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Investigation of the reaction mechanism of blended fly ash and rice husk ash alkali-activated binders

Fernando Sarah, Gunasekara Chamila, Law David W., Nasvi M. C. M., Setunge Sujeeva, Dissanayake Ranjith, Ismail M. G. M. U.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 1

art. no. e24, 2022

This study investigates the influence of the chemical and physical properties of two abundantly available waste by-products in Sri Lanka, fly ash and rice husk ash (RHA) as precursor materials for the synthesis of alkali-activated binders. The suitability of the two types of fly ash and the replacement of fly ash by RHA (10% and 20% by weight of the binder content) were assessed. The study reports the development of compressive strength together with an in-depth analysis of the reaction mechanism of the blended RHA alkali-activated binders. The 100% fly ash mortar achieved the optimum compressive strength of 38.9 MPa at 28 days. Replacement of the fly ash with 10% and 20% RHA reduced the compressive strength by approximately 14% and 43%, respectively. The higher specific surface area of RHA and relatively higher unburnt carbon content in RHA were identified as the major factors influencing the low compressive strength obtained. Furthermore, the addition of RHA increases the reactive silica in the gel matrix and leads to an increase in the Si/Al ratio (3.70–3.89), which has a negative effect on the compressive strength. The difference in solubility rate of precursor fly ash and RHA negatively affect the formation of the gel matrix which is hypothesized as a further reason for the lower compressive strength observed in the RHA mixes.

Creep, shrinkage and permeation characteristics of geopolymer aggregate concrete: long-term performance

Seneviratne Charitha, Gunasekara Chamila, Law David W., Setunge Sujeeva, Robert Dilan

Archives of Civil and Mechanical Engineering

2020

Vol. 20, no. 4

633--647

The long-term impact on creep, drying shrinkage, and permeation characteristics of an innovative concrete produced with manufactured geopolymer coarse aggregate (GPA) has been investigated and compared with quarried Basalt aggregate concrete. Microstructure and pore-structure development up to 1 year were examined through scanning electron microscopy, nanoindentation, and X-ray computed tomography. Compressive strength and elastic modulus of GPA concrete varied from 34.6 to 50.8 and 18.5 to 20.5 GPa, respectively, between 28 and 365 days. The 1-year creep strain of GPA concrete was 747 microstrain while the calculated creep coefficient was 0.97, which is significantly lower than the creep coefficient predicted by AS 3600 and CEB-FIP models. Moreover, the 365-day drying shrinkage is 570 microstrain, which is also lower than the maximum permissible limit specified by AS3600. The GPA concrete displayed high water absorption, but lower air and water permeability compared to Basalt aggregate concrete. This is attributed to a porous surface layer with large number of capillaries increasing the water absorption of GPA concrete through capillary suction. The discontinuity in the pore network coupled with a condensed interfacial transition zone formed in GPA concrete could be the reason for lower permeability. Overall, the long-term performance of the GPA demonstrates a potential as a lightweight coarse aggregate for concrete, with the added advantage of reducing the environmental impact utilizing fly ash from coal-fired power generation.