The Cement Industry with annual worldwide production of around 1.5 billion tons is one of the largest users of high-grade limestone, coal, electricity & other minerals. Each ton of cement requires 1.3 tons of limestone, 0.2 tons of coal & around 100 kwh of electricity. Worldwide, cement production now accounts for more than 1.6 billion tons of CO2, or over 8% of total CO2 emissions from all human activities (Wilson 1993).

The increasing failure of our concrete infrastructure to live up to its designed life has focused attention on concrete durability over the last few decades. Although I believe it was the oil crisis of the 70’s that made the world understand the evils of prolificacy of natural resources. The French, the Germans, the Japanese took the lead cause these were the countries blessed with little natural resources.

This led to the resurgence of blended cements. The thrust was clearly on conservance of natural resources, usage of waste materials & on the durability of concrete (or the high performance concrete ‘HPC’) instead of normal/high strength concrete. This is what professor Pierre-Claude Aitcin, PEng; Ph.D. the renowned authority on HPC has to say.

"HPC can be broadly defined as low water/cement (W/C) or low water/binder (W/B) ratio concrete, the term binder being used to qualify any mixture of portland cement and fly ash, slag, silica fume, limestone or silica filler, rice-husk ash, ground pumice, diatomaceous earth, or any two or three of these finely divided materials’ Usual concrete (W/C ~ 0.50 to 0.60) have a rapid chloride-ion permeability of 5 to 6000 coulombs, while a 0.30 to 0.35 w/c HPC have usually a chloride ion permeability lower than 1000 coulombs, which represents a major difference in durability. "

The latest is Rock Powder Concrete (RPC) with a w/c ratio of 0.2 & a coulomb rating of 10.

The benefits of usage of fly ash & silica fume are well known. The incorporation of silica fume/fly ash in concrete produces marked reductions in w/c, the permeability and ionic diffusivity rendering the concrete more resistant to the penetration of harmful species (e.g. chlorides, sulfates, carbon dioxide, acids, etc.).

The American ASTM C 595 allows addition of up to 40% fly ash in cement.

PCA in ‘The Use of Limestone in Portland Cement: A State-of-the-Art Review’. claims "An optimized portland cement containing up to 5% limestone can exhibit improvements in workability as compared to the same cement without limestone". Although most countries (Germany, France, Canada. Japan, Russia. Brazil……). limit the addition of limestone to 5%, Certain European countries now allow addition of up to 35% low grade ground limestone (the specifications are available) in cement.

The Germans not to be left behind produce & market a slag cement containing only 15% clinker named Hochofenzement (Encyclopędia Britannica).

According to figures released by Cembureau (the European Cement Association), blended cement production in Europe during 1994 increased by 21 percentage points to 51% of total cementitious production (from 30% in 1992). Portland cement production during the same period declined to 37% (from 52% in 1992). In the period from 1990 to 1995, the average clinker factor in a ton of cement (worldwide average) was reduced by more than 6%.

Lafarge patented the silica fume cement in 1982. In India, silica fume (a byproduct of the reduction of high-purity quartz with coal in electric furnaces, & still being released into our very own atmosphere) was imported & first used in Mumbai flyovers that are being built today.

With the entry of Lafarge & the imminent entry of other multinationals our Indian Cement Industry has to start looking beyond Lea & Duda which have been our bibles for several decades. R & D spend has to be increased. A paradigm shift is required so as to produce cheaper, durable cement, conserve our natural resources & protect our environment.

ACC has made a start with ACC Suraksha (17-18% fly ash) but that is not enough. Others like Madras Cement (15% pozollan) & Shree (11% fly ash) too are making an effort.

We have to start looking beyond OPC.

Sanjay Agarwal
b.tech (chem) (IIT-Kanpur).
(Cement/fly ash technologist)

written for the journal Cement India in 1998