The ecological cost of concrete

Planet Earth has a definite mass consisting of two types of resources: renewable/recyclable and non-renewable. An abundant amount of solar energy impinges on Earth.

Apart from humans, the planet hosts many other living beings and plant life. Renewable/recyclable resources include water, solar energy, and biomass and biomass-related produce. All other resources are limited, exhaustible and non-renewable.

Humans occupy about 2 per cent of the planet’s surface and consume 75 per cent of the resources. Except for biomass and biomass-derived produce, the exhaustible resources are mined and consumed in an unsustainable fashion.

The dependence on non-renewable material resources for mankind’s survival has increased from zero to over 95 per cent in about 6,000 years since 4500 BC. Indiscriminate mining of resources is causing significant environmental and ecological damage, with some resources becoming scarce and vanishing (an example being river sand).

The pressure on finite natural resources is growing exponentially, mainly due to the demand from explosive population growth (the global population has doubled in the last 50 years).

Concrete has been used for construction since Roman times. Roman concrete was a mixture of lime-pozzolana cement and aggregates. Today, the most consumed material on Earth is concrete made from Portland cement (or its variants) and aggregates (sand and coarse aggregates).

The current annual global per capita consumption of concrete is four tonnes. In contrast, the per capita annual food grain consumption is under 0.30 tonnes. The amount of concrete produced each year far exceeds the mass of all the biomass and fossil fuels we use annually. To use a tonne of cement, we need 6–7 tonnes of aggregates.

Globally, about 105 Gt of materials are consumed across all sectors annually, with the construction sector alone accounting for 62 Gt (~60 per cent).

Hence, the global annual per capita consumption of construction materials amounts to about eight tonnes, over 60 per cent of which is aggregates. Aggregates are also used in non-concrete applications such as roads/pavements and other secondary applications.

The global annual consumption of aggregates is about 40 Gt and it is expected to touch 60 Gt by 1930. The main sources for aggregates are river/stream beds, coastal regions and crushed rock. River sand mining is banned in several regions/countries due to environmental and ecological concerns.

Rocky outcrops are slowly vanishing, and craters are created due to excessive mining of rocks, causing the destruction of many animal and microbial species, apart from environmental damage. Thus, aggregate production is an unsustainable business with heavy environmental costs, causing irreparable damage to the ecology and environment.

Are there alternatives to aggregates? There are expectations to meet the demand through circularity, i.e., recycling non-organic solid wastes generated from demolition of structures, and byproducts of industries and mines.

The total quantity of solid wastes generated that are useful as aggregates does not even meet 15 per cent of the requirement. In the Indian context, the useful non-organic solid wastes (for aggerates) generated annually are less than 0.20 Gt, while the annual aggregate requirement is over 4 Gt. A similar scenario exists across the globe.

The aggregate crisis is imminent and will soon explode. We may not have a dearth of cement and steel at least in the near future, but aggregates for building concrete structures will become unavailable.

Unfortunately, neither the cement/steel industry nor the government think tanks are addressing the aggregate shortage. One can guess that there will be rationing for aggregates (similar to India’s cement rationing in the early sixties) within the next three decades, a drastic fall in cement production, and a temporary shift in the construction sector towards steel structures.

Then we will be pondering over alternatives for building civil infrastructure. Global cement production has been stagnant since 2012, and it is easy to forecast the fate of the concrete industry and concrete structures.

We should look for sustainable alternatives to concrete structures; probably the answer is to derive construction products from biomass. The planet has enough arable land (5 giga hectares aside from forests) to grow biomass without jeopardising food security. Construction products can be made from renewably grown non-woody biomass (not necessarily timber) without cutting trees. Biomass-based structures are not uncommon now. The construction sector can become carbon neutral by substituting 50 per cent of construction materials with biomass-based materials.

Goals have been set to move towards carbon-neutral and green economies. To achieve carbon neutrality, we need to rely on mined materials resources. Not all green initiatives may be sustainable; we need renewable and sustainable materials to attain the status of a sustainable developed economy.

(The writer is a retired professor at the Dept. of Civil Engineering, IISc, Bengaluru)