Mortar for below grade foundation

[mfiore]

I'm putting in a block foundation below grade. What is the best type of Mortar to use for this application? I have two books from the library, one says to use Type S, the other says to use Type M. Does it matter?

[dbhansen]

Here's a good summary of the various mortar types and their properties: link

Either type M or S should work for your foundation wall, but S might be best. This is from the mason contractors website:

"It should be remembered that stronger is not necessarily better when selecting mortar for unit masonry. For example, it is not typically necessary to use Type M mortar for high-strength masonry. Type S will provide comparable strength of masonry, and in fact the Masonry Standards Joint Committee's design standard ACI 530/ASCE 5/TMS 402 does not distinguish between the structural properties of masonry constructed using Type S mortar from that constructed using Type M mortar. Moreover, Type S and Type N generally have better workability, board life, and water retention. As a rule of thumb, the specifier should not require use of a mortar of higher compressive strength than necessary to meet structural design criteria."

What is that strength?

"The five typical mortar mixes designated types M,S,N,O and K are labeled so because each is an alternate letter in the term MASON WORK in descending psi strength....

M 2,500 psi
A
S 1,800 psi
O
N 750 psi

W
O 350 psi
R
K 75 psi

[gjbingham]

Good info there! Strength is a relative term though. Compressive, tensile, shear...