Bond between aggregate and fly ash cements

The overall purpose of the research reported herein was to determine factors affecting bond strength in fly ash concretes, using the portland cement bond as a point of references. The work is important because use of low cost fly ash concretes is increasing throughout the construction industry;Microstructural features of the cement paste-aggregate interface were also studied; this included obtaining information about the chemical and elemental composition of the substances formed at the interface, and the effect of the cement paste-aggregate interaction on the pore size distribution of concrete;Mechanical testing provided answers with respect to the magnitude of bond strengths and factors affecting them, including the effect of trace additives. Substances formed at the interface were investigated by the use of a x-ray diffractometer; an electron microprobe (EMP) was used to obtain elemental composition, and pore volumes were measured by a mercury porosimeter;The tensile bond strength between cement paste and limestone was studied in more than 450 specimens with experimental variables being: fly ash type, water-cement ratio, curing time and trace additives;The tensile strength of paste controlled bond strength between fly ash and aggregate; tensile strength of the aggregate controlled portland cement-aggregate bond. Dibasic ammonium phosphate increased both the paste strength and the bond strength of cementitious fly ash;X-ray diffraction at the paste-aggregate interface indicated that the interface region is distinctly different in chemical compostion and structural arrangement when compared to the constituent paste or the aggregate bulk phase;Electron microprobe tests showed that the elemental composition of the paste-aggregate interface region is different from the paste or the aggregate bulk phase. Interphase elemental diffusion and redistribution of elements within the paste phase are evident;Mercury porosimetry showed that cement paste-limestone interaction caused a reduction in pore volumes over a wide range of pore sizes;Microstructural features of the interface region correlate with the measured bond strength and the mode of tensile bond failure.</p