“Geopolymers” are artifical stone materials that are somewhat like cements, and have interesting applications in many of the same areas. There are, however, some important differences, both practical and theoretical, between a geopolymer, and, say, a mortar or concrete based on Portland cement. Practically speaking, geopolymers show impressive performance, in some tests, above and beyond those of run-of-the-mill cement and concrete mixes. On the theory side, geopolymers are critically different from cements because they don’t depend on the hydration of lime (CaO) to set up. Lime is made by strongly heating limestone to drive off carbon dioxide, a process which, given the huge amounts of lime required by the huge amounts of cement our world consumes, is a major contributor to atmospheric CO2 emissions. Hence, a lot of the excitement about geopolymers, apart from their potential high-performance applications, has to do with reducing our collective carbon footprint.
Anyway, big-picture takeaway: A chunk of cast geopolymer is a very different thing than a chunk of cast cement. If you’re a hands-on type, like me, your first question on hearing about this, or any kind of fancy material, is likely to be how do I get my hands on some? Data, theory, and popular science all have their uses, but ultimately that’s always the best way for me to understand something: Hold it in my hands, maybe poke it with a stick. Best of all, of course, is if can make it myself.
So I started digging for the practical protocols the lab folks were using to make little blocks of geopolymeric stone to run compression tests on. This kind of thing is always frustrating because a lot of the hands-on info is locked away behind academic publishing paywalls, and I have no intention of shelling out $39.95 to download one six page paper from ten years ago, thank you very much Mr. Elsevier. Anyway, perseverance paid, and I eventually found a 2008 Journal of Materials Science paper by Australian researchers J. Davis, et. al., posted (probably illegally) to scribd.
Table 2 is particularly instructive. It includes four formulae for geopolymer compositions, one of which (“SGP”) is both highest-performing, in terms of compressive strength, and simplest to make. Pound for pound, it’s also probably the most expensive, which would be a problem if I wanted to build a viaduct out of it. But all I want to do is cast a couple small pieces. So we’re good.
What follows are my notes on adapting the “SGP” protocol from this paper for garage prep. It is a straightforward formula, and most of the leg-work, as is so often the case in this kind of thing, is in sourcing materials in a way that does not require going through one of the big, expensive chemical supply houses that really doesn’t want to deal with citizen scientists. It’s important to note that I haven’t actually done this, yet, so I can’t vouch for its efficacy or safety. But I wanted to publish the reference and my thoughts about a DIY version before concrete month gets away from me. If you’re interested, read on, and please do comment if you spot anything fishy; if not, stay tuned for a follow-up about how it works out.
Sodium hydroxide – AKA lye. Available at many hardware stores as drain cleaner, e.g. “Red Devil.” This is strong base and you need to understand how to handle it safely.
Metakaolin – This is a form of kaolin, a common clay, that has been chemically changed by heating at about 750° C for several hours. It is commonly prepared this way in the literature, but most people don’t have a temperature-controlled furnace in the garage for performing this operation. Fortunately, so-called “highly reactive metakaolin” is available commercially, online (in the US, anyway) for use in cement countertops. A 25-lb bag will be well more than we need. If this process works, I may divide up my leftovers for sale in small, cheap portions for those who want to play along at home. The MSDS suggests that it is not particularly hazardous, but as with all fine powders, a dust mask is probably a good idea.
Sodium silicate solution – It is probably possible to prepare a homemade solution of sodium silicate that will work in the geopolymer process by using a procedure like this one from NurdRage, in which finely-crushed silica gel desiccant is dissolved in strong lye solution. In the literature, however, geopolymer samples are seemingly always prepared from a ready-made commercial sodium silicate solution in water. Unfortunately, the “grade O” commercial sodium silicate solution specified is only available from specialty suppliers; however, I think I can fudge it by adjusting the composition of the common “grade N” solution used, e.g. to repair mufflers, by adding lye flakes. So we’ll need some “grade N” sodium silicate, to start. “Grade N” is also called “Grade 40” and “water glass,” and Googling turns up several online sources.
Step 1: Prepare sodium silicate solution
The easiest way to measure ingredients for this process is by weight. You’ll need a scale with a capacity of at least 1000g. Put a 250 mL beaker on the scale, record its weight, and, taking appropriate precautions for handling strong base, add 4.4 grams of lye flakes. Now add an additional 62 grams “grade N” sodium silicate solution. Remove from scale and stir to dissolve lye. Once this solution is well mixed, cover it and let it stand, at room temperature, for 24 hours before use. Note that this solution is now slightly more dilute than commercial “grade O” sodium silicate, but the Davis SGP recipe actually calls for diluting the commercial solution just a bit, anyway, and the math works out pretty closely.
Step 2: Add metakaolin
Remove the cover, put the beaker back on the scale, and bring the the total weight of the solution up to 100g by adding dry metakaolin powder. You may want to add it in batches, with stirring in between. The metakaolin will not dissolve; you should end up with a paste or slurry. The polymerization reaction will begin as soon as you start adding the metakaolin, but you should have at least an hour’s pot life. If it’s working, the mixture should begin to give off heat.
Step 3: Casting
Transfer the mixture to a small metal mold. I will probably use a steel muffin tin—they’re cheap, about the right size, and if I want to I’ll be able to cast and cure multiple samples in a single pan. Seal it well with aluminum foil. The idea is to keep water from getting out during the curing.
Step 4: Curing
The “SGP” mixture should polymerize at room temperature, but geopolymer samples are generally cured with mild heat, and if you want to experiment with other aluminosilicates besides and/or in addition to metakaolin (e.g. fly ash), the curing step seems to be necessary. Generally, the protocols call for heating to 60° C for 24 hours. That’s 140° F, and I’m comfortable doing that in my kitchen oven, as long as I’m not going to be leaving the house during that time.
Demolding the cast samples may be a problem that eventually calls for some kind of mold release. But it’s not reported in the literature protocols I’ve seen, so I’m going to start without it. I’ll probably want to experiment with adding aggregates, and sand is convenient and conventional. The Davis paper describes mixtures with 40 wt% and 60 wt% sand additions to the “SGP” formula.