity, then come the rotary kilns burning and the batch-type tie flowsheets. However, the quality of products obtained by the it two techniques is significantly worse compared to those manufac- ed in batch-type kettles.

When water is added to gypsum powder, the semihydrate calcium ifate CaSO₄0.5H₂O contained in the latter dissolves until a satu- ;ed solution is formed; at the same time, said semihydrate hydrates taking up 1.5 molecules of water and converts to dihydrate CaSO₄• H_sO according to the reaction

CaSO₄ >0.5H₂O+1.5H₂O=CaSO₄2Н₂О

The solubility of the dihydrate is approximately one-fifth that of e initial powder, the semihydrate, with the effect at the resultant saturated solution of the semihydrate is found to > supersaturated with respect to the dihydrate. The supersaturated ilution cannot exist under the ordinary conditions and gives a very ne precipitate, the dihydrate calcium sulfate. The fine particles xumulate, bond together to cause the thickening (setting) of the pulp, len crystallize to form a strong gypsum stone. A further increase in le strength of gypsum takes place through the drying of the setting lass and its further crystallization. The setting of gypsum may e accelerated by drying, but at a temperature not higher than 65°C avoid the reverse dehydration of the dihydrate gypsum. Building gypsum is a quick setting and quick hardening binder, etting of building gypsum, according to the State Standard GOST 25-70, should begin not earlier than in 4 min and end not earlier than n 6 min and not later than 30 min after it has been mixed with water.

The prescribed fineness of building gypsum in terms of the total etained on sieve No. 02 (size of sieve mesh is 0.2 mm in the clear) s as follows: for grade I, up to 15%, for grade II, 20% and for grade II, up to 30%. The finer the size content of gypsum, the greater is strength. The bending strength of specimen bars measuring 4x4x x16 cm 1.5 hours after mixing of gypsum with water should not be ess than 2.7, 2.2 and 1.7 MPa for grades I, II and III respectively, the compressive strength of the halves of specimen bars should not less than 5.5, 4.5 and 3.5 MPa for grades I, II and III respectively.

 

The very rapid setting of gypsum makes it difficult to use it for some purposes and calls for setting retarders (keratin or lime-keratin glue and sulphite-alcohol vinasse in amounts from 0.1 to 0.3% of the weight of gypsum). When necessary to accelerate the setting of gypsum, additions (e.g., dihydrate gypsum, common salt, sulphuric acid) are introduced. Setting retarders slow down the solution and the diffusion processes. Setting accelerants act in an opposite manner. Some of the accelerants raise the solubility of semihydrate gypsum, others (dihydrate gypsum) form nuclei around which the entire mass crystallizes.

Building gypsum is used for the manufacture of gypsum and gypsum- concrete structural items for interiors of buildings (partition slabs, panels, plaster boards), for preparing gypsum and complex mortars and for the manufacture of ornamental and finishing materials (e.g., artificial marble).

Extra-strong gypsum is a variety of semihydrate gypsum. Common building gypsum is obtained by heating natural gypsum rock at nor­mal pressure, fine crystals of semihydrate calcium sulphate being formed under these conditions (^-modification). The resultant gypsum is very hygroscopic (60-65%) water. The excess water, i.e. beyond the amount necessary to hydrate gypsum (15%), evaporates and leaves pores, with the effect that hardened building gypsum has a high poro­sity (up to 40%), and, therefore, a low strength.

Heating of natural gypsum by steam at a pressure of up to 0.2- 0.3 MPa followed by drying at 160-180°C, produces semihydrate gyp­sum of a-modification. The process results in the formation of larger crystals, the effect being a lower hygroscopicity (40-45% water) and gypsum stone of greater density and strength. This type of gypsum is called extra-strong: in 7 days it attains a strength of 15-40 MPa.

Extra-strong gypsum is produced in small quantities and used mainly in the metallurgical industry for the manufacture of moulds. However, it can successfully replace common building gypsum for making items of great strength.

2. Anhydrite Binding Materials

Anhydrite cement is obtained by burning natural dihydrate gyp­sum at a temperature of 600-700°C and then grinding the product together with hardening catalyzers (lime, mixture of sodium sulphate with green or blue vitriol, burned dolomite, granulated basic blast­furnace, slag, etc.).

P. Budnikov suggested an anhydrite binder of the following compo­sition: lime, 2-5%; a mixture of sodium bisulphate or sulphate with green or blue vitriol in amounts of 0.5 to 1% each; dolomite burned at 800-900°C, 3-8%; granulated basic blast-furnace slag, 10-15%. Green and blue vitriols consolidate the surface of hardened anhydrite cement, so that the catalyzers do not seep out and discolour the item's surface. The action of the catalyzers is due to the ability of anhydrite to form complex compounds with various salts in the form of an unstable multiple hydrate, which then decomposes yielding CaSO₄×2H₂O.

Anhydrite cement can also be obtained by grinding natural anhyd­rite with the above additives.

Anhydrite cement is a slowly setting binder; its setting starts not earlier than in 30 min and ends not later than in 24 h. By compressive strength, this cement is available in grades 50, 100, 150 and 200.

Anhydrite cements are used for preparing bricklaying and plastering mortars, concretes, heat insulating materials, artificial marble and other ornamental items.

A variety of anhydrite cements is' the high-burned gypsum (estrich gypsum). It is manufactured by burning natural gypsum or anhydrite at a temperature between 800 and 1000°C followed by fine grinding. This results not only in complete dehydration but also in partial decomposition of anhydrite with the formation of CaO (3-5%). When estrich-gypsum is mixed with water, CaO acts as a catalyzer which promotes the hardening of the anhydrite cement in a manner discussed above.

High-burned gypsum is used to prepare bricklaying and plastering mortars, to build mosaic floors, to manufacture artificial marble, etc. Items from high-burned gypsum have low heat and sound conductivity, higher frost and water resistance and a smaller tendency to plastic deformation than products from building gypsum.

3. Some Data on the Manufacture of Gypsum Binding Materials

The decisive factor is the concentration of production, the cost of gypsum at the larger and better equipped works of yearly capacities in excess of 100,000 tons is almost half that at the smaller works (Table 1).

Table 1

 

Table 1 evidences that about 40% of gypsum are produced at works whose yearly capacity is less than 100,000 tons. The wide varia­tions in costs (by 2 to 4 times) indicate the great potentialities for lowering them.

Table 2 presents the costs of gypsum at different works.

 

Of interest is the high relative amount of expenses for raw and auxi­liary materials (33.0 to 53.9%) and of wages with extras (23.6 to 36.8%).

The cost of gypsum at works operating on delivered raw materi­als, according to data supplied by the VNIIstrom Institute, exceeds roughly by a factor of 1.5 that at the works which produce gypsum from own materials, the expenses for the raw materials then differing by 3 to 3.5 times. This emphasizes the importance of minimizing haulage of the gypsum stone and improving quarrying operations.

There are as yet vast untapped resources for bringing down the labour requirements through mechanization and automation of pro­duction processes.

The prospects for wide introduction of gypsum items are governed to a great extent by the availability of the building gypsum stone and the suitability of the deposits for industrial quarrying.

The industrial reserves of the raw materials (about 6 milliard tons) are sufficient to supply the existing works of gypsum binders and items, ensure their further development and meet the demands for these types of products.

 

4. Magnesian Binding Materials

Caustic magnesite and caustic dolomite are two varieties of magnesian binders.

Caustic magnesite is obtained by burning magnesite rock MgCO₃ in shaft or rotary furnaces at temperatures of 650 to 850°C, with the effect that MgCO₃ decomposes according to the reaction MgCO₃=MgO+CO₂. The solid residue (magnesium oxide) is ground to fine powder.

Caustic dolomite (a mixture of MgO and CaCO₃) is obtained by burning natural dolomite (CaCO₃×MgCO₃) and grinding the product to fine powder. As dolomite is burned, CaCO₃ is not decomposed and remains inert as a ballast, thus reducing the binding activity of caus­tic dolomite as compared to caustic magnesite.

Magnesian binders are mixed not with water, but with aqueous solutions of magnesium sulphate or chloride. Commonly used is a solution of magnesium chloride MgCl₂. Magnesian binders, mixed with solutions of magnesian chloride, provide greater strength than those mixed with magnesium sulphate solutions.

Caustic magnesite hardens relatively fast. It should begin to set not earlier than 20 min and harden not later than 6 h from the time it is mixed. Caustic magnesite is available in grades 400 through 600, which are determined according to the compressive strength of cubic specimens from 1 : 3 hard mortar solution after 28 days of ageing in the air.

Caustic dolomite binding material is of a poorer quality as it con­tains much calcium carbonate. It should run not less than 15% of magnesium oxide and not more than 2.5% of calcium oxide. Caustic dolomite is available in grades from 100 to 300.

Magnesian binders, being air-setting, have a poor resistance against water. They may be used only in an atmosphere of a relative humidity of not more than 60%.

Caustic magnesite readily absorbs humidity and carbon dioxide, contained in the air, to yield magnesium oxide hydrate and magne­sium carbonate, and this clearly indicates why it should be stored in air-tight containers.

Xylolite (a mixture of sawdust and binder), used to make floors, as well as fibrolite and other heat-insulating materials, are manufac­tured from magnesian binders which are also used for fabricating items for interior facing of buildings, subflooring, sculptures.

5. Acid-Resistant Cements

Acid-resistant cements are used for lining chemical apparatus and for building towers, tanks and other installations for chemical in­dustry. Acid-resistant cements consist of an aqueous solution of so­dium silicate (soluble glass), an acid-resistant aggregate and an additive (hardening accelerant). The microaggregates are quartz, quartzites, andesite, diabase and other acid-resistant materials; the hardening accelerant is sodium fluosilicate.

Soluble glass is melted from quartz sand, ground and thoroughly mixed with soda ash, sodium sulphate or potassium carbonate in glass tanks at a temperature between 1300 and 1400°C. Melting takes from 7 to 10 h. The resultant glass mass flows from the furnace into cars where it cools rapidly and breaks up into pieces called "silicate lumps". This glass is soluble in water under normal conditions, but when exposed to the action of high-pressure steam (0.5-0.6 MPa at about 150°C), it fairly readily becomes liquid.

Soluble glass hardens in the air because atmospheric carbon dioxide causes amorphous silica SiO₂×2H₂O [Si(OH)₄] to settle out and dry according to the reaction

Na₂SiO₃+CO₂+2H₂O×Si(OH)₄+Na₂CO₃

But this process is very slow in the air. Hardening of soluble glass is accelerated by adding a catalyzer sodium fluorosilicate (Na₃SiF„) which interacts rapidly with soluble glass to produce a silicate gel

Na₃SiF₆+2Na₂SiO₃+6H₂O - 6NaF+3Si(OH)₄

Soluble glass is also used for preparing acid-resistant and heat- resistant coatings. It should not be used in constructions subjected to the action of water, alkalis and phosphoric, hydrofluoric or fluosilicic acids for long periods of time.

Silicate lumps may be shipped in containers or in bulk. Soluble glass, which is of a syrupy consistency, is transported in barrels or glass bottles.

Acid-resistant cement is not water-resistant: it fails when attacked by water or weak acids. To enhance the water-resisting property of cement, 0.5% of linseed oil or 2% of ceresit are added to it. The hydro­phobic cement thus obtained is known as the acid- and water-resistant cement, or КЦБ-cement.

6. Building Lime

Building lime is produced by burning (to eliminate carbon dioxide) calcium-magnesium rocks, such as chalk, limestone, dolomite- and marl-bearing limestones, dolomite and marl-bearing chalk.

Fine building lime is obtained by slaking with water or grinding of the nonslaked lime in the process of which mineral fine additives may be introduced. The building lime is used for preparing building mortars, concretes, and binders, and for manufacturing artificial stones, slabs and miscellaneous building parts.

Depending on setting conditions, a distinction is made between an air-hardening lime which causes building mortars and concretes toharden and retain adequate strength un­der air-dry conditions and a hydraulic lime added to mortars and concretes so that they set and remain strong in the air and in water. By the type of main oxide it contains the air-hardening lime is divided into calcium, magnesium and dolomite varieties. As to its hydraulic properties, the hydraulic lime is divided into weak- and strong-hydraulic classes. In appearance, hydraulic lime is lump or powderlike; the powder hydraulic lime is of two kinds, aground lime and a hydrate lime (fine slaked lime) obtained by the hydration (slaking with water) of calcium, magnesium and dolomite lime.

Building air-hardening lime is manu­factured from calcium-magnesium -bearing rocks containing more than 6% clay im­purities. Lime manufacture consists in its quarrying and preparation (crushing and screening) and burning. After bur­ning, lump lime is ground to unslaked powder lime, or is slaked with water to yield slaked lime.

The main stage in lime manufacture is burning in which limestone decarbonizes and turns into lime according to the reac­tion CaCO₃=CaO+CO₂. The dissociation of carbonate rocks is accompanied by an absorption of heat (1 g-mole CaCO₃ re­quires for its dissociation approximately 190 kJ of heat). The dissociation of calcuim carbonate is a reversible reaction and is governed by the temperature and the par­tial pressure of carbon dioxide. The dis­sociation of calcium carbonate is appre­ciable at temperatures above 600°C. Theoretically, the normal dissociation temperature is considered to be 900°C. In works practice, the temperature of limestone burning is set generally between 1000 and 1200°C with due regard for limestone density, content of impurities, type of burning kiln and several other factors.

The burning of limestone removes carbon dioxide which accounts for up to 44% of its mass, the volume of the product diminishing by approximately 10%, and, in consequence, the lime lumps acquiring a porous structure.

Limestone is burned in various types of kilns (furnaces), as shaft rotary kilns, fluid-bed reactors, flash burning furnaces and other ty­pes of arrangements. Most widely used are shaft shelf-type kilns, which are economical in fuel consumption, but yield a lime contaminated by fu­el ash.

The shaft furnace con­sists of a shaft, a charging and a discharging arrangement and, blast (air) supply and gas exhaust means. Limestone is charged into the shaft ' ^ furnace in batches or continuously from the top. As lime is discharged, the column of materials descends, while hot combustion gases move in a direction counter to that of the material being burned.

By the character of processes oc­curring in the shaft furnace, one may distinguish the heating, burning and the cooling zone. In the heating zone in the top part of the fur­nace where the temperature is ne­ver held higher than 900°C, lime­stone is dried, preheated, and orga­nic impurities are burned out. In the middle part of the furnace - in the burning zone where the tempera­ture lies between 900 and 1200°C- CaCO₃ is dissociated with the evo­lution of carbon dioxide. In the bottom part of the furnace, the coo­ling zone, lime is cooled by the air

coming from below from a temperature of  900 down to 50 or 100°C.

Gas-fired furnaces yield "pure" lime, they are simpler in operation and amenable to mechanization and automation.

Rotary kilns yield high quality lime, but are disadvantageous.as regards fuel consumption.

Highly efficient fluid-bed reactors nave been lately gaining ground in limestone burning. A fluid-bed reactor (Fig.7) is a metal shaft li­ned internally with refractories and divided along its height by grate- type roofs into 3 to 5 horizontal zones. Material is transferred from zone to zone through pipes fitted with a limiter. The height of the fluid bed is from the edge of the transfer pipe to the grate-type roof. Gas or fuel-oil burners are provided about the periphery of the reactor. The multi-zone design of the reactor enables the production of high quality lime against a reasonable consumption of fuel. The product from the burning of carbonate rock is known as unslaked lump lime, or quicklime. This lime is either ground prior to use or directly slaked.

 

Ground unslaked lime. Building air-hardening lime is manufactu­red in three grades-I, II and III. Unslaked lump or ground lime sho­uld meet the requirements listed in Table .3.

 

 

 

 

 

 

 

 

 

 

 

 

 

l - flow of waste gases to cleaning; 2 - grid roof; 3 - overflow pipe; - dis­charge of burnt lime; 5- supply of comp­ressed air; 6 - blast box with grid; 7 - burners; 8 - charging of lime

Fig. 7.Fluidized-bed reactor for lime burning

 

 

As specified in the State Standard GOST 9179-70, unslaked lime should be ground to a fineness of not more than 1 and 10% oversize on screens No. 023 and 008, respectively. Generally, commercial lime is characterized by a screen No. 008 oversize of 2 to 7%, which corre­sponds to a specific surface area of 3500 to 5000 cm²/g.

Ground unslaked lime is transported in air-tight metal containers or bitumen-impregnated paper bags. Ground lime may be stored for 10 to 15 days in dry warehouses. When handling lime, rules prescribed by labour protection should be observed, since particles of ground lime may cause much harm if they penetrate the lungs or set on mucous mem­branes.

Ground unslaked lime may be used as such, this presenting a number of advantages: no unslaked grains of lime are lost; the heat released by the hydration of lime speeds up the setting and hardening processes. Items from this kind of lime are denser, stronger and more water-re­sistant.

Hardening of mortar and concrete mixes incorporating ground unsla­ked lime may be accelerated by the introduction of calcium chloride, or slowed down during the initial (setting) period by additions of gyp­sum, sulfuric acid and sulfite waste liquor. Moreover, the additions of gypsum and calcium chloride enhance the strength of mortars and con­cretes, while retarders prevent cracking, which is otherwise bound to occur when prescribed hardening conditions are not met.

Slaked lime. Air-hardening lime differs from other binders in that it may be powdered not only through grinding, but also through sla­king, or the action of water upon lumps of unslaked lime,with a release of a considerable amount of heat according to the reaction

CaO+H₂O=Ca(OH)₂+65.5 kJ

One g-mole of CaO releases 65.5 kJ of heat, and 1 kg of quicklime. 1160 kJ.

Theoretically, the slaking of lime to powder requires 32% water of the mass of CaO. Practically, the amount of water used is 2 or 3 times greater, depending on the lime's composition, the degree of its burning and the slaking method, since the heat evolved during slaking vapori­zes some of the water. The lime slaking rate is affected by the tempera­ture and the size of lime lumps, the rate increasing with the tempera­ture. Slaking is particularly rapid when performed under elevated pressure in closed drums.

Depending on its slaking rate, air-hardening lime is divided into the following kinds: rapid slaking lime with a slaking time of not more than 8 min; medium-slaking lime, of up to 25 min; slowly-slaking li­me, of not less than 25 min.

Lime is slaked to powdered state in special apparatus called hydra- tors.

Quicklime is slaked to lime paste in a type "ЮЗ" slaker in which lump lime is both ground and mixed with water to form lime water; the latter is transferred to a separator where it is settled out to yield lime paste. Lime paste with a relatively high amount of unslaked grains

Content of active CaO + MgO in hydrate lime in terms of dry matter, %, not less than: without additives with additives Content of CO₂, %, not more than Moisture content of lime, %, not more than Fineness: oversize, %, not more than, on screens with wire mesh of: No. 063 No. 008 must not be used, as these grains may slake in the brickwork and thus cause cracking of the hardened lime mortar. The breaking of lime lumps in the "ЮЗ" slaker contributes to a practically complete slaking of lime, whereas in other types of apparatus the proportion of unslaked grains (waste) may be as high as 30%.

Hardening under air-dry conditions only is typical of what is called- air-hardening lime. The evaporation of water, which takes place in the process, causes the fine particles of Ca(OH)₂ to agglutinate into coarser aggregates followed by their crystallization. The resulting Ca(OH)₂ crystals coalesce into a carcass that encloses sand particles. This is paralleled by the carbonization of calcuim oxide hydrate thro­ugh absorption of carbon dioxide and water according to the reaction Ca(OH) ₂+CO ₂+?гН ₂O=CaCO ₃+(n+ 1)H ₂O

The hardening of lime mortars thus results from their drying and forming a crystalline aggregate Ca(OH)₂, and from the crystallization of calcium carbonate on the surface of items. Slaked lime hardens slow­ly and the strength of lime mortars is low because the crystallization of calcium oxide hydrate proceeds slowly and the crystals are weakly bonded to one another. Moreover, a crust of CaCO₃ formed on the sur­face prevents air from penetrating further inside the lime mortar and thus inhibits further carbonization.

Calcium oxide hydrate crystallizes the faster the greater the evaporation of water, and therefore an above-zero temperature is essen­tial for the hardening of lime.

Application, transportation and storage. Air-hardening lime is wi­dely used in the preparation of mortars for bricklaying and plastering, in the manufacture of lime-puzzolana binding materials, artificial stone materials, such as lime-sand brick, lime-sand and foam lime- sand items, slag concrete blocks and also as painting compositions.

Lump lime is transported in bulk, care being taken to protect it against humidity and contamination; ground lime is generally packed in special paper bags or closed metal containers. Lime paste is transpor­ted in specially fitted dump truck bodies.

Unslaked lime should be stored in closed warehouses and protected against moisture. Hydrate lime may be stored for a short period of time in bags in dry storage areas.

Ground lime should not be stored for more than 30 days, as it is gradually slaked by air moisture and loses its activity.

7. Lime Manufacture Performance Data and Means for Lowering Costs

The cost of 1 ton of construction lump lime is approximately 12 $, of ground lime, 12.5 to 13 $, of powder slaked lime, more than 14 $, i.e. higher than the average cost of cement. This is due to poor technical standards of lime manufacture, an insufficient concentration of production, greater expenses for raw materials and fuel (Table 5).

 

Table 5

 

The decisive factor in lime costs is the works average capacity (Table 6). The smaller, poorly equipped works account for more than half of the lime output; the cost of lime at the smaller works of yearly output of up to 10,000 tons is twice as high as that at the larger works. It is therefore economically advantageous to build large- capacity lime production plants according to new standard designs in­corporating improved lime-burning kilns.

At lime-sand items works, lime is manufactured generally in shaft furnaces and rotary kilns. The yearly average of shaft furnace operation time in the whole of the varies over a very wide range, from 187 to 346 days.

 

 

 

By their output, the furnaces may be characterized by the data listed in Table .7.

Table .7

Classification of Lime Works by Output per m³ of Shaft Furnaces

 

The improvement in the operation of shaft furnaces in terms of out­put and on-blast time makes it possible to raise the output of commerci­al lime by 10 to 12% and to lower its cost. Materials and equipment account for more than half of the lime cost, of which the share of the raw material expenses is particularly great. The investigations by the VNIIstrom Institute have indicated that these expenses at works operating on their own raw materials are 2.5 times less than at works using limestone supplied from elsewhere. In most of the instances, this limestone is nonfractionated and thus needs crushing and sizing.

The concentration of quarries and the quarrying mechanization make it possible to lower the cost of limestone by 30 to 40% and to reduce the transportation and handling expenses. Improvements in quarrying enterprises and limestone-burning shops of lime-sand items works may lower production shop cost of lime by another 7 to 8%.

Fuel is a large item in lime cost (22.5%). Conversion of works to gas fuel not only reduces the cost (1.5 to 2 $/t), but also enhances the quality of lime. A major prerequisite for reducing the consumption of fuel and improving the quality of lime is to ensure optimum com­bustion of fuel in limestone-burning furnaces.

Characteristic of lime works is a low level of mechanization (30 to 60%). It is of utmost importance to mechanize the loading and unloa­ding operations and the transportation of limestone and fuel to the furnaces.

The introduction of measures aimed at lowering the cost of limestone burning, the organization of the supply of sized limestone and coal to lime works, the mechanization of labour-consuming operations and the improvement in labour organization are bound to bring down lime costs at existing lime works by not less than 20 to 25%.

The production of lime is certain to rise in the next few years. The steps to be taken in the near future are as follows: increase in the degree of concentration and improvements in production facilities through ere­ction of specialized limestone works of annual output of 300,000 tons and more; gradual closing of the smaller and the unprofitable works and shops equipped with obsolete types of lime furnaces; commissio­ning of large specialized quarries to supply lime works with sized limestone; improvement in the quality of lime binders and expansion of their range. The major means to increase the production capacity is to build new highly-mechanized works with efficient burning furna­ces (kilns).

 

 

Unit 6. HYDRAULIC BINDERS

Hydraulic Lime

Hydraulic lime is a product of moderate burning (at temperatures between 900 and 1100°C) of marly limestone containing from 6 to 20% of argillaceous impurities; part of CaO resulting from the decom­position of calcium carbonate combines in solid state with oxides SiO₂, A1₂O₃ and Fe₂O₃, contained in the clay minerals, to form silicates (2CaO-SiO₂), aluminates (CaO×Al₂O₃) and calcium ferrites (2CaO•Fe₂O₃) that are capable of hardening not only in the air, but in water as well.

Since hydraulic lime contains free calcium oxide CaO, it slakes like air-hardening lime upon contact with water. The more is the amount of free CaO in hydraulic lime, the less is its ability to harden in water.

Construction hydraulic lime is manufactured as a fine powder, in which the amount of screen No. 008 oversize should not exceed 10%. In addition to clay and sand, the marly limestones commonly contain 2 to 5% of magnesium carbonate and some other impurities. Hydraulic lime should be manufactured from limestone with as uniform distri­bution of clay and other inclusions as possible, since the quality of the resultant product greatly depends on this factor.

Hydraulic lime is available in two kinds: a weak hydraulic lime with a modulus of 4.5 to 9 and a strong hydraulic lime with a modulus of 1.7 to 4.5. If the burning product has a hydraulic modulus of less than 1.7, it is referred to Roman cement (m=l.l to 1.7), and greater than 9, to air-hardening lime.

Hydraulic lime, after it is mixed with water and begins to harden in the air, continues to harden in water as well, in which case the physi­cal and the chemical air-hardening processes combine with those of the hydraulic hardening. As water evaporates, calcium oxide hydrate gradually crystallizes and also carbonizes due to the carbon dioxide of the air. The hydraulic hardening of lime takes place through hydration of silicates, aluminates and ferrites of calcium in the same manner as in Portland cement.

Construction hydraulic lime is required to meet the specifications listed in Table 8. Hydraulic lime is used as a fine powder in terms of dry sub­stance, %: not less than not more than Content of active MgO, %, not more than Content of carbon dioxide (CO₂), %, not more than Ignition losses, %, not more than Compressive strength of specimens, 10^-1 MPa, not less than: after 7 days after 28 days Size composition after grinding, screen oversize, %, not more than: No. 02 screen No. 008 screenring construction mortars intended for service in dry or moist surro­undings, as an admixture to lower grades of concretes, etc. Unlike air-hardening lime, hydraulic lime offers less plastic mortars, which harden more rapidly and uniformly throughout the wall thickness and possess a high strength.

Roman Cement

Roman cement is a product of fine grinding of pure and dolomite bearing marls which contain not less than 25% of clayey impurities and which have been burned just below the caking point. The properties of Roman cement are adjusted by the introduction of up to 15% of active mineral additives and up to 5% of natural dihydrate gypsum.

The raw materials for the manufacture of Roman cement are marls which are a natural mixture of calcium carbonate and clays. Most desirable are marls of such a combination of limestone and clays, which, when burned short of caking, yield a product carrying no free calcium oxide. These are marls with a rather low calcium carbonate content and a hydraulic modulus lying between 1.1 and 1.7. The manu­facture of Roman cement consists in quarrying marl, crushing it to lumps of a specified size, burning it and, finally, grinding the burnt material. Raw materials are burned mostly in shaft furnaces and some­times in rotary kilns at temperatures from 1000 to 1100°C. Burnt mate­rial is best ground in ball mills with an admixture of gypsum and acti­ve mineral agents, as this yields a more uniform product.

Setting and hardening of Roman cement involves the hydration of calcium silicates and aluminates, similar to ones contained in the hyd­raulic lime, which are the main constituents of Roman cement. Ro­man cement should set in 15 min and harden not later than 24 hours after it has been mixed with water.

Roman cement is a slow hardening binder with relatively low strength grades of 25, 50 and 100. Roman cement size composition is defined by a screen No. 02 oversize of not more than 5%, and a screen No. 008 oversize of not more than 25%.

Roman cement is used for plaster and bricklaying mortars, low grade concretes, manufacture of wall stones and smaller slabs, in particular of steam-treated ones.

Portland Cement

Composition of portland cement. Portland cement is a hydraulic binding material which hardens in water and in the air. It is manufactured by fine grinding of a mixture of limestone and clay that has been burned until the components have caked; presence of clay ensures the predominance of calcium silicate in the clinker, which is a mixture of raw materials burned to fusion and composed of grains up to 40 mm in size. The quality of clinker governs the major properties of cement: strength and rate of strength gain, durability, resistance to various service conditions.

Setting time of cement is controlled by adding gypsum during grinding in amounts from 1.5 to 3.5% of the weight of cement in terms of sulphur trioxide SO₃. Portland cement may be manufactured with or without active mineral admixtures, which are added in amounts of up to 15% of the weight of cement.

The quality of clinker depends on its chemical and mineralogical compositions. Portland cement clinker is manufactured from limestone and clay. Limestone consists chiefly of two oxides (CaO and CO₂) while clay is composed of various minerals containing basically three oxides (SiO₂, A1₂O₃ and Fe₂O₃). Carbon dioxide CO₂ is removed by burning the raw mixture, the four remaining oxides CaO, SiO₂, Al_aO₃ and Fe₂O₃ forming the clinker material.

 

The content of oxides in ce­ment is approximately as follows (in %):

Calcium oxide CaO    62-68

Silica SiO₂                 21-24

Alumina A1₂O₃              4-8

Ferric oxide Fe₂O₃         2-5

Aside from the above chain oxides, portland cement clinker may also carry other oxides, e.g., magnesium oxide MgO, alkali oxides K₂O and Na₂O, whose presence affects adversely the quality of cement. Magnesium oxide, burned at a temperature of about 1500°C slakes very slowly when mixed with water, and causes cracks after mortar or con­crete hardens, and, therefore, its content in portland cement should not exceed 5%. Presence of more than 1% of alkali oxides may cause failure of the concrete made from the cement.

The chief oxides already mentioned are not free in the clinker but combine in the process of burning into four principal minerals whose relative amount in portland cement is as follows (in %):

Tricalcium silicate 3CaO×SiO₂ (alite) 45-65

Dicalcium silicate 2CaOSiO₂ (belite) 15-35

Tricalcium aluminate 3СаO×А1₂O₃, .... 4-14

Tetracalcium alumferrite 4CaO×Al₂O₃×Fe₂O₃  10-18

The above minerals are designated for short as C₃S, C₂S, C₃A and C₄AF.

When a raw mixture has been correctly calculated and thoroughly prepared and burned, the clinker should contain no free calcium oxide CaO since lime that has been overburned at about 1500°C slakes very slowly, similar to MgO, and expands, a fact which may cause cracking of concrete as it hardens.

Portland cements obtained industrially from various kinds of natu­ral material and by a variety of production techniques differ both in chemical and mineralogical compositions and in properties. Standard specifications do not cover fully all the structural properties of cement, such as resistance to corrosive media, frost resistance, intensity of heat release, deformation capacity, etc. However, of great' help in this respect is the knowledge of the mineralogical composition of clinker, which bears direct relation to chief physical and mechanical properties of cement; this allows to predetermine the properties of port- land cements and to design cements for preparing concretes for speci­fic working conditions.

Manufacture of portland cement. Raw materials for the manufacture of portland cement should contain 75-78% of CaCO₃ and 22-25% of clayey substance. Rocks meeting the above requirements are seldom found in nature. Therefore, portland cement is usually manufactured from limestone and clay, which are complemented by the so-called correcting admixtures, containing a considerable amount of oxides that are missing in the raw mixture. Thus, a deficit of SiO₂ calls for an introduction of high-silica substances (opoka, diatomite, tripoli). The content of iron oxides may be increased by the addition of pyrite cinders or ore. The content of alumina A1₂O₃ is raised by introducing high-alumina clays.

In addition, an ever wider use is being made in the cement-making industry of various by-products, such as waste from a number of indu­stries, e. g. blast furnace slags, nepheline slime and others. Nepheline slime-an alumina production waste-contains 25 to 30% SiO₂, 50 to 58% CaO, 2 to 5% A1₂O₃. 3 to 5% Fe₂O_sand3 to 8% of other oxides. If 15 to 20% of limestone are added, the composition of the mixture becomes similar to that used in the manufacture of portland cement.

The fuel used is pulverized coal (or anthracite), fuel oil and natural gas. At present, the cement-making industry operates mainly on gas fuel.

The flowsheet for manufacture of portland cement involves the follo­wing main operations: quarrying limestone and clay, preparing raw materials and blending them; burning the mixture; grinding the resul­tant clinker to a fine powder together with gypsum and sometimes with additions.

There are two chief techniques for manufacturing portland cement: the wet and the dry methods, which differ by the manner of preparing the raw material mixture. In the wet method the source materials are ground and mixed in the presence of water, and the mixture is burned in the form of a pulp (slime of liquid consistence); in the dry method, the materials are ground, mixed and burned dry. Along with the two main techniques, there is an increasing trend to use a combined method, which incorporates the advantages of the dry and the wet methods. According to this technique, the raw mixture is prepared by the wet method, then slime is dewatered and processed to granules which are burned by the dry method.

Each of the methods has its own advantages and shortcomings. In water, the materials are readily ground and rapidly homogenized, but the consumption of fuel in burning is 1.5 to 2 times greater than in the dry technique. The dry method had long been unpopular due to a poor quality of the clinker. However, advances in grinding and homogeni­zing of dry mixtures have contributed to the manufacture of high quality dry-method portland cement, and in the last decade the meth­od has gained considerable ground. At present, the world trend is to convert cement works to the dry method through introduction of. high- capacity rotary kilns (Japan, FRG). As compared to the wet technique, the combined method lowers the consumption of fuel by 20 to 30%, but requires more electric power and labour.

In the RK, up to 88% of cement are manufactured by the wet me­thod; in the USA, up to 58%; in Canada, 66%; in Great Britain, 94%. In Japan, FRG, France, Italy, Sweden and Mexico, where the raw ma­terials are crystalline limestones of a low natural moisture content, the dry method prevails.

By the wet method (see flowsheet in Fig.) lump source material transported from the quarry to the plant is crushed to a size not larger than 5 mm. Hard rocks are broken up in crushers, whereas softer rocks (clay, chalk) are ground by stirring with water in blungers. A blunger is a round reinforced-concrete tank, 5 to 10 m in diameter and 2.5 to 3.5 m high, lined with cast-iron plates. A crosspiece with steel rabbles suspended from it on chains for breaking down clay lumps revolves about a vertical axis in the blunger. The resultant pulp with a moisture content of 50% is discharged through an opening, protected against plugging by a wire-mesh, and is pumped into a tubular rotary mill which is also continuously fed with crushed limestone.

A ball mill (see Fig. bellow) is a steel cylinder up to 15 m long and up to 3.2 m in diameter, revolving on hollow trunnions, through which the mill is charged at one of its ends and discharged at the other. Inside, the mill is divided by perforated partitions into three chambers.

 

1 - supply of limestone from quarry; 2 - limestone crusher; 3 - supply of elay from pit; 4 - supply of water; 5 - clay kneading tank; 6 - mill for raw materials; 7 - slurry tanks; 8 - rotary kiln; 9 - cooler; 10 - supply of pulverized coal to furnace; 11 - elevator for transporting coal from crusher to bin; 12 - rotary kiln for drying coal; 13 - coal mill; 14 - coal storage; 15 - gypsum storage; 16 - elevator for supplying gypsum from crusher to bin; 17 - clinker storage; 18 - ball mill; 19 - cement silos; 20 - cement packing

The first and the second chambers are loaded with steel or cast iron balls, and the third, with small cylinders. Pulp enters the first chamber of the rotary mill through a hollow trunnion. As the mill revolves, the balls are pressed aga­inst the walls by the action of centrifugal and friction forces and thus caused to rise to a cer­tain height, then fall to crush and grind the material. Tubular mills are devices of continuo­us action. Finely ground material of a creamy consistency (pulp) is pumped to reservoirs (which are cylindrical reinforced-concrete or steel vessels) where the chemical composition of the pulp is given a final correction, and a certain amount of material is stored to ensure uninterrupted operation of rotary kilns. Pulp is pumped from reservoirs to tanks, and then is fed uniformly to a rotary kiln for burning.

A rotary kiln is a long cylinder from sheet steel, lined on the inside with refractories. Kilns measure 150, 185 or 230 m in length and 4, 5 or 7 m in diameter. The cylinder is sloped 3.5 to 4° and revolves about its axis at a speed of 0.5 to 1.4 rev/min. Pulp is injected at the top end of the kiln and moves toward the bot­tom end.

Fuel gas or pulverized coal is injected toge­ther with the air at the opposite end of the kiln and burns to give a temperature of abo­ut 1500°C. Combustion gases are exhausted from the top end.

The pulp moving along the length of the cy­linder comes into contact with hot gases, which flow in the opposite direction, and gradually heats up, with the effect that the mechanically bound water vaporizes and the pulp dries to lumps. Next, organic matter burns out and dehy­dration takes place (removal of chemically bound hydrate water). Calcium carbonate de­composes at a temperature between 800 and 900°C according to the reaction

CaCO₃=CaO+CO₂

Resultant carbon dioxide is exhausted together with the combustion products, whereas CaO interacts chemically at about 1000°C with clay oxides to form dicalcium silicate, tricalcium aluminate and tet- racalcium alumoferrite. At 1300°C the two latter compounds melt, the resultant liquid dissolving some of the CaO and 2CaO×SiO₂ until saturation is achieved and the two components interacting in molten state give tricalcium silicate ЗСаО -SiO₂, the ma'ir mineral constituent of portland cement.

The hot clinker thus obtained is transferred to a cooler, where it is rapidly cooled by a countercurrent of cold air. Clinker, discharged from rotary kiln coolers at a temperature of about 100°C is transported for final cooling and ageing to storage areas, where it remains for 15 days. If free lime is present in the clinker, it is slaked during ageing by the air moisture. At highly mechanized plants with an efficiently organized production process, the quality of clinker is so high that there is no need for ageing.

Clinker is ground together with admixtures in tubular multi-cham­ber mills.

Finished portland cement (at a temperature of 100°C and more) is conveyed by pneumatic transport to silos for cooling. Once cooled it is packed in batches of 50 kg in multilayer paper bags or loaded into special shipping facilities. Portland cement is manufactured by the dry method whenever the raw materials are marls or mixtures of hard limestones and clays of low moisture content.

The manufacture of cement by the dry method is simpler as compared to the wet technique: slime preparation is omitted; individual flow­sheet operations may be performed in a single unit (autogenous grin­ding or aerofall mills, homogenizing storage facilities, grinding and drying mills for raw materials, etc.).

In the dry method, the raw materials-marls, limestones, and clay- are crushed in type C-776 crushers to 2.5 mm undersize (clayey material is processed in crushing-and-drying units). Once crushed, the raw mate­rials are transported by conveyors to storage facilities where they are homogenized with the aid of blending machines to specified chemical composition and fed to mill bins. From there, the charge ingredients, together with additives, are fed via weigh-feeders to receiving arrange­ments of grinding units where they are ground to required fineness, dried by off-gases of the rotary kilns and separated.

The gases, whose flow is induced by mill fans, entrain the fine ma­terial (flour) to cyclones for discharge. The flour is fed to correction silos for homogenization and transfer to service silos. From the silos, the raw mixture is transported by pneumatic lifts to charging arrange­ments fitted with weigh-feeders and then into cyclone heat exchangers of rotary kilns. In the heat exchangers, the raw charge is heated by counter-current hot gases of the rotary kiln to a temperature of 750- 800°C, partly decarbonizes, and then enters the burning kiln. The resultant clinker is cooled to 60-80°C in a grate cooler and then fed to separator mills for grinding.

Cement is transported to silos, and from there it is discharged for shipment in bulk or fed to a bag machine for packing prior to shipment in bags to consumers.

In the combined method, the raw materials prepared by the wet method and the slime having a moisture content of up to 40% are dewatered in filters down to 16-18% moisture content. The resultant cake is pelletized, and the pellets are burned according to the dry me­thod procedure.

Hardening of portland cement.

 Mixing of portland cement results in a sticky cement paste, which gradually thickens and becomes stone­like.

Hardening of portland cement involves very complicated chemical and physical phenomena. When water is added, each of the minerals reacts with it to yield new compounds. All the interactions of the indi­vidual clinker minerals occur simultaneously, are superimposed one upon another and influence each other. The resultant compounds may interact with one another and with the clinker minerals and produce ever new compounds. All this creates problems in the study of the hardening of portland cement. Reactions characteristic of the harde­ning of portland cement and other binders are hydration reactions which involve the addition of water molecules. The reactions proceed either with or without the decomposition of the main substance (hydrolysis).

Hardening of portland cement is thus governed in the main by the hydration of silicates, aluminates, and alumoferrites of calcium.

In case of complete hydration the interaction of C₃S with water at room temperature proceeds according to the pattern

2(3CaO SiO₂)+6H₂O=3CaO2SiO₂3H₂O+3Ca(OH)₂

Since the liquid phase of the hardening system is rapidly and fully saturated with calcium oxide, it is assumed that the initial reaction results in calcium hydrosilicate C₂SH₂, which converts to CSH(B) as lime precipitates as a solid phase. A beneficial effect upon the process has the passage of alkalis to solution, which reduce the concentration of lime therein.

The hydration of β-C₂S under the same conditions follows the above pattern, except for a lesser amount of the lime precipitate.

The interaction between C₃A and water proceeds very rapidly at the mixing temperature of 21 °C with the evolution of a substantial amount of heat

ЗСаО×A1₂O_з+6Н ₂O=3CaO×A1₂O₃×6H₂O

C₃AH₆ is the only stable compound among the calcium hydroalumi- nates.

As it interacts with water in the presence of dihydrate gypsum and hydrates at ordinary temperatures, tricalcium aluminate forms comp­lex compounds known as calcium hydrosulfoaluminates. This reaction proceeds according to the pattern

3СаО×A1₂O₃+3CaSO₄×2H₂O+25H₂O=3CaO×Al₂O₃×3CaSO₄×31H₂O

The natural mineral of the same chemical composition is called ettringite.

The alumoferrite phase, represented in ordinary portland cements by tetracalcium alumoferrite (C₄AF) when portland cement hydrates, i.e. when the solution is saturated with lime, interacts stoichiometrically at ordinary temperatures with water according to the equation

4CaO×A1₂O₃×Fe₂O₃+2Ca(OH)₂+10Н₂O=3CaO×A1₂O₃×6H₂O+3CaO×Fe₂O₃×6H₂O

Very stable combined crystals C₃(AF)H_e are formed as a result of the reaction.

In addition to the chemical transformations that occur during c.ement hardening, other important physical and physicochemical processes accompany the chemical reactions and result on the whole in the conversion of cement, on mixing with water, to a plastic paste and then to a strong hardened stone.

A great many investigations were devoted to the study of the chemi­cal and the physical transformations of the ceftient paste as it hardens.

In 1882, Le Chatelier put forward the so-called crystallization theory of hardening according to which the starting anhydrous cement mine­rals, possessing a greater solubility in water as compared to their hyd­ration products, give rise to solutions supersaturated with respect to the new hydrate formations. The latter precipitate from the super­saturated solution as crystals, forming as the process continues a cry­stalline aggregate having a sufficient strength.

V. Michaelis, who suggested in 1893 a colloidal theory, did not dis­claim the formation of crystalline products on hardening of portland cement, but considered that of importance to cements is not the stre­ngth, but the "hydraulic properties". He assumed that a strong, water impervious stone results mainly due to the formation during hardening of hydrogels of calcium silicates, aluminates and ferrites. According to Michaelis, the hydrosilicates fail to crystallize during the mixing of cement with water and that the hardening of concrete comes to the formation of gelatinous compounds (gels) due to swelling of cement grains under the influence of water and to subsequent hardening and growth of crystalline formations in these gels.

As already mentioned, A.A. Baikov advanced in 1923 a binder hardening theory, which generalizes the views of Le Chatelier and Mi­chaelis. According to Baikov the hardening of cement is due to the combined crystallization and colloid-formation processes. Baikov stated that any substance which hardens through hydration is bound to pass through a colloid stage even if, in the final analysis, it produces a clear-cut crystalline growth (e.g. dihydrate gypsum). Let us recall that according to Baikov the hardening of portland cement comprises three steps. The first step involves dissolution of the binder in water until the formation of a saturated solution; the second step is the co­lloid formation or setting that is characterized by the direct addition of water to the solid phase of the binder and the appearance of hydrate compounds of extreme colloid fineness without the intermediate disso­lution of the starting material, the process being accompanied by the concrete mass setting; the third step is crystallization and hardening, when the gel-like formations recrystallize and turn into a crystalline growth, so that the system hardens and increases its strength.

A team of researchers under the guidance of P.A. Rebinder carried out comprehensive investigations which have greatly expanded the knowledge on hardening of binders. By their concept hardening invol­ves the dissolution of the unstable starting substances and the crystalli­zation of the thermodynamically stable new hydrate formations out of the solutions that are supersaturated with respect to them. First, these new formations and nonhydrated particles give rise to a coagula­ted structure, characteristic of which is a small strength and thixotropy, which is the property of the gel to restore its structure when disturbed. Next, the crystals gradually grow stronger in two stages: a framework of the structure is formed, and crystals of the new formations come into contact with one another; the crystals grow about the framework and thus enhance the structure's strength, though this may cause under certain conditions undesirable internal stresses.

The strength of the hardening system is at its highest when the inter­nal stresses acting upon the crystals of the new formations are the least.

Thus, as in the case of construction gypsum, there are two main viewpoints upon the mechanism of hydration of portland cement: hydration proceeds through the solution, wherefrom the new forma­tions precipitate, which are less soluble than the starting substances; and hydration takes places in the solid phase.

S. D. Okorokov considers the "colloidation" of clinker grains to be a spatial regrouping of elements of the crystal lattice, similar in me­chanism to polymer transformations. At the same time he notes that the interaction of the solid clinker grains with water is only possible at points of contact between the solid and the liquid phase, rather than throughout the body of the substance.