Understanding Plaster

The primary ingredient in pool plaster is cement, which hardens into concrete. Just like concrete sidewalks and driveways, this product needs time to fully dry. It might look finished on the surface, but underneath, a chemical process called hydration is still occurring.

Plaster’s cure time is essential because it needs to develop:

• Structural strengthening
• Water tightness
• Colour fastness

What’s cool about plaster is that it cures under water. Yes, it does and shouldn’t be exposed to the air during this period! The pool is actually filled the same day that plaster is finished.

So what happens if you ignore your fresh pool plaster? All sorts of issues can pop up. Structural damage is a huge concern, especially spalling (flaking of the top layer) and crazing (a network of cracks like on an eggshell). There can also be aesthetic problems, such as inconsistent plaster colour, stains, and leftover impressions from trowels.

The good news is that the cure time for pool plaster is fairly quick, around 85% of the plaster’s cement cures within the first month. It’s why this initial 30 days is when you need to take preventive steps to protect the plaster.

How to Protect Fresh Pool Plaster

The most important step is to brush, brush, brush for the first two weeks. Twice a day, just like brushing your teeth.

This is because as plaster cures, it leeches calcium into the water. If it’s not removed, the efflorescence will cling to surfaces and eventually harden into a white scale. Loosening this dust into the water allows it to circulate and get filtered out.

Best of all, these steps won’t prevent you from using your new pool!

The most common type of nodule is the “delamination” nodule. These nodules grow because of a void (usually a bond separation) between plaster and its substrate. Here is the sequence:

Under normal conditions, the plaster (white) is exposed to the water (blue), is bonded to the gunite substrate (speckled), which in turn rests on dirt (tan). (Picture not to scale…)

Sometimes, an air cavity (a bond failure) can form between the gunite and the plaster, or between multiple layers of plaster. This is referred to as “delamination” and the cavity is referred to as a “void.” As long as the void is not connected to the surface of the plaster, the fact that the void even exists may not be known. Unless the plaster completely breaks free from the surrounding plaster, creating what is referred to as a “pop–off,” or unless the delamination is extensive, this is not considered by the plaster industry to be a defect.

In some cases, however, the void is connected to the surface by a small pinhole or hairline crack. Pinholes and cracks are often created from structural flexing of the delaminated plaster.

Gradually, water from the pool penetrates the void via the hole or crack, and creates a localized chemistry environment completely separate from the water balance in the pool. As calcium hydroxide bleeds into the void water, it creates a calcium–rich, high pH solution like a little “calcium/pH factory” beneath the plaster surface. The pinhole or crack is not of sufficient size to create a rinsing effect like that which occurs at a new plaster surface.

Calcium–rich void water and pool water containing bicarbonate alkalinity, in contact with each other at the plaster surface, react with each other, which results in the production of an insoluble calcium carbonate by-product.

If the conditions are right, this insoluble calcium carbonate can build up a “slag pile” around the exit point, thus forming a nodule.

Attempts to remove a nodule through scraping, sanding, or a direct acid treatment, can be successful – but the node may reform on the same site once or twice, until all of the source hydroxide is used up. Then it shouldn’t come back.

Acid washing nodules off usually isn’t the best response, since the calcium carbonate it is made of is the same stuff much of the plaster is made of. So to dissolve the one, you also dissolve some of the other.

Plugging the hole, by drilling and epoxying, has been successfully used to stop nodule formation, since water can then no longer travel in either direction. BUT REMEMBER, DRAINING THE POOL MAY CAUSE MORE DELAMINATIONS TO OCCUR, AND IT IS LIKELY THAT MORE NODULES WILL FORM AGAIN AFTER THE POOL IS FILLED WITH WATER. So this repair should be performed under water.

It is important to keep in mind that nodules are a symptom, rather than the disease itself. The disease is bond failure, cracking or some other imperfection that allows water “to boldly go where no water has gone before”

Nodules that form on horizontal surfaces like pool floors end up round shaped:
While those that form on walls drip downward, reminiscent of some stalactites:

Although the calcium carbonate is white, and thus pure calcium nodules are white, when the crystalline matrix incorporates or absorbs dirt, metals, or other contaminants the nodule takes on a color:

Another type of nodule is associated with crazing (small cracks in the plaster) rather than with delamination. Craze cracking can form from high heat, excessive wind, low humidity, or from shrinkage due to excess water or calcium chloride in the mix.

If the surface is sanded or acid washed, the existing nodules may be removed. However, more of the craze network is exposed, and many more nodules may then be formed.

Nodules can even grow on tile grout! This is actually two nodes – a large one to the upper left and a smaller one to the lower right. Notice the “volcano hole” where the continuing process happens!

But the mechanism is the same. In this instance, the source of the calcium- and hydroxide-rich solution is a delamination in the grout/plaster/gunite interfaces:

Nodules can also form on fiberglass surfaces! However, research has shown that in these instances, pinholes existed in the fiberglass, and the source of the node material (calcium) was the plaster/gunite beneath the fiberglass coating:

For decades, it was understood among plasterers that calcium nodules primarily formed due to bonding failure (delamination) of a new coat of plaster, and occasionally from severe craze cracking, and that they were responsible for this plaster defect. Plasterers learned that bonding failure rarely occurs on fresh gunite substrates (new pools); however, bonding new plaster (replaster) to old plaster surfaces can occasionally be difficult to achieve. This is why most calcium nodules occur in replaster jobs.

Unfortunately today, some in our industry (primarily plasterers) will claim that faulty water chemistry maintenance is the direct cause of nodules. That is absolutely incorrect. High water hardness and high pH will cause a uniform layer of calcium scale throughout a pool surface, not in isolated and individual spots (bumps). Service techs and pool owners should not accept the blame that their chemical treatment caused this plaster defect. Aggressive water would actually prevent nodules from forming, and properly balanced water will not prevent nodules from forming, but actually facilitates the visible growth that exposes the underlying problem.

What is calcium scaling in a swimming pool?

Calcium scaling refers to the buildup of calcium deposits on the walls and surfaces of a swimming pool, resulting from high calcium levels in the water.

What are the two types of calcium scaling in pools?

The two types of calcium scaling are calcium carbonate and calcium silicate. Calcium carbonate is relatively easier to remove, while calcium silicate requires professional intervention.

How can I distinguish between calcium carbonate and calcium silicate scaling?

To differentiate between the two, you can place a drop of muriatic acid on the scaling. If it reacts and foams, it’s likely calcium carbonate. If nothing happens, it’s probably calcium silicate.

What causes calcium scaling in swimming pools?

High calcium levels, high alkalinity, high pH, and elevated water temperatures create the perfect conditions for calcium scaling.

Is muriatic acid safe to use for removing tough scaling?

Muriatic acid is a strong acid and can be hazardous if mishandled. It’s best to leave its use to professionals who have experience in handling such chemicals.

Why is it essential to remove calcium silicate scaling promptly?

Calcium silicate buildup can cause damage to your pool’s pipes and filter if left untreated for too long.

How often should I check my pool’s water chemistry to prevent scaling?

Regularly test your pool’s water chemistry and make necessary adjustments at least once a week to maintain proper chemical balance and prevent scaling.

Can I prevent calcium scaling in my pool?

Yes, you can reduce the likelihood of scaling by maintaining proper water chemistry, using appropriate pool chemicals, and regularly cleaning and maintaining your pool.

How can I maintain my pool to prevent scaling?

Regularly balance your pool water, clean and vacuum the pool, and address any issues promptly to prevent scaling and maintain a clean and inviting swimming pool.

 

What Are They?

Nodules are small white growths of hard calcium mineral de¬posits. They are generally round, mound or volcano-shaped, and usually located on fine and sometimes almost invisible cracks in the plaster. If they are located on walls or a vertical surface, they generally “drip” down the plaster surface, somewhat in the shape of an icicle. Nodules can range in size from very small (pencil-tip) to very large (12-15cm), but the average is in the dime-size range or smaller.
It happens when there is delamination (bond failure) of the plaster surface with the gunite substructure, excessive craze cracking, or other similar defects, which allows water to penetrate either behind or into the interior of the plaster surface and dissolve calcium hydroxide, which then hardens and grows back up toward the surface of the plaster, thus evolving into volcano shaped mounds.
It’s more common in replastered pools where the new plaster surface sometimes has difficulty bonding to the chipped out underlying sur¬face, and bond failures can cause a void which can lead to the surface nodule formation.
Smaller nodules may form on craze cracking that can result from an overly wet mix, an accelerated drying of the new plaster, such as from excess calcium chloride (a set accelerant), or from environmental factors such as excessive heat, low humidity, wind, or excessive sun exposure before the pool is filled.

How do you fix it?

Nodules can be sanded off to make the pool cosmetically pleasant. But they will begin to return within several months of the pool being refilled with water. To completely solve the problem the pool will have to be replastered.

Nodules are not the result of poor water chemistry or anything that might suggest that the water has been out of balance.

Common Pool Plaster Problems

Pool Plaster Crazing

Craze cracks are small clustered cracks that occur when the pool plaster shrinks. Plaster tends to shrink as water in the plaster mix gets absorbed during the curing process, but if there is too much shrinking, craze cracks can appear.

Pool plaster crazing is a pretty common problem, and the consequences are usually minor. When the crazing is too bad, however, it can cause staining, algae issues, and calcium nodules. A bad case of crazing will require you to replaster your swimming pool.

Exhibit A: Craze cracks appearing on pool steps

Pool Plaster Etching and Scaling

You might have a problem with etching if your pool surface becomes excessively rough. Etching happens when lines and dents are literally carved into your pool plaster over time. This is caused by improperly balanced pool chemistry, particularly low pH, alkalinity, or calcium, and it’s one of the reasons why we emphasize the importance of taking care of your pool correctly.

Scaling is another problem and can occur as a result of high pH, alkalinity, or calcium. This can lead to a buildup of calcium and other minerals in your pool. Not pretty.

Here’s how to avoid scaling in your pool

• Brush the pool walls regularly
• Pre-dilute acid before you add it to the water
• Don’t add salt within 30 days of replastering your pool
• Never allow salt to sit on fresh plaster

The most important takeaway: make sure your pool is clean and your water chemistry is balanced. Regular and consistent maintenance goes a long way and can preserve your plaster longer.

Pool Plaster Spalling

Your plaster might become a victim of spalling if your pool is trowelled incorrectly. For example, if your plaster is over trowelled or if the trowelling is improperly timed, a thin layer of plaster (about 3.175mm) can begin to flake and peel off, which can set your project back another month. This can also occur as a result of adding too much water while trowelling.

So, how can you fix it? You can often fix plaster spalling by sanding the area down to reveal a much smoother underlayer. If spalling has taken over a large area of your pool, it might be better to start fresh and have your entire pool replastered.

Exhibit B: Spalling plaster

Pool Plaster Delamination

Delamination is another unfortunate problem that can plague fresh plaster. This happens when the plaster completely separates from the concrete underneath. Separation occurs when the bond between the plaster and the concrete is too weak and is typically a result of the surface being poorly prepared before the plaster is introduced.

Your pool plaster repair specialist might be able to fix any delamination with a simple patch if the area is small. If delamination is affecting a large area of your pool, the best course of action would be to have the entire surface replastered.

Exhibit C: Plaster delamination

Pool Plaster Fading

All swimming pools fade over time with persistent exposure to chlorine and sunlight. You’ll probably see a huge difference in your plaster’s colour by the 8 to 12 year mark (unless you went with traditional white plaster). If you decided to integrate colour into your plaster and you don’t want to see it fade so quickly, you can try an alternative sanitizing system like hydrogen peroxide, bromine, or even a natural system.

Pool Plaster Stains

It’s inevitable – most concrete pools will develop stains over time. Stains in the plaster can be caused by organic material in the pool, like leaves and dirt, or inorganic things like metals. How you treat or remove your pool stain will depend on the culprit.

How to clean pool plaster stains

If the stain is caused by dirt or leaves, a treatment of chlorine and some brushing will often do the trick. However, if it was caused by metal, you need to identify the type of metal before you can deploy your plan to remove the stain.

Stubborn stains can also be removed with regular acid washing. Just keep in mind that your pool can only be acid washed so many times before the plaster needs to be completely refinished.

Exhibit D: Stains on the bottom of a swimming pool

Final Lesson To summarize

Do’s and Don’ts for New Pool Plaster

Hydrochloric acid and muriatic acid are closely related, but there are subtle differences between them

1. Chemical Composition
   • Hydrochloric Acid (HCl) This is a strong acid composed solely of HCl molecules. It is colourless and highly corrosive when concentrated. In its aqueous solution, hydrochloric acid fully dissociates into hydrogen ions (H+) and chloride ions (Cl-).
   • Muriatic Acid Muriatic acid is essentially the same as hydrochloric acid, with the chemical formula HCl. However, it contains impurities, which give it a slightly yellow colour. These impurities do not significantly affect its applications.
2. Colour
   • Hydrochloric Acid Colourless.
   • Muriatic Acid Slightly yellow due to trace amounts of iron impurities.
3. Applications
   • Hydrochloric Acid
     • Widely used in laboratories and industries.
     • Used for refining metals because most metals dissolve easily in it.
     • Used for pickling steel (removing rust from iron or steel).
   • Muriatic Acid
     • Also used for similar purposes as hydrochloric acid.
     • Commonly used in masonry and cleaning.
     • Despite its impurities, it remains effective for its intended applications.

When the pH levels in your pool are too low, it can have detrimental effects on the appearance of the plaster. Here’s what you might observe

1. Etching and Marbling
   • Etching Acidic water slowly eats away at the pool plaster, leaving behind a rough and worn-looking surface.
   • Marbling The low pH levels cause a marbling effect in the plaster, making it appear aged and worn out.
   • Colour Changes Darker coloured plasters will show these effects more prominently than lighter coloured ones.
2. Visible Signs
   • Worn Appearance The pool plaster may not look its intended age and might seem prematurely aged.
   • Blemishes Marbling and etching can create visible blemishes on the plaster surface.
3. Choosing the right plaster
   • Lighter Colours Opt for lighter plaster colours like white or gray. They tend to hide imperfections better than dark gray or black plaster.

 

Pool plaster delamination occurs when there is a bond failure between the concrete or gunite and the pool plaster. This separation allows air to fill the cavity, resulting in bumps and bubbles forming on the pool plaster’s surface. These small air-filled cavities are also called calcium nodules. While pool plaster delamination usually occurs in smaller areas, it can even take up an entire side of the pool if left untreated. Over time, these delaminated areas can grow bigger and begin to crack open, allowing algae and other bacteria to thrive inside the concrete or gunite of the pool structure.

Here are some common causes of pool plaster delamination

1. Improper Bond Coat Application
   • If the bond coat (the layer between the concrete/gunite and plaster) isn’t thick enough or wasn’t applied correctly during plastering, it can lead to delamination.
   • A poorly executed bond coat is a sign of a bad plaster job.
2. Exposure Before Refilling
   • Leaving the pool plaster exposed for too long before refilling the pool can contribute to delamination.
   • The plaster should be refilled promptly after application to prevent air pockets from forming.
3. Structural Flexing
   • The pool structure can shrink and expand due to environmental factors or ground shifting.
   • Small hairline fractures may form in the pool plaster due to structural flexing.
   • These cracks allow water to seep beneath the plaster layer.
4. Environmental Factors
   • Humidity and Sun Exposure High humidity and excessive sun exposure can create structural fractures in the plaster.
   • Regular inspection of the pool surface is essential, especially in hot and humid areas.

White spots or blotches on a pool’s plaster can be quite concerning. Let’s delve into the causes and potential solutions

 

1. What Are the White Spots?
   • These spots may appear within a year of new plastering.
   • They are typically round or blotchy in appearance.
   • Colours can range from white to off-white or gray.
   • Sometimes, they form patches on the pool’s surface.
   • If your pool has dark-coloured plaster, these spots become more noticeable due to the contrast.
2. Causes of White Spots
   • Improper Plastering Practices
     1. Calcium Chloride Excessive use of calcium chloride during plastering can lead to white spots.
     2. Too Much Water Overly wet plaster or late trowelling can also cause these blemishes.
   • Incorrect Water Chemistry
     1. Aggressive Water Unbalanced or aggressive pool water can contribute to plaster etching.
     2. Soluble Calcium Ions Dissolution of soluble calcium ions in the plaster can make the surface porous and lead to whitening.
   • A combination of Both
     1. It’s possible that both improper plastering and water chemistry play a role.
3. Fixing White Spots
   • Unfortunately, these spots cannot be fixed without replastering.
   • Properly addressing the root cause during replastering is crucial to prevent recurrence.

Yellow stains in your pool can be attributed to various factors. Let’s explore the possible causes and solutions

1. Organic Matter Stains
   • These yellow stains are often remnants of a now-dead algae infestation. Algae can leave behind organic matter that discolours the plaster.
   • Broken-down pieces of larger natural debris, such as leaves or grass clippings, can also contribute to these stains.
2. Iron Oxidation
   • Another common cause of yellow stains is iron oxidation.
   • Iron present in the water can react with the pool plaster, resulting in yellowish discoloration.

Brown stains in your pool can be caused by various factors. Let’s explore the potential reasons and how to address them

1. Organic Debris
   • Cause Brown stains often result from organic debris accumulating in the pool. This can include leaves, dirt, oils, and even worms.
   • Treatment
     • Scrubbing For minor spots, scrub the affected areas with a pool brush.
     • Vacuuming Use a pool vacuum to remove debris settled on the pool floor.
     • Stain Remover Consider using a stain remover for stubborn spots.
2. Heavy Metal Oxidation
   • Cause Brown stains can also be due to heavy metal oxidation. Metals like iron can react with the pool plaster, causing discolouration.
   • Identification Determine if the stain is organic or metallic. If it responds to chlorine shock or brushing, it’s likely organic.

Calcium hardness plays a crucial role in maintaining the integrity of your pool plaster. Let’s dive into the impact of calcium on your pool surface.

1. Calcium Hardness and Plaster
   • Beneficial Contrary to common misconceptions, calcium hardness is actually your ally for water balance.
   • ;Stability Calcium is remarkably stable and helps maintain the Langelier Saturation Index (LSI) balance year-round.
   • Bedrock of LSI The LSI determines water balance based on how saturated it is with calcium carbonate (CaCO3).
   • LSI Factors Six factors influence the LSI: water temperature, pH, alkalinity, calcium hardness, total dissolved solids (TDS), and cyanuric acid (CYA).
2. Effects of Low Calcium Hardness
   • Plaster Damage In a plaster pool, low calcium levels cause water to draw calcium directly from the plaster.
   • Consequences This leads to pitting, roughness, and eventual crumbling of the plaster.
   • Expensive Repairs Replastering due to calcium-related damage is a significant expense and hassle.
3. All Pool Surfaces Are Impacted
   • Common Factor Whether your pool has plaster, pebble, tile grout, fiberglass, or vinyl, calcium saturation affects them all.
   • Exception Vinyl and fiberglass pools do not donate available calcium to water like plaster pools do.
4. Maintain Sufficient Calcium Hardness
   • Regularly test and adjust calcium levels to prevent damage to your pool surface.
   • Remember that proper water chemistry ensures your pool remains beautiful and durable.

So, embrace calcium as your pool’s steadfast companion, and keep your water balanced for a pristine swimming experience!

Excess calcium in your plastered pool can lead to various issues. Let’s explore the effects

1. Damage to Pool Equipment
   • High calcium levels can cause scaling and deposits on pool equipment such as filters, heaters, and pumps.
   • These deposits reduce efficiency and may even clog pipes.
2. Cloudy Water
   • Excessive calcium can contribute to water cloudiness.
   • Cloudy water affects water clarity and overall aesthetics.
3. Rough Pool Surface
   • Too much calcium can lead to a rough feeling on the pool surface.
   • The plaster may become pitted and uneven.
4. Stains and Discoloration
   • Calcium deposits can cause unsightly stains on the pool walls and floor.
   • These stains are challenging to remove.
5. Corrosion and Leaching
   • If your pool has metal fittings or heat exchangers, excess calcium can corrode them.
   • In vinyl or fiberglass pools, the water may attack metal fittings, leading to pinhole leaks and surface stains.

Balancing Calcium Levels

1. Aim for a calcium content in the range of 300-400 parts per million (PPM).
2. Use a pool water hardness increaser (calcium flakes) to raise calcium levels if needed.
3. Avoid extremes—neither too much nor too little calcium is ideal.

Remember, maintaining the right balance ensures your pool remains beautiful and functional.

Plastered pools can sometimes cause discomfort or minor injuries to swimmers, especially if the plaster surface is not in optimal condition. Let’s explore why your feet might get cut in a plastered pool.

1. Surface Roughness
   • Normal Wear and Tear Over time, pool plaster naturally wears down due to exposure to chemicals, sunlight, and water.
   • Roughness As the plaster ages, it can become rough, developing small pits, bumps, or irregularities.
   • Impact on Feet Swimmers walking or standing on the pool floor may experience abrasions or cuts if the surface is uneven.
2. Chemical Imbalance
   • Low pH Levels When the pool water’s pH is too low (acidic), it can cause the plaster to erode.
   • Erosion Eroded plaster loses its smooth patina, becoming coarse and potentially abrasive.
   • Foot Contact Swimmers’ feet can come into direct contact with the roughened plaster, leading to cuts or scrapes

Preventive Measures 

• Regular Maintenance Properly maintain your pool’s water chemistry to prevent plaster erosion.
• Smooth Plaster If your pool’s plaster is excessively rough, consider resurfacing or replastering.

Common mistakes regarding pool maintenance

• Regular Maintenance
  • Adding more chlorine than recommended may seem like a quick fix for a green pool, but it can make your pool cloudy. Add chlorine in smaller doses each day instead.
• Neglecting Regular Brushing
  • Regularly brushing your pool walls and floor helps prevent algae growth and keeps the surface clean.
• Using the Wrong Automatic Pool Cleaner
  • Not all automatic pool cleaners are suitable for every pool type.
• Infrequent Water Testing
  • Test your pool water weekly to maintain proper chemistry levels. Ignoring regular testing can lead to imbalances and water quality issues.
• Inadequate Filter Pump Runtime
  • Running your filter pump for too short a time can result in poor water circulation and filtration. Ensure it runs long enough to keep your pool clean.
• Ignoring Calcium Hardness
  • Proper calcium hardness is essential for plastered pools. Too little or too much calcium can damage the plaster. Maintain the right balance