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Effect of pH value on strength of plaster

We found that one plaster sample whose pH value was 8- the strength was found alright. To the other sample of the same bldg. where plastering was done about one month back -found stregnth poor- surface scratchable when the plaster gets dry-the ph was about 11. Does increase in pH value have any relation with decrease in strength . Pl. note the water source was same for both the plasters.can you please give any direction /answer to proceed in this regard. (Question asked by Mr Sanjay Kumar Deedwania)

Unfortunately the referred problem doesnt come out with test values for strength, nor does it comment on quality of bricks & length of curing period.

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

*****

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