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Abstract:
For possible application of gamma radiation in processing of fresh concrete, like surface processing at precast plants or 3D printing, it is essential to evaluate effects of irradiation on the early hardening of cementitious binders. The influence of gamma irradiation with the dose rate of 4.6 kGy/h on the early strength, pore size distribution and qualitative phase composition of mortar was investigated. The range of investigation comprises the effects selected micro- and nano-additions to Portland cement mortar used to control the kinetic of setting and early hardening of cement. Results show that gamma irradiation of hardening mortar results in the early strength increase of about 56–100%, in reduction of capillary porosity and pore size refinement. The gamma-irradiation sensitivity of early strength of cement mortar is equivalent to increased temperature wet curing. A correlation of heat evolution characteristics of hardening mortar and the temperature-equivalent of gamma irradiation is discussed.

Keywords:
boron minerals, early hardening, gamma irradiation, pore size distribution, strength, temperature equivalent

Abstract:
The effect of gamma-ray irradiation on cement mortar properties is investigated in this study in order to understand the mechanism behind the strength and stiffness reduction, which may be significant according to the available researches. 60Co irradiation facility with the generating dose rate of 0.1–10 Gy/s and the total activity of 4.4·10^15 Bq (120 kCi) was used to perform the irradiation, so that the total observed dose of the irradiated samples reached the values ranging from 12.0 to 15.0 MGy. An identical set of control samples was placed in the same laboratory conditions away from gamma radiation. The results of nanoindentation, X-ray diffraction analysis and mercury intrusion porosimetry of the irradiated and the control samples are shown and explained in detail in this study. The nanoindentation creep compliance and the nanoindentation elastic modulus of the irradiated and the control samples do not show any significant difference. The mineral composition obtained using the X-ray diffraction analysis of the irradiated and the control samples is also similar. The pore structure rearrangement and microcrack occurrence, which were evidenced by the mercury intrusion porosimetry and scanning electron microscopy, led to the porosity increase and may be attributed to the significant decrease of compressive strength.

Keywords:
cement mortar, drying, experiment, gamma-ray, nanoindentation, porosity, X-ray diffraction

Abstract:
The effects of gamma irradiation on concrete properties during early hardening were studied towards radioactive waste storage or accelerated processing at precast plants. Concrete mixtures containing different mineral aggregates (baryte, magnetite, amphibolite) were investigated. During initial 16 hours of hardening the mixes were irradiated using 60Co gamma source at the rate of 3.5 kGy/h. The mechanical properties and microstructural features of irradiated early-age concrete were tested: the secant elastic modulus, the compressive strength, the porosity and pore size distribution. XRD and SEM analysis were also performed. The results indicate both the stiffening and pore refinement in concrete due to early gamma irradiation. Effects of early irradiation on microstructural features of cement matrix were found in the subsurface layer up to the depth of 2 mm. The influence of different mineral aggregates in concrete on the radiation-induced changes of early age properties is discussed.