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.