They are concretes of a sort, masonry mortar and stucco, that is. They both contain portland cement and aggregates, but then they separate into distinct compositions and purposes. Concrete, to all kinds of structures; masonry mortar, mainly in joints and as parging for brick, block (concrete masonry units, or CMU), and stone construction; and stucco in its primary role as wall coverings.
Concrete needs to be placed and molded to shape, which is done by restraining its potential fluid movement, using forms so restraint against gravity is along the bottom and sides. When initially placed, masonry mortar is restrained in joints by flanking overlying, underlying, or adjacent masonry units, but not along front, back, and all sides. Masonry mortar needs to have “fat” to retain its shape, support overlying masonry units, and not “flow” laterally the moment it is loaded by the weight of overlying masonry units. Stucco for walls also needs “fat” so it can defy gravity and hang in place, which is also true for parging mortar when applied to the backside of walls. The “fat” is obtained using hydrated lime or relatively high levels (e.g. 14%) of air-entrainment (both taboos for concrete) also provide water retentivity needed for cement hydration.
Hydration of portland cement provides their main strength. Concrete strength correlates mainly to water-cement ratio and to attainment of as much portland cement hydration as the water-cement ratio will allow—under the circumstances of its curing environment. On the other hand, although cement hydration is important to mortar and stucco, equally, if not more important, is the “fat” that allows them to accommodate masonry units placed on their surfaces (mortar), and to stay in place. An additional need is to maximize strength by retaining water that can so quickly depart into absorptive flanking masonry units or from front and rear evaporative joint surfaces. Concrete doesn't need that assistance. Water-retention of mortar and stucco is important, and is a property imparted by their constituents.
Little recognized is the relationship of specified mortar compressive strength in standards (which are determined in the laboratory under strictly controlled water content, component batch weights, and moist curing) to in-place field strength. Portland cement will continue to hydrate as long as water is available. Purposeful curing does that for concrete. Mortar and stucco do that, as best they can, by retaining mix water, thus, the need for water retention by the mortar and stucco.
Later, when atmospheric carbon dioxide causes a shell of carbonation products to envelop cement particles, hydration stops—permanently. Because of the relative thinness of mortar and stucco, and their invariably high-water contents that increase permeability to atmospheric carbon dioxide, carbonation more readily happens to stymie later strength development, which does not occur to concrete. Also field masonry mortar, for example, can undergo retemperings for up to two and a half hours, according to usual specifications, and there is no numerical limit on the amount of initial water and tempering water that can be used. So in-place field mortars cannot be expected to reproduce strengths of laboratory-prepared test mortars.
The initial chemical properties of concrete, mortar, and stucco are similar; that is, they each maintain a high pH, never less than the pH of a saturated calcium hydroxide (Ca(OH)2) solution, 12.4 to 12.5. Atmospheric carbonation of concrete affects only its skin, whereas, with time, it usually deeply penetrates mortar and stucco, and thus drops the pH quickly. Carbonation results in shrinkage, and also creates sensitivity to corrosion of embedded reinforcement, which usually is galvanized to resist corrosion. Not so for concrete because if reinforcing steel is buried deeply enough, the high pH maintained protects it from corroding. All of the above assumes the chloride nemesis is not present.
Some major differences between these are: (1) restrictions on water content of concrete versus an almost cavalier freedom for mortar and stucco; as long as their “fat” is retained; (2) concrete slump versus the mason's “artfulness” in controlling “fatness” of mortar and stucco; (3) prolonged cement hydration for concrete and an almost guaranteed cessation of hydration over a relatively short period for mortar and stucco; (4) carbonation, limited to the concrete “skin” but throughout mortar and stucco; and (5) better field strength control for concrete versus mortar and stucco.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for more than 48 years.
