Why does ordinary plaster mortar crack?

A

Allen

Member
Here is the formula,
P.042.5 cement: 144KG fly ash II grade:36KG
yellow sand fineness around 2.0 : 820KG
hydroxypropyl methyl cellulose ether :130 g K12:35g water: 154
wet density is 1698

The plastering has wall cracks in the inner layer of the construction site. The basement is intact. Workers report cracks in the first layer, and the second layer also appeared before the initial setting.

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The sand is too fine so it cracks when applying too thick. Coarser sand will be fine.
 
First, the sand is too fine; second, moisture inside is not kept and it is easy to dry. Add some water-retaining agent and thickener will be better.
 
Too much cement and the sand is too fine.

Since it cracks in first layer before the initial setting, there are some factors need to be considered:Water retention rate, admixture amount, and silt content in the sand.
 
Change the amount of sand to 1350 with half machine made sand and half river sand, cement 200, fly ash 80~100, liquid admixture 10. That will be ok.
 
90% of the cracks that appear immediately are due to manual problems, such as applying too thick, the foundation being too slippery to absorb water.
 
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