Post by Deleted on Jun 4, 2016 8:47:06 GMT -8
Ordinary portland cement (OPC) contains intentionally far more calcium hydroxide (CH)
than required for hardening, as construction steel requires a highly caustic environment
to be protected from corrosion.
CH forms from free Calcium oxide CaO, Dicalciumsilicat (Belit) C2S and Tricalciumsilicat (Alit) C3S
and occupies about 15%, some papers say 20-25%, of the volume of OPC paste.
CO2 from the air penetrates the concrete reacts with with the CH and forms calcium carbonate CaCO3 and water.
Ca(OH)2 has a molar mass of ~ 74 CaCO3 has a molar mass ~ 100.
CaCO3 will react with water that is saturated with CO2 to form Ca(HCO3)2, which is solubtle, with a molar mass of ~162.
OPC can chemically and physically bind an amount of water of approximately 40% of its mass.
The chemically bound proportion is about 25%, physically bound about 15%.
The carbonation in the outer layer binds CO2 gas and increases the percentage of chemicaly bound water.
Both gas and water will be released at high temperatures and expand immediately by several thousand times,
which will destroy the concrete if gas and gaseous water cannot escape or find at least space in pores.
High strength concretes from cements with high Blaine numbers are even more prone to spalling, due to lower porosity.
It does not matter if the high Blaine numbers are achived by grinding or super fine additives like silica fume or metacaolin.
Metakaolin may decrease porosity too, but reduces chemicaly bound water and increases general heat resistance.
Lime cements as used by the Romans have no problem with carbonation as they do not contain excess CH.
Vitruvius a Roman engineer specified 1 part lime to 3 parts pozzolan for regular cement and 1 part lime to 2 parts pozzolan for underwater cement.
Free silica eg sand has multible temperature dependant chrystal phases, which each occupy different amounts of space.
The volume changes by phases changes contribute to deterioration by repeatet termal cycles.
The obvious solutions for making any kind of concrete more heat resistant:
1.) Prevent excess CH.
2.) Ensure sufficient porosity.
3.) reduce the percentage of chemicaly bound water.
4.) limit the free silica.
Mix OPC with a pozzolan to bind excess CH (40% or more).
Porous pozzolan ensures sufficient porosity.
Use pozzolan with high alumina content.
Calcium Aluminium silicates will bind a lower mass percentage of water
and increase the general heat resistance.
Use aluminium silicate sand instead of quartz sand.
Clay as an aggregate can increase strengs at very high temperatures by sintering.
Behavior of blended cement pastes at elevated temperature
www.doiserbia.nb.rs/img/doi/1451-9372/2006/1451-93720602133K.pdf
BTW
Wood ashes have pozzolanic properties too, usually more than natural pozzolans but less than Class C coal fly ash.
Engineering Behavior and Characteristics of Wood Ash and Sugarcane Bagasse Ash.
www.mdpi.com/1996-1944/8/10/5353/pdf
Greener concrete using recycled materials
ejournal.narotama.ac.id/files/Greener%20concrete%20using%20recycled%20materials.pdf
Evaluation of wood ash as a partial replacement to cement
ijsetr.org/wp-content/uploads/2013/10/IJSETR-VOL-2-ISSUE-10-2009-2013.pdf
than required for hardening, as construction steel requires a highly caustic environment
to be protected from corrosion.
CH forms from free Calcium oxide CaO, Dicalciumsilicat (Belit) C2S and Tricalciumsilicat (Alit) C3S
and occupies about 15%, some papers say 20-25%, of the volume of OPC paste.
CO2 from the air penetrates the concrete reacts with with the CH and forms calcium carbonate CaCO3 and water.
Ca(OH)2 has a molar mass of ~ 74 CaCO3 has a molar mass ~ 100.
CaCO3 will react with water that is saturated with CO2 to form Ca(HCO3)2, which is solubtle, with a molar mass of ~162.
OPC can chemically and physically bind an amount of water of approximately 40% of its mass.
The chemically bound proportion is about 25%, physically bound about 15%.
The carbonation in the outer layer binds CO2 gas and increases the percentage of chemicaly bound water.
Both gas and water will be released at high temperatures and expand immediately by several thousand times,
which will destroy the concrete if gas and gaseous water cannot escape or find at least space in pores.
High strength concretes from cements with high Blaine numbers are even more prone to spalling, due to lower porosity.
It does not matter if the high Blaine numbers are achived by grinding or super fine additives like silica fume or metacaolin.
Metakaolin may decrease porosity too, but reduces chemicaly bound water and increases general heat resistance.
Lime cements as used by the Romans have no problem with carbonation as they do not contain excess CH.
Vitruvius a Roman engineer specified 1 part lime to 3 parts pozzolan for regular cement and 1 part lime to 2 parts pozzolan for underwater cement.
Free silica eg sand has multible temperature dependant chrystal phases, which each occupy different amounts of space.
The volume changes by phases changes contribute to deterioration by repeatet termal cycles.
The obvious solutions for making any kind of concrete more heat resistant:
1.) Prevent excess CH.
2.) Ensure sufficient porosity.
3.) reduce the percentage of chemicaly bound water.
4.) limit the free silica.
Mix OPC with a pozzolan to bind excess CH (40% or more).
Porous pozzolan ensures sufficient porosity.
Use pozzolan with high alumina content.
Calcium Aluminium silicates will bind a lower mass percentage of water
and increase the general heat resistance.
Use aluminium silicate sand instead of quartz sand.
Clay as an aggregate can increase strengs at very high temperatures by sintering.
Behavior of blended cement pastes at elevated temperature
www.doiserbia.nb.rs/img/doi/1451-9372/2006/1451-93720602133K.pdf
BTW
Wood ashes have pozzolanic properties too, usually more than natural pozzolans but less than Class C coal fly ash.
Engineering Behavior and Characteristics of Wood Ash and Sugarcane Bagasse Ash.
www.mdpi.com/1996-1944/8/10/5353/pdf
Greener concrete using recycled materials
ejournal.narotama.ac.id/files/Greener%20concrete%20using%20recycled%20materials.pdf
Evaluation of wood ash as a partial replacement to cement
ijsetr.org/wp-content/uploads/2013/10/IJSETR-VOL-2-ISSUE-10-2009-2013.pdf
