More Than You Ever Wanted To Know
About Alkaline Resistant Glass

 

Here’s the story on glass. It has to be a special type of glass to put in cement.

Since it is special, it is expensive - over two times as much as E glass (the conventional type used in most glass reinforced products).

This special glass is called AR Glass. That stands for Alkaline Resistant, which is the specific trait required if it is to be used in a cement based matrix.

It has that catchy name - Alkaline Resistant Glass - because it was coined by British engineers.

The cement mortar matrix is alkaline which means it is chemically basic (as opposed to acidic). This occurs because the by product (or leftovers) of cement becoming stone again is lime, which is mainly calcium hydroxide, which is very alkaline or basic. (Remember how the bad guys in the movies got rid of the bodies? They put them in a hole and covered them with what? That’s right, lime, the stuff that eats up dead bodies...).

When the BERDs (British Engineers Remaining Dull, rhymes with ....) went to work on the problem of glass strength reduction over time in glass reinforced concrete, they discovered a simple (but expensive) solution - add zirconia to the furnace melting the sand into glass.

It is expensive because the Australian’s have most of the zirconia on the planet, and essentially, they just think "controlled substances" should be expensive.

Not only that, but the furnace needs to be dedicated to only making zirconia enhanced glass. It takes a couple of years to set one of these up and then it can only do one thing - make AR Glass. Therefore, capital intensive dedicated equipment = risk = return on investment ASAP = high market price.

If you aren’t bored yet, this last part is guaranteed to force you to click out of here.

Really, why can’t regular inexpensive glass be used? Actually, it is a mystery of sorts.

Initially all we knew was that adding glass to cement mortar made a great composite - the environmental resistance of stone with the strength of glass fibers. BUT, it did not last for long. Those great physical and mechanical strengths (link to page) decreased in a few months - faster in warmer climates.

Why?

Crime scene photographs have been helpful in unraveling the mystery.

Fig. 1* Open space around filaments after curing conventional GFRC

 

Fig. 2* Dense lime deposits around filaments of conventional
GFRC after 1 year under water.

* Photomicrographs courtesy of Cem-FIL Corporation

1- AR glass fiber
2- Lime crystals

EXPLANATION #1

Since strong alkalis are used to etch or frost glass, the fibers must be being notched by this etching phenomena. These notches make the normally strong fiber more sensitive to breaking under load - essentially it allows a tear to start across the fiber. As more notching occurs, the overall strength decreases.

EXPLANATION #2

The lime by product of cement becoming limestone again is initially in solution because of the excess water in the concrete mix. Under these conditions, it can migrate into and around the fibers of glass. As the water migrates to the surface and evaporates, the lime crystallizes in intimate contact with the glass. When the glass is stretched under load it is like a rope being pulled over the edge of a sharp rock.

Back to the movies - remember the close-up of the rope starting to fray and be cut by the rock edge....so you’ve got the picture.

Here is the BERD description of the same thing (for our more technically oriented reader). It is two paragraphs, then back to the reality most of us share.

Standard GFRC is known to lose some of its initial strength and become less ductile over long periods of time. Although the design process accounts for this change in properties, and there is little evidence to show that this is a problem, it is still perceived by some as being a disadvantage and is frequently used as an argument against GFRC products in competitive situations.

There is now strong evidence to show that, when AR glass fibers are used, this loss of strain capacity is not due to alkali attack on the fiber by the cement. It has been established that it is due to the development of lime crystals and calcium silicate hydrates (CSH) around and within the bundles of glass filaments. The lime (calcium hydroxide, Ca(OH) 2) is a by-product of the cement setting and hydration processes and it is initially present as an aqueous solution. This solution is drawn by capillary action into the fiber bundle and later crystallizes, filling the pores and locking the fiber into the cement matrix, thus preventing the slippage that exists in young GFRC. It is this energy absorbing slipping of the fibers that gives GFRC its remarkable toughness characteristics (see Figs. 1 and 2).

Okay, we are at the "so what" stage.

What’s so, is that adding this special glass to cement mortar makes an engineering composite that is much lighter in weight and stronger than conventional steel reinforced concrete.

What’s so, is that regardless of a modest decrease in initial strength, the fiber is stable over long periods - 100+ years in real time correlated with accelerated aging tests without any further decrease in strength characteristics of the composite.

Still, what's so is that not all glass / cement composites are created equally - you want the long strand fibers (1 1/2") version made by STONEWEAR. Any body can throw short strand fibers (1/4 - 1/2") in the precast bucket and call it GFRC - be careful.

In short it’s okay! Use it with confidence. Specify STONEWEAR and be certain, you are dealing with the decorative application pioneers in this material technology.

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