Due
to limited available data, the details
as we understood/assumed are:
- Same
quality of material (cement, water & sand) were used in both the
cases
- Same
standard of workmanship was maintained for both the cases
- Period
of curing is not provided; we have assumed it to be 28
days (since the owner will continue curing for a longer period if
he has
noticed scratchable surface...)
Notes:
- No
data on existence (or otherwise) of efforescence is available
- The
statement of "poor strength" is not elaborated upon, other than
the mention of surface being scratchable. The presumption of poor
strength due
to surface being scratchable is not necessarily valid.
Comments:
New
masonry is often highly alkaline. Water put on the surface
allows water soluble
salts to migrate to surface, carbon dioxide in the air converts calcium
hydroxide & calcium sulphate (both of which may have pH close to
14) to calcium carbonate (which is pH neutral).
If
pH value is still about/more than 11 after 28 days of curing, then only
thing
that is going to bring it down is time/moisture.
In
your case, pH value is still 11 even after more than 28 days. This
suggests either of the
following possibilities (either one or more in conjunction):
(i)
Curing
, in this case, was not done properly. However, if you are sure,
that curing was done properly then continue it for a longer period
(say
about 1 more week)...spraying with a garden hose thrice a day would be
fine
(ii)
May be that bricks (in second
case) are having a high
suction
rate. Such bricks will absorb water from the mortar very
quickly,
potentially leaving the mortar too dry to hydrate
properly
and bond to the brick. Wetting the brick prior to laying or using a
mortar with
high water retentivity, will compensate for the high suction
brick.
If
bricks are also of same quality in two cases, then poor curing seems to
be the
cause.
In
case curing was not done properly or mortar becoming too dry to hydrate
(due to
high suction rate of bricks), the strength will be affected adversely.
However,
if there is efforescence...there would not be any strength problem
(though
surface peeling will occur)."
Hope
that answers query to your satisfaction
