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Hydrated lime, limestone difference

I summarized the information from various sources on hydrated lime and limestone.

Let's begin with a quote from TMC, chapter 9:

"Buffering agents are used to counter the acidic effects of peat and other casing materials. Calcium carbonate (CaCO3) is most commonly used and comes in different forms, some more desirable than others.

1. Chalk: Used extensively in Europe, chalk is soft in texture and holds water well. Chunks of chalk, ranging from one inch thick to dust, improve casing structure and continuously leach into the casing, giving long lasting buffering action.

2. Limestone Flour: Limestone flour is calcitic limestone mined from rock quarries and ground to a fine powder. It is the buffering agent most widely used by Agaricus growers in the United States. Limestone flour is 97% CaCO3 with less than 2% magnesium.

3. Limestone Grit: Produced in a fashion similar to limestone flour, limestone grit is rated according to particle size after being screened through varying meshes. Limestone grit is an excellent structural additive but has low buffering abilities. A number 9 grit is recommended.

4. Dolomitic Limestone: This limestone is rarely used by Agaricus growers due to its high magnesium content. Some researchers have reported depressed mycelial growth in casings high in magnesium.

5. Marl: Dredged from dry lake bottoms, marl is a soft lime similar to chalk but has the consistency of clay. It is a composite of clay and calcium carbonate with good water holding capacity.

6. Oyster Shell: Comprised of calcium carbonate, ground up oyster shell is similar to limestone grit in its buffering action and its structural contribution to the casing layer. But oyster shell should not be used as the sole buffering agent because of its low solubility in water."

As you can see, Stamets relies on limestone flour = CaCO3

A few other quotes:

Hydrated Lime: Solubility in Water: Slightly soluble in water. pH=12.4@25°C

Calcium Carbonate: Solubility in Water: Not soluble in water. pH = 8-9 @ 25oC

Minimum Calcium Carbonate Equivalent 126.94%
CAUTION: Prolonged contact with hydrated lime may cause irritation or burns to wet skin. Keep lime out of contact with eyes, flush thoroughly with water if contact occurs and call a physician. Keep away from open cuts or sores. Wash hands after use. Keep out of reach of children. '''

OK, let's sum up:
Hydratead lime obviously works as a casing ingredient as many have proved.

It is stronger than calcium carbonate in raising the pH, according to the above source it has an Calcium Carbonate Equivalent of 126.94%.

Then it also says 1,480 lbs. of this material equals one ton of standard liming material.
If we assume the standard liming material is limestone flour, then instead of using 1000kg limestone flour one can use 670 kg of the hydrated lime, which is 2/3 by weight

Now, the major problem I see is the pH of the hydrated lime.

When you use limestone flour, you can add as much limestone flour to your casing mix as you want, but still the pH won't rise aboove 8-9.

With hydrated limestone once you neutralized the pH by adding exactly the proper amount the hydrated lime, pH will rocket towards 12.4.
I see this as a major source of failure when people use hydrated lime since not every peat has the same pH and also measuring in cups for something like lime or peat is rather not reproducible.

Now what can we do about it:

  • Easyest thing, switch to limestone flour. It's not so critical in the amount used, it allows for some variation in the amounts
  • Also good, measure the pH of the casing. Add as much hydrated lime or limestone flour to obtain a pH of 7-8.
  • clearly point out the difference between "limestone flour" and "hydrated lime(stone)"

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Page last modified on May 02, 2010, at 03:48 AM