Anyone who has worked with mortar knows how it dries out on a board. Summer’s heat and winds accelerate the drying effect. Retempering, the practice of adding water to mortar to restore workability, is considered good practice because it keeps mortar in the right condition for laying brick, block, and stone.
Retempering is done on the mortar board by the mason, usually by dribbling water into the mortar pile, then reworking with a trowel or shovel. This replaces water lost by evaporation. It keeps the mortar plastic so that units can be laid and positioned in it.
To those familiar with concrete, adding water seems like it ought to be prohibited. It’s true that concrete is not supposed to have water added to it—except for a one-time adjustment if the water-cement ratio isn’t exceeded. Producers can hold back some water to allow field adjusting the mixture. But with mortar, the approach is different. Mortar will be placed into contact with absorptive units. As the mortar contacts the units, an immediate “tug of war” begins for the water: the mortar tries to hold it and the units try to draw it out. This process works best when both the mortar and unit are trying to get the water. It actually improves bond.
The characteristic of mortar that describes its ability to hold water is water retentivity. The measured property is water retention (WR). All mortars specified by ASTM C270, Standard Specification for Mortar for Unit Masonry, are supposed to have a minimum water retention of 75 percent. When mortar loses its water too fast, it becomes stiff and unworkable. Then units can’t be positioned or adjusted and bond can be impaired.
The characteristic of units that describes how water is drawn into them is absorption or initial rate of absorption (IRA). If water isn’t drawn into the units, good bond may not develop. Balancing the water equation is essential to getting good results on masonry projects.
Water additions to mortar are permitted only when time limits have not been exceeded (beginning from the initial introduction of water). When water and cement come into contact, there is a delay before hydration, or chemical combining of cement and water, begins. Masons use time limits to help guide them on mortar’s acceptability rather than try to guess if mortar is unworkable because water has evaporated or cement hydration has occurred. A general recommendation is to limit a mortar’s useable life to two and one-half hours from the introduction of water, but this can be shorter in hot or dry weather. Old mortar should be discarded.
Of course, good practice also means mixing mortar quantities to match the mason’s pace, and preventing evaporation of moisture from the mortar. This could mean covering the mortar on the board, working at cooler times of the day, or leaving the mortar in a covered tub. Although retempering is allowed, it can affect mortar strength and color, and is not the first or only defense against water loss.
Colored Mortars and Retempering
While a reasonable amount of retempering can be good practice, a difference of opinion exists on the topic of retempering colored mortar. Many specifications prohibit this in order to maintain consistent color. Adding water can lighten the color. However, two contractors we spoke to believe that retempering colored mortar within reason can be good practice. Jay Jacob of J. Construction Company, Inc., Cincinnati, Ohio, and Jerry Painter, Painter Masonry, Inc., Gainesville, Florida, weighed in on the subject. They both said that good practice is necessary to minimize all of the various influences that play into the resulting mortar color.
When water evaporates from mortar, it does not carry color pigments with it. The water that is added back during retempering simply replaces lost water. But it is possible that water could leak off the board before being mixed in with the mortar. Or pigment could bleed onto the board. So the first “good practice” step is to work with a reasonable amount of water and avoid the need to retemper.
That said, it seems reasonable that adding water to maintain workability should lead to good placeability and bond. This added water should be thoroughly remixed into the mortar to get uniform consistency on the board. As already noted, preferred practice is to prevent as much evaporation as possible to eliminate the need for retempering. Abusing the practice of retempering can lead to problems. Manufacturer recommendations printed on bags of color mortar mixes might have statements to the effect of “avoid excessive retempering.”
Other factors play into the joint color as well, such as temperature and humidity conditions during construction and time of tooling. To demonstrate the effect of tooling time on joint color, this masonry prism was constructed using one batch of mortar. The top mortar joint was tooled immediately after placement of the units. Remaining mortar joints were tooled at progressively greater time intervals and thus stiffer consistency. The relationship between mortar consistency when tooled and mortar color is quite apparent; earlier tooling leads to lighter joint color. The higher water content means that there is a greater water-cement ratio.
Both contractors note that the timing of striking mortar joints impacts the overall appearance. It affects how much paste is brought to the surface and how sand is exposed. These factors affect the texture, which affects how light is reflected from the joint, and what its apparent color is. Once the mortar is in place, color differences can be quite noticeable on newly constructed masonry. The contractors note that cleaning and sealing of the masonry can impact the wall’s appearance. As time goes on, they say color differences may even out.
Mortar accounts for a large part of a wall surface. Colored mortars, in particular, can have a dramatic impact on masonry’s appearance. To get the best results, both plain and colored mortars should remain plastic and workable while being used to lay units. During hot or dry weather, good practice becomes even more important, and that means preventing evaporation of water from mortar, and perhaps, retempering to maintain workability.
For more on retempering mortar from Portland Cement Association:
Concrete Masonry Handbook for Architects, Engineers, Builders, EB008