I'm kind of skeptical about this. Methods of stopping or diverting lava flows have been tried for centuries, with varying success. The Neapolitans called on their patron San Gennaro to turn away lava flows produced in the 1631 eruption of Mount Vesuvius (although one would think that the flows "spared" Naples more because their supply was cut off than because of the intervention of a 4th-century martyr). The Icelandic town of Heimaey was able to use seawater cannons to partially divert lava flows from closing off the entrance to their harbor in the eruption of Eldfell in 1973. (I seem to remember there were some questions as to whether the flows were diverted because the method actually worked, or if there was some change in the eruption that slowed down the effusion rate anyway - I'll have to check on that.) And in 1935, 1940 and 1942, Mauna Loa lava flows threatening Hilo were bombed in an attempt to keep them from reaching the city (with no significant effects, although studies since then have suggested that better targeted aerial bombing "has a substantial probability of success for diversion of lava"[Lockwood and Torgerson, 1980]).
"Schuiling believes a geochemical approach might be effective in controlling lava flows across the globe. He explains that certain common rocks, namely dolomite, or limestone, will react strongly with hot lava at 1100-1200 Celsius. The chemical reaction that ensues is highly endothermic, which means it requires heat, and this would be supplied by the hot lava.
"The decarbonation of limestone by the hot lava will therefore rapidly cool the volcanic outpourings, making it far more viscous and quicker to solidify. The reaction will leave behind solid calcium and magnesium oxide mixtures - pyroxenes or melilites depending on the specific type of lava. The process would also release some carbon dioxide.
"He suggests that large chunks of dolomite or limestone blocks could be thrown on to lava from the sides, or from above by helicopters or airplanes, or even by an aerial cable system passing over the flow. An alternative approach might be to quickly build a wall of limestone blocks in the path of the advancing lava flow. In places where a future lava flow would cause great material damage, such walls could even be constructed as a forward defense before a new eruption." (Eurekalert)
(Above, USGS image of ships pumping seawater onto the forward margin of the Heimaey lava flow, Photograph courtesy of Sigurgeir Jónasson, March 1973. To right, B-18s over Mauna Loa eruption, April 1940, from the Hawaii Aviation website.)
And, of course, there's always the Jersey barrier solution employed by Tommy Lee Jones in Volcano.
While I'm sure that the chemistry behind the new idea is sound, I have questions about the physical aspects of the situation. A lava flow, even a really fluid one, has a lot of mass, and the source of the flow is constantly adding more. A flow doesn't stop unless its source stops producing lava. It can sometimes be diverted (naturally or not), but more often than not it's simply going to go around or over an obstacle, and keep on moving in the same direction as before. Then you have a whole new set of problems: Where is the diverted lava going to go? How long will the barrier last, and if it fails, are you going to have lava going in multiple directions? This is the very reason that Volcano's Jersey barrier solution was unrealistic - even if they could hold back the lava in the first place, the lava had to go somewhere, and with that kind of force, the barriers wouldn't last very long anyway.
Knowing the capability of lava flows for pretty much bulldozing over anything in their way, I don't think that building a limestone wall in the way of one would be very effective. I could be wrong, though; basaltic lava flows on Hawaii become channelized all the time by their own debris, and I suppose if you were able to dump enough limestone on one in strategic locations, you might be able to divert it. I do think, despite being skeptical of the idea, that it would certainly be really cool if it did work. Not to mention that it might actually be affordable, since limestone is common in nature and used widely for construction. (More affordable than bombing, anyway.)
Come to think of it, Jersey barriers are made out of concrete. Concrete contains cement, and cement is made partially out of limestone. Hmm. Maybe TLJ had something going there...
"Volcano taming: Could geochemistry save lives during volcanic eruptions?" (Eurekalert)
Schuiling, R.D., 2008, How to stop or slow down lava flows: Int. J. Global Environmental Issues, 2008, vol.8, p.282-285 (Link to abstract)
Man Against the Volcano: The Eruption on Heimaey, Vestmannaeyjar, Iceland (USGS Publication)
Lava-Cooling Operations During the 1973 Eruption of Eldfell Volcano,
Heimaey, Vestmannaeyjar, Iceland : U.S. Geological Survey Open-File Report 97-724
Lockwood, J.P. and Torgerson, F.A., 1980, Diversion of lava flows by aerial bombing — lessons from Mauna Loa volcano, Hawaii: Bulletin of Volcanology, vol. 43, no. 4, p.727-741. (Link to abstract)