The suggested reasons for superior performance of blended Pozzolanic cements over those of ordinary Portland cement (OPC).
Pozzolanic additives are responsible for so called pozzolanic reaction where calcium hydroxide, which is one of hydration products of alite and belite, reacts with amorphous silica oxide from pozzolan forming additional C-S-H gel. Overdose of this sort of additive can reduce portlandite (calcium hydroxide) amount increasing risk of steel corrosion, but right amount can increase compressive strength due to additional C-S-H. Also some researches have concluded that in presence of additional silica oxide increase alite hydration percentage also increases so OPC hydration is increased.
Pozzolana material specially Rice husk ash having high amount of reactive silica as compare to Fly ash therefore it reacts rapidly with available lime or produced during hydration of cement and makes additional C-S-H as main phase but in OPC cement, during Hydration produced huge amount of lime and heat with create cracks and voids and lime may reacts with atmospheric CO2 and produced CaCO3 which fall down from cement surface and lime may reacts with iron also due to high reactivity of calcium as compare to iron
The suggested reasons for superior performance of blended pozzolanic cement are related to the reaction between pozzolana and calcium hydroxide that produces more hydration products C-S-H.
This reactio is almost slow, therefore the rate of heat liberation will be slow and the resistance to thermal cracking will be improved.
Moreover the pozzolanic reaction is lime consuming which enhances the durability of concrete to chemical attacks such as by sulfate water and alkali aggregate reaction.
Cappilary voids in cement paste are reduced in size due to the reaction between the pozzolanic particles and lime because of the formation of secondary hydration products that will fill the large capillary voids.Therefore the ultimate strength of concrete and its impermeability will be improved. Pozzolanic reaction results in replacement of large and oriented crystals of lime by numerous , small less oriented crystals. Thus the transition zone between aggregate and cement paste will be strengthen.
I completely agree with that, pozzolanic additives are responsible for so called pozzolanic reaction where calcium hydroxide, which is one of hydration products of alite and belite, reacts with amorphous silica oxide from pozzolan forming additional C-S-H gel. Overdose of this sort of additive can reduce portlandite (calcium hydroxide) amount increasing risk of steel corrosion, but right amount can increase compressive strength due to additional C-S-H. Also some researches have concluded that in presence of additional silica oxide increase alite hydration percentage also increases so OPC hydration is increased.
The superior performance of blended cements over those of PC is mainly due to the following: i) The pozzolanic reaction of SCMs with free lime (CH) produced by cement hydration, which reduces gypsum formation; ii) The reduction of C3A content, i.e. dilution effect; iii) The reduction of pH value therefore, the ettringite becomes less expansive; iv )The formation of additional amounts of CSH, which produces a coating film on the alumina-rich and other reactive phases, thereby hindering the formation of secondary and lastly ettringite; and v) The formation of secondary CSH also results in pore size refinement, which reduces the permeability as well as the ingress of aggressive ions.
Yes, the cappilary voids in cement paste are reduced in size due to the reaction between the pozzolanic particles and lime because of the formation of secondary hydration products that will fill the large capillary voids.Therefore the ultimate strength of concrete and its impermeability will be improved. Pozzolanic reaction results in replacement of large and oriented crystals of lime by numerous , small less oriented crystals.
Portland cement is widely used in the world nowadays, with a global consumption. Due the environmental problems related to its use, such as CO2 emission and use of non-renewable raw materials. These information shows how important is to reduce cement Portland consumption due the environmental problems caused by its production. Hence, alternative materials are being researched to diminish these problems. Reduction of energy usage can be taken place even by design methods or using supplementary cementitious materials (SCMs) (e.g., granulated blast furnace slag (GBFS), fly ash (FA), and silica fume) in cement concrete, either as a mineral admixture or a component of blended cement. Nowadays, the use of SCMs is not only for ecological reasons, but also technological, because these materials increase the mechanical properties and improve the durability of cement mortars and concretes. It was concluded that, the superior performance of blended cements over those of PC is mainly due to the following: i) The pozzolanic reaction of SCMs with free lime (CH) produced by cement hydration, which reduces gypsum formation; ii) The reduction of C3A content, i.e. dilution effect; iii) The reduction of pH value therefore, the ettringite becomes less expansive; iv )The formation of additional amounts of CSH, which produces a coating film on the alumina-rich and other reactive phases, thereby hindering the formation of secondary and lastly ettringite; and v) The formation of secondary CSH also results in pore size refinement, which reduces the permeability of cement paste.
Most SCMs (eg. ggbs) help to refine the pore structure of cements, making the cement material less susceptible to attack by destructive compounds such as chloride and sulphate. SCMs also consume excessive portlandite and contribute more strength bearing C-S-H. However, optimal replacements should be considered to achieve best results.
We should try to make a cement with stoichiometric way it means cement is a mixture therefore some amount of ions or free lime or reactive silicates or silica are available for further reaction with mixture or surroundings, if cement is free from these species than may be, no problems of permeability with chloride and sulphate ions
Yes, Pozzolanic additives are responsible for so called pozzolanic reaction where calcium hydroxide, which is one of hydration products of alite and belite, reacts with amorphous silica oxide from pozzolan forming additional C-S-H gel. Overdose of this sort of additive can reduce portlandite (calcium hydroxide) amount increasing risk of steel corrosion, but right amount can increase compressive strength due to additional C-S-H. Also some researches have concluded that in presence of additional silica oxide increase alite hydration percentage also increases so OPC hydration is increased.
I completely agree with that, the superior performance of blended cements over those of PC is mainly due to the following: i) The pozzolanic reaction of SCMs with free lime (CH) produced by cement hydration, which reduces gypsum formation; ii) The reduction of C3A content, i.e. dilution effect; iii) The reduction of pH value therefore, the ettringite becomes less expansive; iv )The formation of additional amounts of CSH, which produces a coating film on the alumina-rich and other reactive phases, thereby hindering the formation of secondary and lastly ettringite; and v) The formation of secondary CSH also results in pore size refinement, which reduces the permeability of cement paste.
We should try to compare Industrial OPC cement vs lab prepared cement only with calculated amount of lime and reactive silica. Hopefully we can find solutions of many theoretical views
The use of supplementary cementitious materials (SCM) in concrete has become very common due to their technological, economical, and environmental benefits. The use of SCM such as silica fume and fly ash in concretes has been found to improve the resistance of concrete to sulfuric acid attack because of the reduced presence of calcium hydroxide, which is most vulnerable to acid attack
The reduction of energy usage can be taken place even by design methods or using supplementary cementitious materials (SCMs) (e.g., granulated blast furnace slag (GBFS), fly ash (FA), and silica fume) in cement concrete, either as a mineral admixture or a component of blended cement. Nowadays, the use of SCMs is not only for ecological reasons, but also technological, because these materials increase the mechanical properties and improve the durability of cement mortars and concretes.
Nowadays, the use of SCMs is not only for ecological reasons, but also technological, because these materials increase the mechanical properties and improve the durability of cement mortars and concretes. It was concluded that, the superior performance of blended cements over those of PC is mainly due to the following: i) The pozzolanic reaction of SCMs with free lime (CH) produced by cement hydration, which reduces gypsum formation; ii) The reduction of C3A content, i.e. dilution effect; iii) The reduction of pH value therefore, the ettringite becomes less expansive; iv )The formation of additional amounts of CSH, which produces a coating film on the alumina-rich and other reactive phases, thereby hindering the formation of secondary and lastly ettringite.
The suggested reasons for superior performance of blended Pozzolanic cements over those of ordinary Portland cement are: 1) The pozzolanic reaction of SCMs with free lime (CH) produced by cement hydration, which reduces gypsum formation; 2) The reduction of C3A content, i.e. dilution effect; 3) The reduction of pH value therefore, the ettringite becomes less expansive; 4 )The formation of additional amounts of CSH, which produces a coating film on the alumina-rich and other reactive phases, thereby hindering the formation of secondary and lastly ettringite; and 5) The formation of secondary CSH also results in pore size refinement.
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