A comprehensive analysis of the causes and solutions for cracking in lightweight plaster at different stages!-HPMC/PVA

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I. Initial Cracking of Lightweight Plaster (0-30 min); Cracking begins a few minutes after application to the wall, with cracks appearing during construction. It is often accompanied by white spots and numerous, messy cracks.

Causes: Typical plaster contains excessively high levels of anhydrous gypsum, leading to excessive water absorption and reaction; the plaster layer is too thin, resulting in insufficient water retention; the substrate absorbs water too quickly; the amount of water added to the plaster is too high.

Solutions:

(1) Measure the anhydrous gypsum content and allow it to age appropriately;

(2) Confirm the standard consistency and viscosity. If the particles are too fine, add a water-reducing agent to reduce the amount of water used in mixing;

(3) Add an appropriate amount of cement or lime (not exceeding 5% of the total formula);

(4) Increase the dosage of thewater-retaining agent;

(5) Apply a bonding agent to the substrate.

II. Early Cracking (30 min - after hardening): Cracking occurs in the middle to late stages before hardening, with large and wide cracks.

Causes: High ambient temperature, high wind speed, and rapid surface water loss leading to shrinkage cracking. Excessive amount of fine powder or fine filler. Excessive retarder setting time, resulting in prolonged gypsum hardening. Insufficient effective gypsum content, preventing prolonged drying and hardening. Secondary mixing with water in wet mortar, leading to severe powdering after drying.

Solutions:

(1) Avoid construction in hot, windy weather; increase the proportion of aggregate or widen the particle size range;

(2) Confirm the retarding time, especially in summer, control the retarding time carefully;

(3) Use a thickener from the Youwei gypsum product system;

(4) Confirm the effective gypsum content and replace the gypsum;

(5) Strictly prohibit the reuse of mortar with failed molding properties after secondary water addition.

III. Post-Cracking (After Full Hardening): Cracking after full hardening mainly manifests as through-cracks, and light tapping reveals hollow areas.

Cause Analysis: The formula design may be unreasonable, with excessive cement content leading to excessive shrinkage of the plaster plaster later. Poor adhesion strength between the mortar and the substrate, such as low surface strength of the substrate, can also cause this problem. Using an interface agent with excessive PVA content can also cause this issue.

Solutions:

(1) For minor hollow cracks, repair by cutting 10-15cm along both sides of the crack;

(2) For severe hollow areas, the entire plaster needs to be removed and redone;

(3) Before construction, confirm the condition of the substrate, including loose dust and release agents;

(4) The formula should be redesigned based on the current condition of the raw materials.