Materials - Cement/Concrete
Like wood, cement is ubiquitous in construction, being a part of concrete, brick mortar, and fiber cement products. The manufacture of cement accounts for five percent of the world's total energy consumption, and the average home requires 76 cubic yards of concrete (about 120 tons), which takes the equivalent energy of 950 gallons of gasoline to manufacture1.
Of course cement is also an incredibly flexible product for which there is no substitute.
Pure Portland cement is an excellent glue, but is otherwise not strong at all, so cement is always mixed with other materials and is only the glue that hold them together. Concrete is a mixture of gravel, sand and cement; mortar is a mixture of sand and cement, and fiber-cement products use various fibers (wood, fiberglass) mixed with cement. In all cases, water is added to cement and it undergoes a chemical reaction which causes all the cement molecules to bind to each other. Not every cement is the same, and there are hundreds if not thousands of formulas for concrete. The main identifying feature of concrete is its compressive strength (the amount of weight it can hold up), and that is largely due to how much Portland cement is put into it. Because concrete is not flexible, it has little resistance to bending, and as a result almost all applications of concrete require some kind of metal reinforcing: either re-bar or wire mesh is typically used.
Since the biggest environmental impact of concrete is the energy used to make the Portland cement, the focus of environmental building has been to use less of it. There are three approaches typically used: substitute flyash, use insulating concrete forms and substitute a completely different material.
Flyash
Flyash is a byproduct of coal burning power plants or waste from blast furnaces (iron making) and has properties similar to Portland cement, and since it is a waste product it makes a better environmental choice. Flyash comes in two varieties, called "class C" and "class F", with the difference being that class C can directly replace Portland cement, while class F can reduce the amount of Portland cement used, but not replace it. (See EBN,V8#6 for a more complete discussion) Different kinds of coal produce different flyash. In both cases, flyash typically makes a stronger, more chemical resistant concrete, but achieves its strength over a longer period (Concrete strength is usually given as the strength is gets in 28 days: flyash concrete can take twice as long to achieve that strength. In residential construction, flyash concrete is still strong enough in a couple of days to begin framing, and so does not impact schedules) Flyash is often added to concrete to make it flow better, and in that case it does not reduce the amount of Portland cement used. Unfortunately much of the flyash produced has too many impurities in it (typically carbon) to make a good cement, and in addition since much of it is the result of burning coal, it is clearly not a renewable resource.
Assuming cement quality flyash is available, is can be used to replace anywhere from ten to fifty percent of the Portland cement needed for a particular concrete mix2. The downside is that it finishes different from normal cement, and so the concrete installers have to be experienced with it- particularly when using it for slabs. Flyash concrete also doesn't reach it full strength for much longer than normal cement: the standard is that it reach full strength in 28 days, while flyash concrete can take up to three months. In spite of these drawbacks, many people are learning to work with it, and some are experimenting with using greater than fifty percent. As a rule of thumb, using 15-20% flyash as a replacement for Portland cement is easy to do.
When using a flyash mix, it is important to make sure the flyash is replacing cement, not sand and gravel. Compare how many sacks of cement were used to how many are normally used, rather than the percentage of flyash. A typical mix uses five sacks of cement (500 pounds).
Alternative Foundations
Insulating Concrete Forms (ICFs), are a material that acts as both the form for a concrete wall, but then stays in place to act as insulation. Using an ICF wall instead of a regular poured wall can use 50% less concrete. ICFs are typically made of polystyrene foam, and there is a product made with recycled polystyrene and cement, as well as one made with waste wood chips and cement. More on ICFs is in the construction section.
If a basement isn't needed, the house can be built on a crawlspace instead, using either a post and pier support system or a pin foundation (find out more about these), but since there are other issues in using a crawlspace, these must be considered also (see healthy home section). When using any kind of post system, larger diameter pieces of wood are used to replace the one piece foundation, so there is a tradeoff between wood and concrete.
Adobe/Cob: A more radical idea to reduce concrete usage it to construct the floor slab out of earth instead. Cob (a mixture of straw, sand and clay) is the ideal material, because once dry it hardens like concrete. Because Cob does not set permanently like concrete, it must be protected from water, by a sealer of some kind (eg linseed oil). The biggest limitation on cob/adobe floors is that they take months to dry and are prone to cracking during drying, plus you end up having to use a lot of concrete for footing anyhow, since neither the code department or the majority of structural engineers will allow anything else.
Green Cements: there is a resurgence in interest in earthen plasters, lime plasters, and other alternative cements. Although earthen and lime plasters are not nearly as durable as Portland cement, there are a variety of cements being developed that apparently are. They all seemed to be based on some combination of magnesium oxide and calcium oxide, and their primary benefit is a lower embodied energy.
In any attempt to use less concrete, a creative structural engineer is almost a must, both in getting past the building department and making sure that the building will still last for a long time.
Other concrete products
Fiber-cement products have recently become very popular both for siding and roofing. They have the advantage to being relatively rot-proof (although they do wear out, and acid rain would be a significant enemy of these products) and dimensionally stable, which means that paint will typically last much longer on fiber cement siding than it does on wood. The downside is the high content of Portland cement.
Stucco, like fiber cement is fire proof, rot proof and long lasting. It also contains a lot of Portland cement, although acrylic stuccos probably contain less (research needed). Since stucco is usually applied quite thickly (up to 1.5" thick), it will contain more Portland cement, and hence a higher embodied energy than fiber-cement siding (although fiber cement siding may have been trucked a long way, negating any energy benefits)
Concrete block: in residential settings, concrete block is used mostly for landscape. In terms of energy use, the delivery of the product probably uses more energy than manufacture, so when comparing to stone, the product with the least travel is probably the greenest.
Notes
1: I found the number on cement.org, who refers to a NAHB study I can't find on NAHBs website, but 76 yards is probably not that far off. The energy value is assuming 1.5mBtu/yard embodied energy and 120KBtu/gal gasoline. This figure includes walkways.
2: in limited tests, its been used as a 100% replacement for Portland cement, but its only possible with the right flyash.