9.6. Properties of Portland cement. Corrosion of cement stone

Composition and properties of Portland cement. Properties of Portland cement depend on the composition and structure of the clinker. An enhanced calcium oxide content in the clinker, bound in minerals, results in the production of cement with high activity and speed of strength gain with age. The content of free CaO in a clinker ranges from 0 to 2%. As a rule, effort is made to keep the CaO content to the minimum during the reactions of clinker formation. This is because free calcium oxide remaining in the clinker may cause volume instability and may lead to reduction in the strength of a cement stone.

A high Magnesium oxide content may have a negative influence on the properties of cement. Therefore, the MgO content in Portland cement must not exceed 5%.

Harmful influence of free calcium oxide and magnesium oxide are characterised by their capacity for the slow slaking and development of internal tensions in hardened concretes and mortars.

The most important mineral compounds in the cement clinker are called alite and belite. An alite - is a solid solution of tricalcium silicate (C₃S) and little amount of Al₂O₃, MgO and others. Solid solution in this case is the result of interstitial of the indicated oxides into the crystalline lattice of tricalcium silicate. Alite determines the basic properties, of the Portland cement, the early strength and rate of strength gain.(Fig. 9.6).

Belite is the second most important of the clinker minerals. It is the solid solution of dicalcium silicate (C₂S) and tenuous amount of Na₂O, K₂O, Fe₂O₃ and others. It hardens slowly but steadily increases its strength over a long time.

Microscopic examination of cement clinker reveals clearly discernable prismatic crystals of alite and round-belit. The aluminate and aluminoferrite phases are the complement of intermediate matter which lies between them.

The aluminate phase is present in the form of tricalcium aluminate 3СаО∙Аl₂О₃ (C₃A). It is a high-early-strength mineral which is characterized by fast and highly exothermic reaction.

The aluminoferrite phase is the solid solution of different aluminoferrites and for many clinkers near on composition to tetracalcium aluminoferrite (C₄AF). It contributes very little to the strength gain.

Usually, the mineralogical composition of Portland clinker varies within the following percentage range: C₃S – 45-60; С₃А – 5-15; C₂S – 15-30; C₄AF – 10-20. However, the mineralogical composition for some special types of Portland cement may not correspond to these limits. Increased content of mineral-silicates (especially alite) improves strength and other properties of cement, but complicates the burning of the clinker. The rational compositions of clinker are selected at the production of cement, which provide both high quality of cement and optimum conditions of rotary furnace working.

Grinding cement clinker to a high fineness is the necessary condition to ensure high reactivity and binding properties. To ensure adequate fines, not less than 85% by weight of cement must pass through when sifted through a №008 sieve. Increasing the fines of cement also increases the surface area of the cement particles which enhances chemical reactivity and strength gain in proportion to the specific surface area. The specific surface area for Portland cement ranges from 2500-3500 cm²/g and is determined based on the speed of air passing through the layer of cement powder.

The absolute density of Portland cement varies between 3 – 3.2 g/cm³. The bulk density of cement depends on the degree of powder compression: in the loose state, it is 960-1300, but varies between 1600-1840 kg/m³ in the compacted state.

Cement, mixed with water, forms a plastic paste. The water demand of cements is estimated by the amount of water (in percents by weight of cement), which is necessary for forming the paste with normal consistency. The concept of normal consistency is conditional and determined by the standardized penetration of pestle of Vicat apparatus in cement paste. Portland cement is characterized by the comparatively small water demand. Its normal consistency ranges between 24 to 29%. High content of aluminates and mineral additives of sedimentary origin (tripoli powder, diatomite and others) and increased milling fineness increases the water demand of cement. Introduction of surface-active additives, such as so-called superplasticizers, can reduce the water demand. Increasing of water content affects the properties of cement such as: strength, shrinkage deformations, freeze-resistance and others, unfavorably. This is because increasing the water content beyond the theoretical necessity for good consistency increases the total porosity of cement stone.

Setting is the first stage of hardening of the cement paste. All the period of setting is divided into initial and final. Initial setting time of cement paste is considered to be the time period from the moment of mixing till the moment, when the needle of Vicat apparatus will not reach the plate on which a ring is set on 1-2 mm. Final setting time is considered to be the time from the beginning of mixing till that moment, when penetration of the needle into the paste will not be more than 1 mm. Initial and final setting of cements is rationed within the limits, comfortable for mortars and concretes preparation. Initial setting time does occur before, a period of between 45-60 minutes for cement. As a rule, it is observed within 2-4 hours from the moment of mixing. The final setting time for cement should not occur later than 10 hours. The indicated requirements are provided as a result of adding of gypsum additive to Portland cement. Gypsum – dehydrate slows the setting of Portland cement. The slowing action of gypsum is caused by the formation on the surface of grains of C₃A (the most fast-hardening phase of cement), protective coats of a new compound – hydrosulphoaluminate. This compound is the product of interaction of gypsum, tricalcium aluminate and water.

Strength of cement is determined after 28 days of hardening of specimens. The standard prism(beam) specimen, 4040160 mm in size, is made from cement-sand mortar 1:3 (1 part of cement, 3 parts of normal sand) using standard conditions. The cement strength increases intensively at early age, but the rate of strength gain slows considerably at later ages, The strength of high-early-strength cements is determined also after 2 days of hardening. Mechanical and physical requirements of cement accordingly to EN 197-1:2000 are shown in the Table 9.3.

Таble 9.3

Mechanical and physical requirements for cement

Strength class	Compressive strength, MPa			Initial setting time, min	Soundness (expansion), mm
	Early strength		Standard strength
	2 days	7 days	28 days
32,5 N	-	16.0	32.5	52.5	10
32,5 R	10.0	-
42,5 N	10.0	-	42.5	62.5
42,5 R	20.0	-
52,5 N	20.0	-	52.5	-
52,5 R	30.0	-

Note: N - normal early strength, R - high early strength.

Cement plants should determine strength of cement also at steaming in the age of one day and to specify it in the certificate of cement quality.

Also another accelerated methods can be used for approximate determination of cement strength.

The cement strength is dependent on the multiplicity of complex of factors. Cement composition is one of the basic factors. Not only content of separate minerals, but also their microstructure influences the indexes of the cement strength. In the last years the additives which promote the cement strength has received great attention.

T he compressive strength of cement, especially at early ages, increases as the finnes and specific surface area increases. Figure 9.7 shows that in ultra-high-early-strength cements, made with very fine particles below 10 m, 82% of the ultimate strength was reached after 7days whilst for the mixture made with particles between 25-50 m, only about 30% of the ultimate strength was developed (Fig. 9.7).

Conditions of storage, using and hardening of cement substantially influence on the strength of cement concretes and mortars. It is not desirable to hold in stock Portland cement for a long time, as its activity goes down under the action of humiduty and carbonic acid of air.

Hydration and hardening of cement. New chemical compounds - hydrates appear as a result of interaction of minerals, which are contained in cement, with water. Hydrosilicates are basic compounds, which are formed during the hydration of minerals – silicates. It is possible at certain conditions to describe the process of hydration of C₃S by equation:

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

Composition of hydrosilicates depends on temperature and on the concentration of calcium hydroxide.

Tricalcium aluminate in the presence of gypsum, which is contained in cement, and water, forms calcium hydrosulphoaluminate (ettringite), which slows the process of setting of cement paste:

3CaO∙Al₂O₃+3(CaSO₄∙2H₂O)+25H₂O=3CaO∙Al₂O₃∙3CaSO₄∙31H₂O

After the formation of hydrosulphoaluminate, another hydrate, calcium hydrosulphoferrite or solid mortar of these two compounds appears. Chemical reactions begin just after mixing of cement with water. Ettringite and calcium hydroxide are the first new hydrate formed.

The mechanism of cement hydration is very complex. Accordance to modern views, new formations crystallize from oversaturated solution in two stages. There is the formation of framework with the origin of contacts of accretion between the crystals of new formations during the first stage. Growing of crystals, which are joined between themselves, is also possible. During the second stage, new contacts does not develop, but there is only the growing of the framework which has appeared already. It results in the increase of the strength of the cement stone, but internal stretching tensions can develop. A decisive role is played by oversaturation of solution. At low oversaturation the amount of crystals is small and they are not accreted. For the maximum strength of artificial stone, the optimum conditions of hydration are needed, which provide the origin of new hydrate compounds of sufficient size at minimal tensions.

Hardening of Portland cement largely depends on the temperature and moisture conditions. So, the decline of temperature from 20 to 5°C slows the rate of hardening by 2-3 times. Increasing the temperature to 80°C increases the speed of hydration by upto 6 times. At a temperature below -10°C, the hydration of cement become practically halted.

The normal progress of the processes of hardening is possible only in conditions of sufficient environmental humidity and temperature, but temperature increase must not be accompanied with drying. The acceleration of physical and chemical processes of hardening of Portland cement by means of thermal treatment (steaming-out, electro-warming and others like that) allows obtaining concrete and reinforced concrete products with the required strength in the short terms.

Cement stone corrosion. A cement stone is the basic component of concrete, and it corrodes under act of aggressive environment.

Natural and industrial water solutions, which contain the different amounts of dissolved matters (acids, salts, alkalis), and also some organic liquids are the most widespread liquid aggressive environments.

Atmospheric water can contain the enhanced amount of salts in seaside areas and saline lands. Chemistry of river water depends on the type of sources and river bed rocks. By the degree of mineralization, that means content of salts, river water may be devided on four groups: I - small mineralization (to 200 mg/l), II - middle (200-500 mg/l ), III - enhanced (500-1000 mg/l), IV - high mineralization (over 1000 mg/l). Natural and ground waters also substantially differ by composition. The soft ground water, which formed as a result of snow melting and rain falls, is characteristic for the north and mountain areas. The high mineralized ground waters are often meet in the south areas.

Aggressive to cement concretes gases contain vapors and aerosols of different acids and salts. The solid corrosive environments are dry mineralized soils and different granular chemical matters: fertilizers, paints, insecticides, fungicides, herbicides, etc. Corrosive processes in gaseous and solid environments occur only in the presence of liquid phase.

Corrosive processes under the action of different environments, which operate on concrete and reinforced concrete structures, may be divided into three types.

Corrosion of the first type is conditioned by the dissolution of some components of cement stone and mainly the hydroxide of calcium - the product of hydrolysis of tricalcium silicate (corrosion of leaching). It goes intensively in soft waters, especially during filtration of water through a concrete. At leaching of Са(ОН)2 disintegration of other hydrates, steady only at the certain concentration of Ca(OH)2, begins and take place reducing of density and violation of structure of cement stone.

Corrosion of the second type is based on the exchange chemical reactions of interaction between components of cement stone and aggressive water solution with formation of easily soluble salts, which are eluted, or amorphous products without binder characteristics. Acid (i.e. carbonic acid) and magnesia corrosions are the kinds of this type of corrosion.

At carbonic acid corrosion hydroxide of calcium, which is contained in a cement stone, at first, interacts with CO2 with formation of sparingly soluble CaCO3, than the soluble hydrocarbonate appears at the excess of carbonic acid, which is eluted from concrete. For the concrete in sandy or gravelly soils which filter water at a speed of 0.04-0.1 m/sec, corrosion develops at content of CO2 in water over 15-20 ml/l. At the increase of carbonate water hardness, that means the content of soluble calcium carbonates and magnesium carbonates, the amount of free CO2 diminishes and carbonic acid corrosion becomes less dangerous.

The other acids: hydrochloric, sulfuric, nitric, acetic and other have also a destructive influence on a cement stone. Acids interact mainly with hydroxide, and then with the calcium hydrosilicates. The more soluble are salts, the more quickly cement stone and accordingly concrete are destroyed. So at the same concentration of hydrogen ions, the speed of corrosion under the action of solutions of sulfuric acid is lower than hydrochloric acid that is explained with the greater solubility of calcium chloride comparatively with a sulfate.

All natural waters contain some amount of the carbonic acid. The sulfur and some organic acids can be met in the peat waters. The large amount of free acids can be contained in sewage of industrial enterprises, and also in underground waters, fouled with the products and waste of different productions.

The magnesium salts (mainly magnesium chloride and magnesium sulfate) are contained in sea water, and also often are in underground waters. At interacting of them with calcium hydroxide besides to soluble calcium salts, the amorphous magnesium hydrate appears and mechanical properties of concrete are decreased.

Corrosion of the third type is also caused by the reactions of exchange of matters, dissolved in water with the components of cement stone, however their products are poorly soluble salts, crystallization of which takes place in pores and capillaries with the volume increase. The typical example of corrosion of this type is sulfate corrosion. It is caused by the ions of SO42-, the source of which are calcium, magnesium and sodium sulfates. The varieties of sulfate corrosion are sulphoaluminate and gypsum corrosion. At sulfo-aluminate corrosion take place a reaction between the calcium sulfate, which is contained in water, and hydroaluminate in a cement stone with formation of ettringite, that is accompanied with the considerable increase of volume and origin of destructive tensions. At the large concentration of sulfates (over 1000 mg/l SO42-) gypsum is crystallized in capillaries of a cement stone (gypsum corrosion).

Corrosion of cement stone can be also caused by interacting of alkalis of cement with active silica which are contained in such minerals, as an opal, chalcedony and other, which are met in the concrete aggregates. Harmful tensions are the results of jelatinous alkaline silicates formation.

Finding-out of reasons and mechanism of corrosion of cement stone allows to choose the method of increase of its resistance. An increasing of concrete density due to diminishing of water content and water-cement ratio of concrete mix, and also adding of plasticizers, polymeric and other additives is positive in all of cases. More resistant to the corrosion of leaching and sulfate corrosion are sulfate-resistant cements with specified amount of tricalcium aluminate and also cements, that contain active mineral additives, which bound Ca(OH)2 in slightly soluble compounds.