Properties of high volume fly ash concrete.

This thesis presents a detailed investigation on the engineering properties and microstructural
characteristics of concrete containing a high volume of fly ash (HVF A). The purpose of the
project is to evaluate the concept of using relatively large volumes of fly ash in normal
portland cement concrete, and hence enhance the beneficial use of fly ash in value-added
products and construction.
A total of eight concrete mixtures with and without fly ash was investigated. The
proportion of fly ash in all the HVF A concrete mixtures varied from 50 to 80 % by weight of
the cementitious materials, with a constant water-to-cementitious ratio of 0.40 for all the
mixtures. A high degree of workability was maintained by the use of a superplasticizer. To
optimize the pozzolanic activity in the HVF A concrete, silica fume was used in some of the
mixes. The total cementitious materials content was kept constant at 350 kg/m3 and 450
kg/m3 respectively. The influence of the different replacement materials and two curing
regimes was studied.
The study consisted of two parts. The first part is an extensive study of the
engineering properties such as strength development, modulus of elasticity, ultrasonic pulse
velocity, swelling, and drying shrinkage at various ages up to 18 months. The depth of
carbonation of HVF A concrete under different curing regimes was also investigated. A study
of the microstructure of HVF A concretes forms the second part of the investigation. Pore
structure, air permeability and water absorption of HVF A concretes with different
replacement mixtures were studied. A detailed discussion dealing with the change of the
morphological phase under different curing regimes is also presented.
The results show that HVF A concretes exhibit excellent mechanical properties with
good long-term strength development. Compressive strength in the range of 40 to 60 MPa
"as achieved for all the HVF A concretes at the age of 90 days. The dynamic modulus of
elasticity reached values of the order of 55 GPa at 90 days. Under similar conditions,
concretes made with both fly ash and silica fume had engineering properties which were as
good as those made with cement replaced by fly ash alone. The use of fly ash to replace both
cement and sand has the advantage of mobilizing and combining the benefits and effects of
both separate replacements. The HVF A concretes also have low permeability and exhibit
good potential characteristics to resist water penetration.
Reduction in the volume of large pores was observed with the progress of the
pozzolanic reaction. Higher HVF A concrete strength was generally associated with a lower
volume of large pores in the concrete. A decrease in the levels of calcium hydroxide was seen
with progressive water curing and age in all the HVF A concretes, providing evidence of
continued pozzolanic reactivity of the fly ashes.
Various empirical relationships and design equations are presented and conclusions
are drawn at the end of each part. It is recommended that further research is required to
determine the influence on HVF A concretes of extreme curing conditions such as high or low
temperature and low moisture availability, and to improve the early strength properties of the
HVF A concretes.