Obviously, lime as binder is the most adapted one.

Useless type of fly ash in cement and concrete industry is environmentally massive waste. Although lime can be better choice in terms of adaptation to the environment, it is a industrial product not waste. Moreover,  waste bacterial treatment used in self-healing applications can be give more satisfactory results rather than fly ash, so it can be think as an option.

lime is the best material but you may think for flyash mixed with lime..filyash - lime mixtures is best alternate to other kind of stabilizers..

Lime is probably the best "stabilizer" in terms of environmental footprint, depending on the nature of the soil or the particular soil problem.

Gypsum is another product that could rank with lime as a potential benevolent soil stabilizer for many applications.

Cases could also be made for fly-ash (good economy) which is by nature inert, used in moderate concentrations (due to potential leaching of heavy metals) and should be especially viable for road layer stabilization applications.

The likes of cement could also have a role specifically with respect to "stabilization" of road materials, but cost could become a factor.

Other materials might also be useful in special applications, but the appropriateness of any materials considered as a stabilizer should ideally be assessed by means of an appropriate Environmetal Impact Study in every case.

I think lime plus sodium carbonate is reasonable option. The reaction Ca(OH)2+Na2CO3-> NaOH + CaCO3 

NaOH react with the clay in the soil and precipation of limestone is good for early strength. Such system is used in ancient times (Egypt) - rock natron and lime. Be aware of mature period which is crucial. 

Have you seen this article : Environmental Friendly Soil Stabilization Materials Available in Iran

http://jefm.kiau.ac.ir/article_537951_4eb4ca96a6d24b7d6c8143c55ff391e3.pdf

I recommend to use lime or fly ash. Some chemicals for small areas we can use sodium silicates with calcium chloride which is also a great option, please read my published paper in journal of hazardous materials vol 359 entitled (Electrokinetic Nondestructive In-situ Technique for Rehabilitation of Liners Damaged by Fuels). This paper is a great asset to help using such chemicals and reagents for the purpose of soil stabilization.