The use of portland cement repointing/rebuilding mortar on masonry construction originally built with lime or natural cement is problematic because:
1. Portland mortars are typically harder and more rigid than natural cement and lime mortars. If you replace the outer 1/2" of mortar with something harder, then as the structure expands and contracts, all the stress is concentrated at the outer 1/2" of surface, and this can lead to spalling of the brick.
2. Lime and natural cement structures were usually built without expansion joints because the mortars were flexible enough to accommodate movement related to thermal cycling. Portland needs expansion joints, and if you don't provide them, the structure may crack, thereby providing its own joints.
3. If portland mortars are proportioned to be fairly dense, as modern mortars tend to be, they will be slower to release moisture than their traditional counterparts. That tends to force the moisture to seek alternate routes of escape, i.e., through the masonry units themselves. The masonry suffers for it. So does the older back-up mortar.
4. If portland mortars require repointing more frequently than natural cement mortars, then portland represents a less sustainable approach, as every round of repointing induces damage and loss of fabric to the original structure.
Mike E
---------- Original Message -----------
From: [log in to unmask]
To: [log in to unmask]
Sent: Fri, 4 May 2007 10:37:32 EDT
Subject: Re: [BP] Pulaski Still Being Pounded
> In a message dated 5/4/2007 8:52:27 AM Eastern Standard Time, [log in to unmask] writes:
The portland work is bad, everyone knows it's bad, yet it continues.
> Mike, educate me ... how is it bad? Christopher
>
>
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