Acid Attack - Factors affecting reaction
Factors affecting this process include the volume of solution to attack area ratio, the acidity of the solution, time of exposure, and temperature. Several empirical relationships have been established to illustrate the overall effects on the CH dissolution rate by varying these factors.
When the time of exposure is increased there is an increase in the amount of dissolution thus causing an increase in the cement paste deterioration. The empirical equations below relate the exposure time to both the degree of dissolution and the depth of the dissolution front. (G Harold, 1995)
| Dissolution vs. Time | Corrosion Depth vs. Time |
| CaO m(t) = 12.50 x t 0.5773 | Ca x(t) = 168.8 x 10-5 x t 0.57773 |
| Al2O3 m(t) = 0.811 x t 0.6104 | Al x(t) = 116.0 x 10-5 x t 0.6104 |
| MgO m(t) = 0.365 x t 0.5859 | Mg x(t) = 125.6 x 10-5 x t 0.5859 |
Note: Relationships obtained for pH values of 1
Dissolution rates are also a function of time and solution concentration. The greater the time exposed and the more concentrated the solution the more dissolution occurs. This relationship is expressed in terms of the dissolution front depth using the equations below. (G Harold, 1995)
cm(t,c) = K x t 0.5572 x c 0.4074
K1 = (pH 1.0) = 39.03 |
| t = time (min) K2 = (pH 1.5) = 30.45 |
| c = concentration (mol/l) K3 = (pH 2.0) = 33.71 |
| K = calculated coefficient K4 = (pH 2.5) = 30.15 |
| K5 = (pH 3.0) = 32.64 |
Temperature is also a major contributing factor of the acid degradation process. Increases in temperature cause an increase in the dissolution rate. As a rule of thumb an increase in temperature equal to 40 degrees Celsius will double the rate of reaction. This characteristic accounts for the an increase in the damage caused by acid attack in warmer climates. (G. Harold, 1995)