Masonry Magazine June 1995 Page. 29
Portland Cement Association
pca
TROWEL TIPS
an aid to the masonry industry
Efflorescence
Efflorescence is a crystalline deposit, usually white, that may develop on the surfaces of masonry construction. Often it appears just after the structure is completed-when builder, architect, and owner are most concerned with the appearance of the new structure. Although unattractive, efflorescence is generally harmless. However, deposits can occur within the surface pores of the material causing expansion that may disrupt the surface. This condition is sometimes termed cryptoflorescence.
Causes
A combination of circumstances causes efflorescence. First, there must be soluble salts in the masonry. Second, there must be moisture to pick up the soluble salts and carry them to the surface. Third, evaporation or hydrostatic pressure must cause the solution to move. If any one of these conditions is eliminated, efflorescence will not occur.
All masonry and concrete materials are susceptible to efflorescence. During the construction process, water used to achieve a workable mortar or flowable grout constitutes an available source of moisture in the masonry system. Additional moisture is often introduced into exposed masonry by rain or snow. Water-soluble salts that appear in chemical analyses as only a few tenths of one percent are sufficient to cause efflorescence when leached out and concentrated at some point on the surface. The amount and character of the deposits vary according to the nature of the soluble materials and the atmospheric conditions.
Efflorescence is particularly affected by temperature, humidity, and wind. In the summer, even after long rainy periods, moisture evaporates so quickly that comparatively small amounts of salt are brought to the surface. Usually efflorescence is more common in the winter when a slower rate of evaporation allows migration of salts to the surface. With the passage of time, efflorescence becomes lighter and less extensive unless there is recurrent moisture movement through the wall. Light-colored surfaces show the deposits much less than darker shades.
In most cases, salts that cause efflorescence come from beneath the surface; but chemicals in the materials can react with chemicals in the atmosphere to form the undesired efflorescence. For example, in concrete masonry, mortar, or stucco, hydrated portland cement contains a substantial amount of calcium hydroxide as an inevitable product of the reaction between cement or lime and water. Calcium hydroxide brought to the surface by water combines with carbon dioxide in the air to form calcium carbonate, which then appears as a whitish deposit. Since calcium hydroxide is much more soluble in water at cold temperatures than at warm temperatures, such deposits are again more common in winter than summer.
Another source of salts is the soil in contact with basement and retaining walls. If the walls are not protected with a good moisture barrier, the salts may migrate a foot or two above grade. Severe efflorescence of this type can be eliminated by following the steps in the section "Prevention"
Prevention
Since many factors influence the formation of efflorescence, it is difficult to predict if and when any will appear. This fact is evident in the lack of any accepted standard test method for measuring the efflorescence potential of masonry mortar. Several experimental methods have been proposed, but none has been accepted as effectively predicting the performance of mortar materials in actual use.
ASTM C67 does include an efflorescence test for clay brick. The test is helpful in indicating whether or not clay units will effloresce by themselves when exposed to moisture. However, it does not address the potential for efflorescence resulting from cement-brick reactions or other external conditions that often occur in service.
Given the characteristics of masonry materials and construction, it is virtually impossible to entirely eliminate all the soluble salts, construct walls containing no free moisture, or completely eliminate paths of moisture migration. However, steps can be taken to minimize the extent of these three contributing factors.