Imagine Freeways Being Carbon Sinks

Round two on green cements here after skimming Novcem’s site, its duly notated that the “British Cement industry view Novcem’s claims with skepticism” as they should until said claims have been fully grounded in field practice. We have no concrete or composite strength data for comparisons with high strength portland based materials like concretes for bridges and high rises, but since this is about Green, Novcem does cite a CO2 absorption figure,its fair game, provided a conventional green binder could be had so we are comparing absorbers to absorbers. Based on magnesium olivine, serpentine (soapstone, talc and magnesium peridotite mineral strengths its difficult for this author to envision a magnesium based cement composite having the strengh of calclium based calcium silicates and aluminates and ferroaluminates being that the old high magnesia (12-14% magnesia)cement of yesteryear that would have absorbed some CO2, they were also much higher in lime content and exhibited considerabley lower ultimate concrete strengths, admitting an abosrptive cement would be a big plus in the new cement’s favor being portland is not an absorber except in the amount of its small free lime content.The only conventional CO2 binder mineral green contender is the oldest, lime that gradually becomes a structural carbonate cement over time unless treateed with carbonic acid mix water or injected with supercritical CO2.

Unlike naturally occurring CO2 absorbing minerals like peridotite all lime can do is reabsorb the carbondioxide qusntity it lost during thermal sublimation from limestone if not complexed with a pozzolan or organic before hand, in which case its a 1:1 proposition, the corresponding reabsorption figure being about 1.1 tones of CO2 per ton of lime. It does not recoup the CO2 generated by process combusion of fuel. (The fuel loss deficit can be mostly eliminated through conversion of existing lime kilns to electrics that add about 20kg fuel derived equivalent CO2/ton as opposed to 295Kg/ton addition for coal fired kilns or the new solar lime kilns that automatically yields a 20% material savings over convention, but currently cost 2-3X more than conventional, that largely places them out of the running until mass produced. So fuel combustion with air lime CO2 total material yields ranges between 1.3 tons CO2 for coal fired to 1.04 tons for solar for ever ton of lime made. At present Novcems patent application has not been reviewed to glean their magnesium cement’e equivalent mineral and energy use quantities sinced this is not a true energy-materials material balance comparison.

This leaves Novcem’s present available absorption data for comparison with lime: its published atmospheric or wastestream capture CO2 figure of 0.6 tons CO2 per ton of cement. Not having read the patent application before going off half cocked here, the author cannot say if the new magnesia cement is a fired cement based on magnesium carbonate-silicate mineral blend, magnesium caarbonate having a much lower sublimation temperature than lime or simply a calcined or sintered magnesium silicate/hydrosilicate capable of hydration and absorption to a carbo hydrosilicate hydraulic cement. If so, the 0.6:1 absorption ratio is just another zero-net carbon tradeoff with the environment as lime’s.

Carbonitization of portland concretes and mortars is adverse, weakens strength and shortens effective life expectancies. Owing to the smaller ion size, it is not known whether or not a magnesium silicate matrix, unlike portland is resistant to carbonatization deterioration. It is known that naturally occurring magnesium silicates and carbonates and magsium silicate cements readily weather to brucite (magnesium hydroxide) and that ground waters containing magnesium ions deteriorate portland concretes. In light of the above it no wonder the British Cement boys remain sceptical.

Another approach at attempting to undestand a lower cost magnesia cement dynamic would be through analysis of known glasses in glass making. For instance there are low fusion point high lime and/or magnesia soda-lime glass systems, some of which should fuse or sinter at approximately the same temperature that magnesite sublimes to magnesia. such might yield a lower cost cement since it is well known that the old high magnesia cement rock cements were all “lightly fired cemennts fired at between 800-1100 C caompared to portland’s 1200-1400C.