Understanding the Performance Differences Between Expanded Perlite and Calcium Silicate

Johns Manville Industrial Insulation Group (JM IIG) offers two high-temperature industrial insulation products with exceptional compressive strength: Sproule WR-1200®, hydrophobic expanded perlite, and Thermo-1200™, water resistant calcium silicate. These materials have a number of similarities that make them ideal for many of the same applications, but there are some unique differences between the two that designers should consider when specifying either of these insulations.

Material Makeup
First, is their material makeup. Thermo-1200 is calcium silicate, a powder that is derived from naturally occurring limestone and diatomaceous earth (a soft, siliceous sedimentary rock). In contrast, Sproule WR-1200 is perlite, a material made from amorphous volcanic glass. The reason perlite insulation is technically referred to as “expanded perlite” is because when it is heated during the manufacturing process (to approximately 1600°F/900°C) the moisture expands, causing it to pop like popcorn at 1600°F. This makes perlite a very low-density material that is light-weight, with low thermal conductivity values.

Compressive Strength
Despite its low density, perlite still offers high compressive strength (> 80 psi/ 550 kPa)1 when compared to other insulations like mineral wool (the strongest of which offers 7.5 psi/ 48 kPa)2 or aerogel (> 3 psi/ 20.7 kPa)3.  It is outmatched by calcium silicate which offers compressive strength that equals or exceeds 100 psi (690 kPa)4. The only other materials typically used in industrial settings that exceed the compressive strength of calcium silicate are some types of cellular glass – which are typically used to insulate the bottom of cryogenic storage tanks. When comparing the cellular glass product that is typically used for pipe insulation (rather than cryogenic storage tanks), the compressive strength falls right between calcium silicate and perlite, coming in at 90 psi/ 620 kPa5.

Sproule WR-1200 is manufactured with a hydrophobic component, which makes it a popular product for many applications in the Gulf Coast region. Bear in mind, however, all hydrophobic treatments on any insulation, not just expanded perlite, will burn off at 450°F. Since the hydrophobicity remains intact until temperatures exceed 450°F, perlite can be a good solution for applications that fall within the corrosion temperature bandwidth (ambient to 350°F). 

By comparison, most calcium silicate insulation, with the exception of Thermo-1200 water resistant calcium silicate, has no water resistant properties. While Thermo-1200 calcium silicate is not hydrophobic (i.e., it does not completely repel all water), it is the only calcium silicate available that is water-resistant. It is engineered to shed water during a typical rainstorm, making installation logistics easier for contractors, as they have a little more liberty to decide how and when to apply the jacketing.

It’s important to remember, however, that all forms of hydrophobicity and water resistance have their limitations. Such features are designed to prevent the insulation from absorbing water and holding it against metal surfaces – which can lead to corrosion under insulation (CUI). However, in humid environments like the Gulf and other tropical locations, we must face the fact that some water, via vapor, will eventually get into the system. At that point it is important to have another deterrent in place to help mitigate CUI.

Corrosion Inhibiting Properties
Inhibiting corrosion is where both Sproule WR-1200 and Thermo-1200 stand apart from competing products. They both contain the XOX Corrosion Inhibitor®, an integral series of components in the formulation that help prevent corrosion. Among other things, XOX Corrosion Inhibitor contains a variety of silicate ions that, when exposed to water, leach from the insulation and gather on the metal surface, creating a passivation layer that both acts as a coating on the pipe surface and neutralizes the pH of the water. This helps prevent water and corrosive substances from reaching the pipe itself.

The XOX Corrosion Inhibitor works like a contingency plan in most applications: if water should penetrate into the insulation, the corrosion inhibiting properties of XOX Corrosion Inhibitor will “activate” to help prevent corrosive damage from occurring on the pipe surface.

It’s important to note that Johns Manville Industrial Insulation Group is the only manufacturer to offer perlite or calcium silicate products with the XOX Corrosion Inhibitor. With the XOX Corrosion Inhibitor, Thermo-1200’s corrosion-inhibiting performance is nearly equal to that of Sproule WR-1200 for most stainless steel applications.

The air pockets in expanded perlite do have a tendency to make it a more brittle material than calcium silicate. As a result, calcium silicate can be a stronger material during shipping or installation than expanded perlite.

The thermal performance and corrosion-inhibiting properties of Thermo-1200 and Sproule WR-1200 make them highly interchangeable in many applications. If you’re unsure which would be best for your application, please contact our technical support line: 800-866-3234.

You can also request a free, on-site technical presentation for you and your team to learn more about crucial science and technology in the industrial industry, or get information about JM IIG products. Click here.


  1. Sproule WR-1200 Data Page
  2. http://rwiumbracortiny-na.inforce.dk/media/1002645/rti_pds_prorox_ps_980...
  3. http://www.aerogel.com/_resources/common/userfiles/file/Data%20Sheets/Py...
  4. Thermo-1200 Data Page
  5. http://www.industry.foamglas.com/__/frontend/handler/document.php?id=106...