Selection of the correct grade of mortar is perhaps the most important factor in the performance of a masonry wall. The mortar must have sufficient strength, be durable, resist rain penetration as much as possible and yet be flexible enough to accommodate slight movement within the wall.
Mortars used in the setting of Cast Stone should meet the requirements of ASTM C270, Type N mortars. These Portland cement/lime mortars generally consist of one part cement (ASTM C150), one part lime (ASTM C207) and six parts of clean, washed masonry sand (ASTM C144). They may also contain iron oxide coloring pigments (ASTM C979).
The 1/1/6 mixture provides good bond strength with desired
weather resistance and moderate compressive strength relative to the
stone when cured. The lime enhances the workability of the mortar
while reducing shrinkage. The practice of wetting the head and bed
joints of the stone will further protect against joint shrinkage.
Proper mixing is essential to good consistency. All materials except pigments are measured by volume. Sands should be placed in the spiral-blade or paddle type mixer first, followed by pigments (if required), pre-water, lime and cement, final water and 5-7 minutes of mixing time. Mortars unused after 2 ½ hours should be discarded. (This time may be affected by temperature.)
Head joints in most hand set stones may be set with the usual wet consistency mortar used in setting brick and block. Stiffer mortar must be used when setting larger stones and shims are recommended for all pieces over 300 lbs. When setting, fill all dowel holes, anchor slots and similar building stone anchor pockets completely with mortar. Non-shrink grout or anchoring cement may be specified for dowel connections. Caution should be used when the bed joint is on horizontal flashing as it will act as a bond breaker. Special anchorage may be necessary to accommodate this condition.
Mortar systems have the ability to carry loads but cannot absorb much joint movement. For this reason, thoughtful designers often require that joints at parapets, copings and other particularly sensitive areas be left open (unmortared) for later closing with sealants.
When using a post setting pointing system, rake all stone-to-stone joints to a depth of ¾" for pointing later. It should be noted that in many cases (and specifically when setting small veneer pieces) it is not practical to rake out and point all joints. In these instances, full bed setting and finishing in one operation can be used. Particular attention must be paid to the waterproofing systems behind and incorporated into the veneer. Stone-to-brick joints are usually struck and tooled to a slight concave. See Technical Bulletin #44 on Pointing. Sponge all mortar smears from face of stone with water. Hardened, smeared mortar is difficult to remove from the surface of cast stone. Clean with a commercial masonry cleaner approved for use with cast stone or with water and a stiff fiber brush. Consult Cast Stone manufacturer first. Power washers should never be used to clean Cast Stone. See Technical Bulletin #39 on Cleaning.
If lug sills, which extend beyond the masonry opening, are fully
set in mortar, the mortar at the ends under the lugs (where the load
is) may, in certain situations, compress or shrink more than the
rest of the mortar (due to overburden forces), causing shear or
bending stress in the sill and possibly leading to failure. One of
two additional precautions may be taken when setting lug sills. The
first is to set only the ends of lug sills in a full bed of mortar;
and point the portion of the sill inside the masonry opening after
the initial mortar bed has cured. The other approach is to shim
under the lugs to bear any compressive loads. The system designer
should examine the lug sill configuration and how it interfaces with
the adjacent and under-supporting masonry units to insure that point
bearing will not occur at the mid-span of the sill. Slip sills,
which do not extend past the masonry opening, do not carry any load
other than themselves and are set in a full bed of mortar.
This Technical Bulletin is provided by the Cast Stone Institute®,
and is intended for guidance only. Specific details should be obtained
from the manufacturer or supplier of the Cast Stone units.