I read Mark Clark's message with interest, because:
1. Mea culpa. It makes me realize that I misread the original posting,
which describes a Type K, not Type O mix and
2. It underscores the confusion in the grey areas surrounding the
borderline between all-lime and cement-lime blends.

It seems to me that Mark approaches the question from the point of
view of someone accustomed to working with pure lime mixes, whereas,
admittedly, our work has almost entirely involved cement-lime blends.
We do millions of pounds of Type N and Type O mortars, smaller and
more infrequent runs of Type K's.

The main reason I am given by conservators for specifying Type K is to
gain early frost and rain resistance, an important consideration in
the Northern states. The curing requirements are also reduced, as are
lime streaking tendencies, which can be of value when engaging
commercial restoration contractors who have not mastered the art of
employing more traditional materials, a commercial reality.

The 3:1 sand to cementitious materials ratio is correct for Type K,
although I would agree with Mark that most all-lime mortar recipes I
have seen use less sand. I disagree with the comments about Type I
White Portland cement, however. White cement is not made white by
higher temperature firing, it is white due to lower iron content in
the rock used as feedstock for the cement plant. Type I White cement
must meet the same performance requirements as Type I grey cement,
under ASTM C150.

What may be confusing to those who rely on feel and observation,
rather than quantitative methods, is that in order to maintain a
certain level of brightness, cement producers often grind their white
Portland to a finer particle size, similar to a Type III grey cement.
This produces a little faster hydration due to increased surface area,
but the ultimate strength is unchanged.

We have used white cement in Type K mortars without any reported
difficulties. While I am inclined to agree that mortars which are too
hard are a potential disaster, I am also inclined to question what
represents "too hard" in many situations. Certainly the age and design
of the building, the hardness of the stone or masonry, and the weather
exposures are important engineering concerns. Sometimes engineering
approaches may point you in opposite directions from typical
preservation philosophies. I tend to encourage the use of critical
analysis as to the true needs of each particluar situation.

Edison Coatings, Inc.
M. P. Edison
President
3 Northwest Drive
Plainville, CT 06062  USA
Phone: (860)747-2220
Fax: (860)747-2280
email: [log in to unmask]
Internet: www.edisoncoatings.com

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