The production of Portland cement contributes significantly to the CO2 emission in the atmosphere. World’s Portland cement production is responsible for 7% of total CO2 emission. For global sustainable development, reduction in Portland cement clinker production without decreasing the needed amount of cement for the construction industry could be achieved by incorporating larger than customary proportion of supplementary cementitious materials into blended Portland cement or by incorporating them as pozzolanic additions in concrete.
A technology, known as High Volume Fly ash (HVFA) concrete, was developed in 1985, incorporating large volume of fly ash. Such concrete has all the attributes of high performance concrete i.e. excellent mechanical properties, low permeability, superior durability, etc. Further, it would be environmentally friendly too.
It is perceived that this HVFA technology is well suited for country like ours, having sub-tropical environment conditions: but the ground realities could be different. A majority of concrete produced in the country is site-mixed, and most of it is volume-batched, using low efficiency mixers leading to inhomogeneous distribution of various ingredients in the concrete mass. HVFA concrete is designed with low water-binder ratio and to maintain the same is toughest challenge in site-mixed concrete production. Incorporation of a superplasticizer is mandatory for HVFA concrete, improper and variable dosages of the same can have undesirable effects on structures. It is for this category (site-mixed concrete) that the use of HVFA concrete may pose a number of problems. HVFA concrete demands very strict attention to quality control and quality assurance with high level of workmanship.
The quality of fly ash, especially its fineness varies considerably and the residue on 45 micron ranges from 12 % to 50. In this scenario use of the site blending option of HVFA concrete could have its limitations. Use of classified/processed fly ash at site in concrete directly (site blending) could also have the drawback of inhomogeneous distribution of the lighter fly ash component with the concrete ingredients and adequate cost-competitive availability of the desired quality of fly ash would be a serious limitation.
On the other hand, considering the site conditions normally available in our country , the option of manufacturing PPC with moderate to high proportions of fly ash, through the intergrinding/co-grinding of fly ash, clinker and gypsum, would provide a more consistent and reliable HVFA concrete with a good control on variability, thus leading to better performance characteristics.
This paper presents comparative data on the performance of pavement quality concrete made with moderate to high volume fly ash (HVFA) blended cement vis- à-vis moderate to high volume fly ash concrete in which the available fly ash and Ordinary Portland cement had been mixed in the concrete mixer. It was observed that regardless of the fly ash incorporation levels, the concrete made with the blended cement developed higher strengths at all ages than that of fly ash concrete (site-mixed).The strength differential between them were distinct from very early ages (01 day) itself.
The authors conclude that considering the site conditions prevailing in the country and for simplifying the production of concrete at site and to eliminate variability due to site conditions, certain operations could be moved to cement plant where there would be better control on the composition and properties of the incorporated cementitious materials, especially where larger than customary proportion are being used. This would ensure the desired performance of concrete.
Some new methods and standards could be developed to characterize such high volume fly ash blended cements, tailor-made for pavement concrete and other civil engineering applications. This would help in enhancing the utilization of this new high performance cement and also have a control on the performance of the resultant high volume fly ash concrete.
I am in full agreement with your view.this inhomogenised concrete mix done @ site will be harmful in the long lasting. Thanks for enriching our knowledge and for sharing this article.