and it is mixed with gypsumtocontrol setting. When mixed with water,the
portland cement forms ah ydrated phase, and it hardens. As the cement
hardens, achemical reaction takesplace. The two most importantreactions
are the generation of calcium hydroxide and of tricalcium silicate hydrate.
The calcium hydroxide generated could, theoretically,b ea sh igha s2 0% of
the weight of the cement, producing an alkalinity that at the solubility of
lime, results in an equilibrium pH of 12.5. Steel that has been coated with
cement is passivated as aresult of the hardened materials. The alkalinity of
the coating is provided by the presence of calcium oxide (lime). Any material
that will cause the calcium oxide or hydroxide to be removed, lowering the
pH, willp rove detrimental and cause solution of the cement hydrates.
Contact with organic or inorganic acids can cause this to happen. Organic
acids can be generated when organic materials ferment.
When carbond ioxide dissolves in water that may be present on the
cement,weakcarbonic acid is produced. The weak carbonic acid lowers the
pH of the cement solution, allowingt he coated steel to corrode. Thisi s
sometimes referred to as the carbonation of cement.
Sulfates willa lso cause portland cement to deteriorate. In addition to
beinga ble to produce sulfuric acid that is highly corrosive to portland
cement,s ulfatesa re reactive with some additives used in the formulations.
Refer to Table 9.2.
Cementitious Coatings
513
CAT8247—CHAPTER 9—6/10/2006—12:51—SRIDHAR—XML MODEL B–pp. 509–513
portland cement forms ah ydrated phase, and it hardens. As the cement
hardens, achemical reaction takesplace. The two most importantreactions
are the generation of calcium hydroxide and of tricalcium silicate hydrate.
The calcium hydroxide generated could, theoretically,b ea sh igha s2 0% of
the weight of the cement, producing an alkalinity that at the solubility of
lime, results in an equilibrium pH of 12.5. Steel that has been coated with
cement is passivated as aresult of the hardened materials. The alkalinity of
the coating is provided by the presence of calcium oxide (lime). Any material
that will cause the calcium oxide or hydroxide to be removed, lowering the
pH, willp rove detrimental and cause solution of the cement hydrates.
Contact with organic or inorganic acids can cause this to happen. Organic
acids can be generated when organic materials ferment.
When carbond ioxide dissolves in water that may be present on the
cement,weakcarbonic acid is produced. The weak carbonic acid lowers the
pH of the cement solution, allowingt he coated steel to corrode. Thisi s
sometimes referred to as the carbonation of cement.
Sulfates willa lso cause portland cement to deteriorate. In addition to
beinga ble to produce sulfuric acid that is highly corrosive to portland
cement,s ulfatesa re reactive with some additives used in the formulations.
Refer to Table 9.2.
Cementitious Coatings
513
CAT8247—CHAPTER 9—6/10/2006—12:51—SRIDHAR—XML MODEL B–pp. 509–513