Mapping Strength and Durability of Cement and Concrete
by
Sanwar. M. Mishra, Ex-UNIDO Consultant
98, Ganeshnagar, Motidoongari
Jaipur, Rajasthan, India
Cement strength and durability optimization provides the much needed quality advantage to meet the performance challenges under the global competition. In this title strength related processes at the manufacturing and construction stages are well focused. Operation parameters contributing to cement strength, beginning at raw-mix design, sintering operation in kiln and at clinker grinding stages, are highlighted. Similarly, the strength and durability factors are briefed for the construction of a concrete structure. Demand for better performance of cement in concrete should take into consideration these aspects in order to fulfill the requirements of strength and durability.
Cement Manufacturing:
Strength is a function of various parameters at the stages of manufacture of cement such as raw mix design, sintering process, clinker-grinding operation etc. These critical processing stages are addressed as follows.
Raw-mix design considerations:
Basic principles of raw mix design are expressed by the following relationships.
Lime Saturation Factor =CaO ÷ SiO2 + Al2O3 + Fe2O3 ( It ranges from 88 to 95 )
Silica Modulus = SiO2 ÷ Al2O3. + Fe2O3 (Commonly used range is 2.2 to 2.8)
Alumina Modulus = Al2O3 ÷ Fe2O3 (Generally it is kept around 1.1 to 2.4)
Important consideration here is that proportioning of raw materials should be targeted at the silica modulus, alumina modulus and lime saturation factor and not at C3S and C3A. Silica modulus will determine the total amount of calcium silicates i.e. C3S and C2S in the clinker and lime saturation factor determines the proportion of C3S in these silicates.
Likewise the alumina modulus will determine the amount of C3A in the clinker. Here, it may be noted that higher the silica modulus higher is SiO2 in the clinker and or lower will be the content of Al2O3 and Fe2O3. It will lead to a higher concentration of silicates in the clinker at the expense of aluminate and alumino-ferrite. As regards the alumina modulus, it must be more than 0.64 to ensure combining with iron oxide in C4AF. Higher than 0.64 will result in the form of C3A in the clinker.
In the case of lime saturation, it is to be ensured that maximum amount of lime is made available to combine with silica, alumina and iron oxide in the form of C3S (3CaO.SiO2), C3A (3 CaO. Al2O3). and C4AF {4Cao (Al2O3.Fe2O3)} respectively. It is believed that the maximum combinable limit of lime in clinker is around 2.8 times the silica content, which is the ratio of lime to silica in C3S. Similarly, the maximum combinable limit of alumina is 1.64, being the ratio of lime to alumina in C3A. In the same way, the ratio of lime to iron oxide is about 0.70 in C4AF. Thus, if the amount of lime present is equal to the maximum combinable limit, then the clinker is one hundred percent lime-saturated. But if the lime is more than the combinable limit then, uncombined free lime will be present in the clinker, which is undesirable. However, in practice some free lime will always be present. In this context, the Lea and Parker formula is CaO % = 2.8 x SiO2 % + 1.18 x Al2O3 % + 0.65 x Fe2O3 % . More than 95 % of the clinker constitutes of lime, silica, alumina and iron oxide. Only the remaining 5 % are alkalies, magnesia etc.
Sintering and Rate of Cooling:
Burning temperature and cooling rate decide the formation of C3S in clinker, which develops early strength in cement. Most important from the point of view of strength, is the formation of tri-calcium silicate (C3S). It requires a thresh-hold temperature of above 1250 C to 1400 C (2552 F). At this temperature, the exothermic reaction yields energy, which self-fuels the kiln and lowers the overall energy input of the process. If this temperature falls below 1250 C, then C3S formation does not take place and partial self-fueling of the kiln is lost. Consequently the kiln cools down rapidly as the energy demand becomes higher than when C3S formation was set in. To restore the situation, it takes a long time and more fuel burning takes place. Once the C3S has formed, then the process of cooling assumes more importance. In fact, C3S does not occur in a chemically pure state; it combines with oxides of Al, Fe, Ti as Al2O3, Fe2O3, TiO2 etc.
Cooling process has a great effect on the quality of clinker as it influences the mineralogical structure of the clinker. If the cooling is slower, C3S begins to decompose into to C2S and CaO. Therefore, rapid cooling must follow once the C3S has been formed to stop decomposition.
Clinker Grinding:
At this stage cement strength is a function of the fineness and particle size distribution. Cement properties are affected not only by the chemical and mineralogical composition but also by the fineness and distribution of particle size. Increase in proportion of fines increases the cement strength.
Raw materials for cement are finely ground before burning for the reason that they react rapidly. After the burning process, the clinker has to be finely ground to react with water more quickly in the process of hydration at the construction stage. Unit of fineness is cm2/gm. For every 100 cm2/gm increase in fineness of cement, the average increase in 28-day compressive strength is about 1N/mm2.
In the context of particle size, the middle range of 3 to 30 micron size fines should be above 50% to gain higher strength. This is done at the cost of fines and coarse particles such that the specific surface of cement is not reduced. Open-circuit grinding mills produce cement with wide particle size distribution, whereas closed-circuit grinding, produces narrow band of particles.
During clinker grinding, gypsum is easy to grind, hence it increases the concentration of finer particles. Likewise, the fly ash does the same. But the blast furnace slag adds to the concentration of coarse fractions.
Gypsum at clinker grinding controls the setting time of cement. Addition of gypsum retards the setting process. This retarding effect is achieved by the reaction of sulfates with C3A. Without it, the cement will set rapidly. Higher the amount of C3A present, more is the sulfate (gypsum) required for reaction. But excess of sulfate will cause expansion. So, there is an optimum quantity of gypsum/sulfate to be added for increasing cement strength. It will affect the water to cement ratio, as discussed in this article. The optimum amount of gypsum addition for the proportion of C3A and alkalies ground to a high degree of fineness, is 5% by weight . For very little or nil C3A small traces of alkalies, the gypsum requirements are 2.5 to 3 % by weight.
Construction Stage
Main purpose of using cement is to economically produce a strong and durable concrete structure capable of withstanding the environmental degradation. In the formation of a concrete structure, the individualities of bricks/blocks, aggregate, sand etc. are unified by the adhesive strength of cement. It acts as the binding agent. For obtaining strong and durable concrete, some of the critical factors among others are as follows.
•Water cement ratio
•Concrete cover
Water Cement Ratio:
There exists a linear relationship between water to cement ratio (w/c) on one side and the 28-days compressive strength on the other. As the w/c ratio reduces, the strength increases proportionately and vice versa. For example, with w/c ratio at 0.34 the strength of a particular quality of cement, with a given mix-design is, say 33 Mpa. Then for the same quality of cement at a w/c of 0.44 the strength will be 25 Mpa; similarly at 0.54 w/c the strength will be 19 Mpa and so on. It concludes that lower the w/c ratio higher is the strength. As far as possible the water to cement ratio should be kept at a lower level to obtain higher concrete strength.
Water to cement ratio has a relation to setting time. Component affecting the setting time of cement other than gypsum is C3A. It controls the water demand and influences the setting time. Formation of Ettringite occurs by the reaction of C3A with CaSO4 in the presence of water.
Concrete Cover:
Provision of an adequate cover of a concrete structure is necessary due to the fact that atmospheric carbon-di-oxide enters the concrete surface, reacts with the hydrated cement constituents and lowers the concrete pH to about 9.5. The CO2 reacted depth is called the carbonation front. The pH can be measured by taking a fractured piece from the concrete structure and impregnating it with the pH-indicator such as phenolphthalein. It gives the pH value.
Generally, the depth of carbonation front is proportional to the square root of the duration of time elapsed since the construction of the structure. Approximately, 1 mm reduction in cover reduces the life by 5 years. Cover thickness is an important parameter of durability.
Idle Storage of Cement:
Cement strength is adversely affected by the period of idle storage after manufacture. Time elapsed between manufacture and use for construction should be as less as possible. Once the cement is produced, it is advisable to use for construction while fresh. The cement should not be allowed to remain unused for reasons of deterioration in its compressive strength. Cement is a hygroscopic material and absorbs moisture from the environmental air, which sets in the degradation.
Available data showing the effect of degradation are as follows. (Courtesy – NCB, India)
Please see Table 1 below.
Among them, PPC is least affected at 18 % reduction, whereas PSC is most affected at 39 % reduction in cement strength over a period of 6 months. It is advisable to store cement in a dry surrounding and without air circulation.
Durability:
Properties of durability and high early strength relate to different types of cement. A single cement quality does meet the requirements of high early strength and durability to some extent. Enhanced durability and corrosion resistant properties compared to ordinary Portland cement are more pronounced in blended cement. Blended cement are comparatively more durable but do not possess the characteristic of high early strength, though the ultimate strength is not lower than OPC. They have a dense microstructure, low heat of hydration, low shrinkage, lower w/c ratio, use additives in addition to clinker, e.g. fly ash, slag, pozzolanic materials etc.
High early strength cement requires a higher percentage of C3S i.e.60 to 65 and around 15 % of C2S, a higher w/c ratio compared to normal strength cement, high heat of hydration and no additives such as, fly-ash, slag etc. It has 95 % clinker.
It has been found that Portland Pozzolana Cement (PPC), Portland Slag Cement (PSC) have a lower early strength as compared to Ordinary Portland Cement (OPC). But PPC and PSC have been found to be more durable than OPC, besides being more resistant to environmental corrosion caused by CO2 and chlorides. A profile of the properties of slag cement shows this difference.
Properties of slag cement:
•Compressive strength of slag cement varies from 40 to 60 N/mm2. It is comparatively higher due to the presence of excess silicates in slag.
•Water absorption reduces from 1.73% to 1.26 % as the slag content increases.
•Corrosion of concrete structure decreases with increase in slag component.
•Chloride penetration reduces.
Some of the materials used along with clinker for producing cement are as under.
•Ground and or glassy granulated blast furnace slag
•Pozzolana/calcined clay – a volcanic rock material
•Lignite ash (contains CaO – can be mixed with raw-meal – reduces CO2 emission.)
Summary:
Durability, high early strength and good ultimate compressive strength are the desired characteristics of cement. But they may not be found in a single quality of cement. Durability implies resistance to atmospheric corrosion and in this context, PPC and PSC are an appropriate choice. Further, for scarce-water areas, PPC can be a better cement for construction, where high early strength requirements are not necessary. It has a low heat of hydration, good ultimate compressive strength, requires less water compared to OPC and is anti corrosive. Use of blended cement reduces the emission of carbon-di-oxide and energy consumption.
Cement Classification:
Under the EN-197-1:2000 standard, cement quality is defined by the criteria of compressive strength falling in 3 categories; 32.5, 42.5 and 52.5 Mpa and the cement composition comprising of 5 categories; cement type I, II, III, IV, and V. It relates to the chemical composition and the additives used as shown in the following exhibit.
General Classification of Cement under EN197-1:2000
Strength & Durability of Cement
Mapping Strength and Durability of Cement and Concrete
by
Sanwar. M. Mishra, Ex-UNIDO Consultant
98, Ganeshnagar, Motidoongari
Jaipur, Rajasthan, India
Cement strength and durability optimization provides the much needed quality advantage to meet the performance challenges under the global competition. In this title strength related processes at the manufacturing and construction stages are well focused. Operation parameters contributing to cement strength, beginning at raw-mix design, sintering operation in kiln and at clinker grinding stages, are highlighted. Similarly, the strength and durability factors are briefed for the construction of a concrete structure. Demand for better performance of cement in concrete should take into consideration these aspects in order to fulfill the requirements of strength and durability.
Cement Manufacturing:
Strength is a function of various parameters at the stages of manufacture of cement such as raw mix design, sintering process, clinker-grinding operation etc. These critical processing stages are addressed as follows.
Raw-mix design considerations:
Basic principles of raw mix design are expressed by the following relationships.
Lime Saturation Factor =CaO ÷ SiO2 + Al2O3 + Fe2O3 ( It ranges from 88 to 95 )
Silica Modulus = SiO2 ÷ Al2O3. + Fe2O3 (Commonly used range is 2.2 to 2.8)
Alumina Modulus = Al2O3 ÷ Fe2O3 (Generally it is kept around 1.1 to 2.4)
Important consideration here is that proportioning of raw materials should be targeted at the silica modulus, alumina modulus and lime saturation factor and not at C3S and C3A. Silica modulus will determine the total amount of calcium silicates i.e. C3S and C2S in the clinker and lime saturation factor determines the proportion of C3S in these silicates.
Likewise the alumina modulus will determine the amount of C3A in the clinker. Here, it may be noted that higher the silica modulus higher is SiO2 in the clinker and or lower will be the content of Al2O3 and Fe2O3. It will lead to a higher concentration of silicates in the clinker at the expense of aluminate and alumino-ferrite. As regards the alumina modulus, it must be more than 0.64 to ensure combining with iron oxide in C4AF. Higher than 0.64 will result in the form of C3A in the clinker.
In the case of lime saturation, it is to be ensured that maximum amount of lime is made available to combine with silica, alumina and iron oxide in the form of C3S (3CaO.SiO2), C3A (3 CaO. Al2O3). and C4AF {4Cao (Al2O3.Fe2O3)} respectively. It is believed that the maximum combinable limit of lime in clinker is around 2.8 times the silica content, which is the ratio of lime to silica in C3S. Similarly, the maximum combinable limit of alumina is 1.64, being the ratio of lime to alumina in C3A. In the same way, the ratio of lime to iron oxide is about 0.70 in C4AF. Thus, if the amount of lime present is equal to the maximum combinable limit, then the clinker is one hundred percent lime-saturated. But if the lime is more than the combinable limit then, uncombined free lime will be present in the clinker, which is undesirable. However, in practice some free lime will always be present. In this context, the Lea and Parker formula is CaO % = 2.8 x SiO2 % + 1.18 x Al2O3 % + 0.65 x Fe2O3 % . More than 95 % of the clinker constitutes of lime, silica, alumina and iron oxide. Only the remaining 5 % are alkalies, magnesia etc.
Sintering and Rate of Cooling:
Burning temperature and cooling rate decide the formation of C3S in clinker, which develops early strength in cement. Most important from the point of view of strength, is the formation of tri-calcium silicate (C3S). It requires a thresh-hold temperature of above 1250 C to 1400 C (2552 F). At this temperature, the exothermic reaction yields energy, which self-fuels the kiln and lowers the overall energy input of the process. If this temperature falls below 1250 C, then C3S formation does not take place and partial self-fueling of the kiln is lost. Consequently the kiln cools down rapidly as the energy demand becomes higher than when C3S formation was set in. To restore the situation, it takes a long time and more fuel burning takes place. Once the C3S has formed, then the process of cooling assumes more importance. In fact, C3S does not occur in a chemically pure state; it combines with oxides of Al, Fe, Ti as Al2O3, Fe2O3, TiO2 etc.
Cooling process has a great effect on the quality of clinker as it influences the mineralogical structure of the clinker. If the cooling is slower, C3S begins to decompose into to C2S and CaO. Therefore, rapid cooling must follow once the C3S has been formed to stop decomposition.
Clinker Grinding:
At this stage cement strength is a function of the fineness and particle size distribution. Cement properties are affected not only by the chemical and mineralogical composition but also by the fineness and distribution of particle size. Increase in proportion of fines increases the cement strength.
Raw materials for cement are finely ground before burning for the reason that they react rapidly. After the burning process, the clinker has to be finely ground to react with water more quickly in the process of hydration at the construction stage. Unit of fineness is cm2/gm. For every 100 cm2/gm increase in fineness of cement, the average increase in 28-day compressive strength is about 1N/mm2.
In the context of particle size, the middle range of 3 to 30 micron size fines should be above 50% to gain higher strength. This is done at the cost of fines and coarse particles such that the specific surface of cement is not reduced. Open-circuit grinding mills produce cement with wide particle size distribution, whereas closed-circuit grinding, produces narrow band of particles.
During clinker grinding, gypsum is easy to grind, hence it increases the concentration of finer particles. Likewise, the fly ash does the same. But the blast furnace slag adds to the concentration of coarse fractions.
Gypsum at clinker grinding controls the setting time of cement. Addition of gypsum retards the setting process. This retarding effect is achieved by the reaction of sulfates with C3A. Without it, the cement will set rapidly. Higher the amount of C3A present, more is the sulfate (gypsum) required for reaction. But excess of sulfate will cause expansion. So, there is an optimum quantity of gypsum/sulfate to be added for increasing cement strength. It will affect the water to cement ratio, as discussed in this article. The optimum amount of gypsum addition for the proportion of C3A and alkalies ground to a high degree of fineness, is 5% by weight . For very little or nil C3A small traces of alkalies, the gypsum requirements are 2.5 to 3 % by weight.
Construction Stage
Main purpose of using cement is to economically produce a strong and durable concrete structure capable of withstanding the environmental degradation. In the formation of a concrete structure, the individualities of bricks/blocks, aggregate, sand etc. are unified by the adhesive strength of cement. It acts as the binding agent. For obtaining strong and durable concrete, some of the critical factors among others are as follows.
•Water cement ratio
•Concrete cover
Water Cement Ratio:
There exists a linear relationship between water to cement ratio (w/c) on one side and the 28-days compressive strength on the other. As the w/c ratio reduces, the strength increases proportionately and vice versa. For example, with w/c ratio at 0.34 the strength of a particular quality of cement, with a given mix-design is, say 33 Mpa. Then for the same quality of cement at a w/c of 0.44 the strength will be 25 Mpa; similarly at 0.54 w/c the strength will be 19 Mpa and so on. It concludes that lower the w/c ratio higher is the strength. As far as possible the water to cement ratio should be kept at a lower level to obtain higher concrete strength.
Water to cement ratio has a relation to setting time. Component affecting the setting time of cement other than gypsum is C3A. It controls the water demand and influences the setting time. Formation of Ettringite occurs by the reaction of C3A with CaSO4 in the presence of water.
Concrete Cover:
Provision of an adequate cover of a concrete structure is necessary due to the fact that atmospheric carbon-di-oxide enters the concrete surface, reacts with the hydrated cement constituents and lowers the concrete pH to about 9.5. The CO2 reacted depth is called the carbonation front. The pH can be measured by taking a fractured piece from the concrete structure and impregnating it with the pH-indicator such as phenolphthalein. It gives the pH value.
Generally, the depth of carbonation front is proportional to the square root of the duration of time elapsed since the construction of the structure. Approximately, 1 mm reduction in cover reduces the life by 5 years. Cover thickness is an important parameter of durability.
Idle Storage of Cement:
Cement strength is adversely affected by the period of idle storage after manufacture. Time elapsed between manufacture and use for construction should be as less as possible. Once the cement is produced, it is advisable to use for construction while fresh. The cement should not be allowed to remain unused for reasons of deterioration in its compressive strength. Cement is a hygroscopic material and absorbs moisture from the environmental air, which sets in the degradation.
Available data showing the effect of degradation are as follows. (Courtesy – NCB, India)
Please see Table 1 below.
Among them, PPC is least affected at 18 % reduction, whereas PSC is most affected at 39 % reduction in cement strength over a period of 6 months. It is advisable to store cement in a dry surrounding and without air circulation.
Durability:
Properties of durability and high early strength relate to different types of cement. A single cement quality does meet the requirements of high early strength and durability to some extent. Enhanced durability and corrosion resistant properties compared to ordinary Portland cement are more pronounced in blended cement. Blended cement are comparatively more durable but do not possess the characteristic of high early strength, though the ultimate strength is not lower than OPC. They have a dense microstructure, low heat of hydration, low shrinkage, lower w/c ratio, use additives in addition to clinker, e.g. fly ash, slag, pozzolanic materials etc.
High early strength cement requires a higher percentage of C3S i.e.60 to 65 and around 15 % of C2S, a higher w/c ratio compared to normal strength cement, high heat of hydration and no additives such as, fly-ash, slag etc. It has 95 % clinker.
It has been found that Portland Pozzolana Cement (PPC), Portland Slag Cement (PSC) have a lower early strength as compared to Ordinary Portland Cement (OPC). But PPC and PSC have been found to be more durable than OPC, besides being more resistant to environmental corrosion caused by CO2 and chlorides. A profile of the properties of slag cement shows this difference.
Properties of slag cement:
•Compressive strength of slag cement varies from 40 to 60 N/mm2. It is comparatively higher due to the presence of excess silicates in slag.
•Water absorption reduces from 1.73% to 1.26 % as the slag content increases.
•Corrosion of concrete structure decreases with increase in slag component.
•Chloride penetration reduces.
Some of the materials used along with clinker for producing cement are as under.
•Ground and or glassy granulated blast furnace slag
•Pozzolana/calcined clay – a volcanic rock material
•Fly ash
•Silica fumes
•Ground limestone
•Crushed brick (contains clay, minerals, silicates etc.)
•Lignite ash (contains CaO – can be mixed with raw-meal – reduces CO2 emission.)
Summary:
Durability, high early strength and good ultimate compressive strength are the desired characteristics of cement. But they may not be found in a single quality of cement. Durability implies resistance to atmospheric corrosion and in this context, PPC and PSC are an appropriate choice. Further, for scarce-water areas, PPC can be a better cement for construction, where high early strength requirements are not necessary. It has a low heat of hydration, good ultimate compressive strength, requires less water compared to OPC and is anti corrosive. Use of blended cement reduces the emission of carbon-di-oxide and energy consumption.
Cement Classification:
Under the EN-197-1:2000 standard, cement quality is defined by the criteria of compressive strength falling in 3 categories; 32.5, 42.5 and 52.5 Mpa and the cement composition comprising of 5 categories; cement type I, II, III, IV, and V. It relates to the chemical composition and the additives used as shown in the following exhibit.
General Classification of Cement under EN197-1:2000
Please see Table in the attachments.
Blending components :
(a)Slag, silica fume, pozzolana, flyash, limestone, calcined clay.
(b)Combination of pozzolana, flyash and silica fume (up to 10%)
(c)Combination of slag and pozzolana or flyash.
Cement Types:
CEM I:
It is Portland cement having more than 95 % clinker.
CEM II:
It is a blended Portland cement consisting of designated additives in the range of 6 – 35% by weight.
CEM III:
It is a blastfurnace slag cement having 36- to 95 % of slag by weight.
CEM IV:
It is pozzolana cement containing 11 to 55% by weight of pozzolanic materials.
CEM V:
It is a composite cement having36 to 80 % by weight of blast furnace slag, pozzolana or flyash.
Strength-wise classification considers 28-day strength in the categories of 32.5, 42.5 and 52.5 Mpa.
They have a suffix “N” or “R”. ‘R’ indicates high early strength and ‘N’ means normal early strength based on the 2-day compressive strength.
-Sanwar. M. Mishra, Ex-UNIDO Consultant, 98, Ganeshnagar, Motidoongari, Jaipur, Rajasthan, INDIA
-Author acknowledges the contribution of Shri Antriksh Jain, General Manager, Process, J.K. Cement Limited, Nimbahera
Attachments
table_2 (JPG)
table_1 (JPG)
sanwar-m.-mishra (JPG)
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