Often, fly ash is looked upon as a measure to bring down the cost of concrete, But fly ash can also enhance properties and reduce cement consumption.
A proper insight into the possible mechanisms of concrete deterioration has led to altering of the properties of OPC concrete through the beneficial use of the blending components like fly ash, GGBS, Metakaolin, etc., so as to achieve an improved performance of resultant concrete.
Fly ash-based cement concrete, made either with the blending component as a part in cement or alternatively as site blended at the ready-mix locations, is becoming an accepted option for a durable civil structure.
There has always been a comparison of the concrete made with factory-produced inter-ground PPC and site-blended fly ash-based concrete, with respect to their performance characteristics. Opinions have always been divided on the issue. Considering that the fly ash as produced is a finer product, inter-blending of the fly ash with ground OPC has been considered to be better in performance as compared to fly ash cement produced by inter-grinding of fly ash, clinker and gypsum. However, this is dependent on the characteristics of the fly ash, such as its particle size distribution, amorphous contents and its extent of variability in terms of its chemico-mineralogical compositions.
It is of proven understanding that the characteristics of fly ash produced in the coal-fired thermal plant are a function of nature of the coal, coal-comminution system, boiler type & efficiency, fly ash collection ESP fields, loading at which the thermal plant operates, etc. As a result, even from the same source, the fly ash characteristics could vary substantially.
In India, the chemico-mineralogical characteristics of dry fly ash produced, have been observed to vary. The mineralogy of fly ash has 15 - 30 per cent Mullite, 15 - 45 per cent Quartz, 1 - 5 per cent Magnetite, 1 - 5 per cent Hematite and around 25 - 35 per cent of amorphous glassy alumino-silicate phase; the fineness of the fly ash from different thermal plants in the country ranges from 12 per cent to 50 per cent residue on 45 microns. In this scenario, use of the site-blended option has its limitations. On the other hand, the inter-grinding route puts forward an opportunity to optimise the fly ash particle characteristics in the Blended Cement product (PPC), so as to maximise the pozzolanic potential of the available fly ash.
Selective use of classified fly ash for producing PPC is limited by the quantum availability of the desired quality of fly ash for the manufacture of PPC through the inter-blending route due to which the inter-ground product is the preferred alternative. Use of classified fly ash at site in concrete directly (site-blending) also suffers from limitations of non-homogenous distribution of the lighter fly ash component with the concrete components; proper control on the proportions of usage of fly ash at such site blending locations is also a cause of concern.
On the other hand, in the inter-grinding (or co-grinding) of fly ash, clinker gypsum provides a means of having a more consistent product with a good control on variability, thus with better performance.
At the R & D Division of ACC Ltd, studies carried out have helped evolve an understanding of the influence of the properties and reaction mechanics of the fly ash component in determining the limiting/favouring conditions for improved concrete properties related to its durability. It could be stated with a high degree of confidence that in the inter-grinding mode, use of proper methods for reducing the variability in chemico- mineralogical composition of fly ash, optimisation of the comminution system (to achieve the desired distribution of fly ash in the size fractions of the composite cement), enhances the pozzolanic activity of fly ash and helps improve the performance of the resultant pozzolanic cement.
The inter-grinding process also helps to mechanically activate the fly ash by creating new reactive surfaces during comminution; it helps to improve the sphericity of the coarse angular fly ash particles (assists rounding of the angular particles).
In the fly ash-based blended cement production by inter-grinding mode, some interactions are reported to occur between the particles of the different ingredients during grinding (mechano- chemical activation). As a result of these interactions, particle size distribution of inter-ground blended cements is different from that of interblended cements of the same composition and fineness.
Turker et al and Popovic et al studied the influence of separate and inter-grinding of cement and pozzolanic materials. Cements having the same composition but produced by inter-grinding and separately grinding were compared from compressive strengths and microstructure of hydration points of view of resultant PPC. It was concluded that for pozzolan-based cements, grinding method affects the microstructure of hydration significantly. Paya et al (4) and Songrpiriyaki report that grinding of fly ash reduces the fly ash particles to a size range at which it reduces the water requirements in concrete; the finer size fractions improve the early age compressive strengths of mortar as well as concrete. The comparison could be extended to performance of concrete made with factory-manufactured PPC and the site blended concrete product.
The differences in performance of concrete made with factory-produced PPC and site-blended fly ash concrete could be due to the following reasons:
The first aspect can be taken care to a large extent in well-controlled RMC operations but the second aspect would still remain unattended in absence of availability of a processed fly ash of desired particle characteristics. Thus even in RMC operations, the quality of fly ash used could bring about a difference in the site-mixed concrete and concrete made with a factory-produced PPC. If in RMC operations, processed fly ash is used of desired particle characteristics and of controlled variability in the chemico-mineralogical characteristics, the differences in the site-mixed and factory-produced PPC based concrete would be minimal.
In order to understand this difference in the two concretes, a comparative study was carried out at an ACC laboratory which is discussed below in some detail, illustrated by the mortar and concrete properties. A study of the differences produced in the reaction chemistry and resultant cement paste microstructure, which determine the performance and durability of the resultant concrete, has also been attempted.
Concrete taken for studies
Concrete - 1 Factory-manufactured PPC with engineered particle size Distribution (PPC) Concrete - 2 Site blended with OPC & fly ash (of the same source with different particle size distribution) (OPCFA).
The chemical composition of clinker and fly ash used in the manufacture OPC and PPC of the same cement plant is summarised in Table -1. The particle-size distribution of OPC, PPC and fly ash is given in Table -2.
The hydration characteristics of the factory-produced PPC (26 per cent fly ash) and the cement mixture OPCFA (OPC + 26 per cent fly ash) represent the expected hydration kinetics of the Cement Paste Matrix (CPM) in the resultant concrete. The evaluation carried was as follows:
Neat cement pastes of the PPCs were cast (w/c=0.38) and de-moulded after one day and cured for different ages of hydration i.e. 1, 3, 7 & 28 days.
At each age of hydration, the neat cement pastes were evaluated for:
The un-reacted fly ash in hydrated cements was evaluated using picric acid methanol method as reported by S. Li et al and was also used by the RCD to study the pozzolanic efficacy of fly ash in the hydrated cement at different age of hydration. The brief method is given below:
A known weight, the hydrated neat cement was treated with aqueous-methanolic picric acid solution with magnetic stirring for a fixed time of 30 minutes. The residue was filtered and washed free of picric acid with methanol and subsequently washed with 300 ml of water at 500 C. The residue was dried and ignited at 1,000o C. The residue was weighed after cooling. The residue in the blending components was determined by the same method separately. Based on the residues of the blending component and the blended cement at each age of hydration, the per cent un-hydrated and per cent hydration of the blending component was computed. The pozzolanic reactions of the fly ash gradually results in consumption of the calcium hydroxide, produced from the hydration of the OPC part of the blending cement. As a result, there is a decrease in the calcium hydroxide contents of the hydrated cements, which in turn is reflected in decrease in the un-reacted fly ash content in the hydrated cement paste.
Thus, the free calcium hydroxide contents of the hydrated cement pastes would be related to the pozzolanic reactivity of the blending component at that age of hydration. This would also correspond to the per cent reaction of the fly ash com-¡ponent in the fly ash-based cement.
Results & discussions
The free calcium hydroxide contents of the hydrated PPC & OPC + fly ash mix, at different ages of hydration (graphically shown in Fig- 1). The un-reacted fly ash in the hydrated cements at different ages of hydration was analysed, and the computed per cent reaction in different hydrated cement pastes was calculated and is graphically depicted in Fig-2.
The per cent free hydrated lime present in the hydrated cement paste (Fig-1) and the per cent reaction of the fly ash (Fig-2) indicates that PPC comparatively shows higher reactivity of the fly ash than OPCFA (site-mix).
The alkalinity (pH) of the hydrated neat cement pastes at different ages of hydration was measured by pH electrode at w/c ratio of 0.38 and is graphically shown in Fig.3. The observation indicates the pH to be highly alkaline (12.7-12.8).
Morphology and Microstructure
The hydrated paste micro-structure of the different cements at 3 & 28 days of hydration is illustrated in the photomicrographs shown in Plate I & Plate II and the salient observations on the microstructure are summarised below:
Hydrated PPC & OPCFA at three days:
Hydrated PPC & OPCFA at 28 days:
Comparative mortar properties of PPC & OPCFA are shown in Fig 4.
The data indicates that PPC has higher strengths at all ages as compared to OPCFA, indicating the merits of improved properties of the PPC (inter-ground fly ash based cement), due to better packing and increased reactivity of the fly ash component. This is substantiated by the observed compacted microstructure resulting from the packing effect of the finer fractions of the fly ash and improved pozzolanic reactivity of the fly ash.
The studies discussed above illustrate that a factory-produced PPC with engineered particle size distribution performs comparatively better than a site-mix concrete at similar levels of fly ash. This is primarily because the particle characteristics of the fly ash in PPC are much more finer than the fly ash used in site mix concrete. The particle characteristics and mineralogy of the OPC & fly ash, alter the chemistry of hydration as well as the pore-filling/packing tendency of the fly ash in the hydrated cement paste matrix, which determines the performance of the resultant concrete. Another aspect of importance in a factory-manufactured PPC is that with the upper limit of BIS going up to 35 per cent, the gypsum component is appropriately adjusted so as to derive the maximum potential from the aluminous component of fly ash (through optimisation of SO3). In a site mix mode, no such gypsum additions are resorted to, and neither are they possible. It is well-studied and reported in literature that at higher fly ash levels, gypsum requirements are higher, in which case, in the site mix mode, one would lose this contribution of hydraulic/pozzolanic potential from fly ash based blended cementitious system. This would not be the case in a factory-manufactured PPC based concrete.
In view of the variations in the characteristics of the fly ash available from the coal-fired thermal plants in the country, the inter-grinding based factory produced PPC option would be more preferable, as it would produce a more consistent product. The site-mix mode of use of fly ash would be suited in RMC locations, if these locations use a processed fly ash of consistent quality. If such a consistent quality of fly ash of desired particle characteristics is made available along with a consistent OPC compatible with the fly ash, the concrete produced could be similar to a factory-manufactured PPC concrete, except for the gypsum deficiencies.
The cements were subjected to concrete evaluation. The M-25 grade concrete was made with PPC & with OPC + fly ash (site mix) and Fig 5 shows the difference in slump of concrete and Fig 6 shows the compressive strength. The results indicate that the concrete with PPC (26 per cent fly ash) shows improved workability compared to the site-mix concrete with same fly ash (26 per cent fly ash). The PPC concrete also shows higher strengths.
(This article has been authored by SA Khadilkar, Director Q & PD, ACC Ltd).