Using South African fly ash as a component of alkali- activated binder

Abstract

This thesis focuses on alkali-activation of South African fly ash with the aim to utilize high volumes of fly ash in construction material, such as concrete. This new type of binder does not contain Portland cement and could reduce carbon dioxide emission related to Portland cement production. At the same time fly ash utilization contributes to growth and economy of the country, conservation of natural resources and improvement of environmental aspects. Although numerous studies have been devoted to alkali-activated materials and geopolymers, only a few studies were performed using South African fly ash. Rational technological parameters such as elevated temperature curing and mix composition of alkali-activated fly ash cements contained South African raw materials were determined in this thesis. Curing at 60 ºC for 16 hours gives the best trade between energy consumption and strength of the alkali-activated fly ash cements. Alkali content should not exceed 12 % Na2O (preferably 9 %) which causes high standard deviation of the compressive strength and coefficient of variation between different batches of the concrete, as well as facilitates efflorescence formation. Produced alkali-activated fly ash cements were characterized by using different techniques (XRD, ATR-FTIR, SEM). Alkali-activated fly ash cements are assumed to be low heat binders but the current study shows that significant amount of energy can be released during elevated temperature curing of these cements. Increased amount of alkali can cause temperatures up to 150 ºC which will negatively affect structure development of alkali-activated fly ash cement resulting in observed strength drop when alkali concentration exceeds 9 % Na2O of fly ash mass. This finding is a valuable addition to the existing knowledge on alkali-activated materials which was not mentioned anywhere previously. For the first time initial shrinkage of alkali-activated fly ash cured at elevated temperature during the first 24 hours was investigated in this thesis. Initial shrinkage of alkali-activated fly ash depends on alkali content and increases with an increase in amount of alkali. The initial shrinkage of alkali-activated fly ash concrete is significantly less, up to twofold, than the shrinkage of fly ash-OPC blended concrete even when the latter was cured at room temperature and 99 % relative humidity in comparison to curing of alkali-activated fly ash concrete at 60 ºC in dry oven. Heat cured alkali-activated fly ash concrete has engineering properties similar to OPC concrete‟s properties. The alkali-activated fly ash concrete with 9 % Na2O, containing 500 kg of fly ash per cubic meter with dolomite aggregates provides adequate workability, and has the following characteristics:  Slump of fresh concrete 50 mm  Compressive strength at 28 days 45.5 MPa  Elastic modulus at 28 days 30 GPa  Poisson‟s ratio at 28 day 0.11  Modulus of rupture at 28 days 5.7 MPa  Split tensile strength at 28 days 3.2 MPa  Initial shrinkage (during first 24 hours) 1400 microstrain  Drying shrinkage after 1 year 300 microstrain  Creep after 1 year 600 microstrain The durability indices indicate that the alkali-activated fly ash concrete is comparable to the conventional concrete, and service life of structures made from the alkali-activated fly ash concrete should meet modern requirements. The alkali-activated fly ash concrete developed can thus be recommended for precast concrete units.