Time concrete turned Green

Obviously  the  concern is about the carbon footprint of concrete. According to a recent report, the annual global  usage of concrete is roughly 30 billion tonnes.  Going by the current trend,  this will rise  upwardly  for decades to come.    Concrete,  an aqueous   slurry  of cement, sand and gravel  can be poured  and  set  in any form.   Once set, the product has enormous strength, stability and durability with absolutely no maintenance cost.   Over centuries concrete has inspired and dared  architects and builders to dream big and build big. 

Burj Khalifa, Dubai

Concrete is 10-15% cement, 15-20% water and the rest an aggregate of gravel and sand. To get high quality concrete, water  to cement ratio must be kept low without compromising the workability of the mixture.  The ingredients are  mixed, set and cured  at room temperature. Seems like a very benign process. So what exactly is our quarrel  with  concrete?  Well our  issue is  not with concrete per se  but with the  cement in the formulation.  Cement is made by the calcination   limestone with clay at 1450 deg.C. This high temperature is achieved using fossil fuel, usually coal which holds a heavily  tarnished environmental  record.  During  the  calcination process multiple reactions take place in sequential  and parallel fashion. First of all  limestone which  is calcium carbonate,  releases  carbon dioxide  and becomes   calcium oxide  and  reacts with  oxides and silicates of  aluminium and iron   in the clay.   The resultant product  is  cement,  a complex mixture of  tricalcium silicate (3CaO · SiO2), dicalcium silicate (2CaO · SiO2), tricalcium aluminate (3CaO · Al2O3), and a tetra-calcium aluminoferrite (4CaO · Al2O3Fe2O3) with small amounts of gypsum (CaSO4 ).  This mixture called clinker  is enhanced with additives if needed and then  pulvarized to yield cement. This is the conventional cement  usually referred to as Portland cement.

Thus cement manufacturing is one of the most energy intensive  industries.  It is one of the highest consumers of fossil fuels.  Approximately   3 GJ of thermal energy and 110 kWh of electrical energy are needed to produce 1 tonne of cement.    In terms of  CO2 emissions  too the cement industry tops the list  with   600 kilograms of carbon dioxide  released into the atmosphere for every  tonne of cement manufactured .  And that  indeed  is our concern. 

We  cannot simply wish away the use of fossil fuels and switch to alternate/renewable energy . Simply because the cost factors are not   yet favourable.  Besides  Coal industries  are  the biggest  employment generators.    There are enormous  hurdles along this  path  because  globally fossil fuels are subsidised at two levels: production level and consumption level both of which  keep the cost to the consumer low.  A recent report in Nature (ref.2) states that  subsidies worth an average of US$555 billion each year were given  from 2017 to 2019.  The figure dropped  $345 billion in 2020 only because of lower fuel consumption and declining fuel prices during the COVID-19 pandemic. But can  the subsidies be withdrawn all  of a sudden?  The answer is No.  Among other factors, rising fossil fuel prices  will be extremely hard on people  in  developing and under-developed countries.     Political  parties and  governments may not want to even tilt the applecart. Along with drastic changes in  political and societal   mindset, cost effective   alternate energy sources must be made available. COP26 in Glasgow is debating this aspect and whole world is listening.

Another question  pops up here : do we have  alternatives for limestone? Perhaps we do. Fly ash, the residue from the coal plants is a possible candidate and so is  slag from the iron and steel industry.  Fly ash is the fine residue  after the combustion of coal   while slag is the residue from iron blast furnaces. Both contain  oxides of silicon, aluminum, iron and calcium and hence  bear an overall chemical resemblance to Portland  cement but not in precise composition or structure.  Nevertheless studies have shown  that  blending  conventional  cement with 20% of either  fly ash or  slag doesn't adversely affect the properties of the final concrete.  Currently blended  cement formulations with varying fly ash/slag  content are available in the market. However here is the catch:   with  a global consensus shaping up to phase out  coal by 2070,  future  availability of fly ash will eventually be an issue.   Slag has its own disadvantages regarding workability,permeability  and durability  which are being looked into.

Tailpiece:

Burj Khalifa :  Having consumed 110,000 tonnes of concrete,  Burj Khalifa, currently the tallest building on Earth, is guilty of releasing  660,000,00 kilograms of  carbon dioxide  into our atmosphere.  The environmental cost factor  of a total of 55,000 tonnes of steel and 103,000 square metres of glass  remains to be calculated.  

References:

1. Concrete needs to lose its colossal carbon footprint: Nature,30 Sept.2021

2. Why fossil fuel subsidies are so hard to kill?

3. Zimmer, M., Kuhanathan, A. & Badre, A. Abolishing Fuel Subsidies in a Green and Just Transition (Allianz, 2021).

4. Agenda for CoP26: Why phasing out coal will not be decided in a jiffy?

4.Optimizing-the-use-of-fly-ash-concrete.pdf

5. Sag Cement Organisation

6. Sustainable cement: the simple switch that could massively cut global carbon emissions

7.