High Volume Fly Ash Cement for Civil Engineering Applications – A Performance Evaluation

The High Volume Fly Ash (HVFA) concrete technology, developed in early 1980s, in which ~50 % of the Portland cement is replaced by fly ash, such concrete has been reported demonstrate all the attributes of a high performance concrete i.e. excellent mechanical properties, low permeability, superior durability, etc. This technology could be considered as an encouraging step towards attaining global sustainable development.

The HVFA technology is being thought to be well suited for country like India, having a sub-tropical environment conditions, but the ground realities are different. Majority of concrete produced in the country is site-mixed, and most of it is volume-batched, using low efficiency mixers which lead to inhomogeneous distribution of various ingredients of a HVFA concrete. HVFA concrete is designed with low water – binder ratio and to maintain the same is a toughest challenge in site – mixed concrete production, incorporation of super plasticizer 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 Quality control and Quality Assurance with high level of workmanship.

On the other hand considering the site conditions normally available in our country, the option of manufacturing High volume Fly ash cement with high proportions of fly ash, through the intergrinding/co-grinding of fly ash, clinker & gypsum, would provide a more consistent and reliable HVFA concrete with a good control on variability thus with better performance characteristics.

This paper presents comparative data on the performance of concrete made with high volume fly ash blended cement vis-à-vis high volume fly ash concrete incorporating fly ash as pozzolanic addition. The parameters investigated include mechanical properties, rapid chloride penetrability, initial surface absorption, ultra sonic pulse velocity and resistivity. It was observed that regardless of the fly ash incorporation levels, the concrete made with High volume fly ash cement outperformed high volume fly ash concrete in which the OPC and as received fly ash had been mixed in the concrete mixer, in each of the parameters studied. The paper also attempts to correlate the superior performance characteristics of High volume fly ash based Cement Concrete to the observed higher pozzolanic reactivity of the fly ash component in such cement paste matrix. This is being attempted through comparative assessment of the hydration characteristics of the high volume fly ash cement paste and the cement pastes of the OPC + Fly ash mixture blended at site.

Thus considering the site conditions prevailing in the country for simplifying the production of concrete at site and to eliminate variability due to site conditions, some of its operations could be moved to cement plant where there would be better control on the composition and properties of incorporated cementitious materials, especially where larger than customary proportion are being used. This would help ensure the performance of concrete as per the intended requirement . This paper was presented and published at 9th NCB Int Seminar