Changes Induced by PVA in the Cement Microstructure Surrounding Aggregate
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Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
number average, 13,000-23,000 mass average. After mixing, the PVA constituted 1.4 parts per one hundred parts cement or slightly over 1 wt% of the combined mass of cement plus water. Two types of specimens were made, and for both, the cement was allowed to harden for 28 d at saturated humidity. In the first type of specimen, the aggregate was cut and ground to generate flat surfaces on which the cement/water and cement/PVA-water solution mixtures were cast. After washing the ground surfaces and casting the cement against them, these specimens were sealed in plastic containers and allowed to harden for 28 d. After the 28 d, some of these specimens were sectioned with a diamond saw and examined with a polarizing optical microscope (Olympus BH2). Other specimens were cleaved with a razor blade inserted along the interface, and a layer of the cement paste within 10 gm of the aggregate interface was collected from each for analysis by a Fourier-transform infrared (FTIR) spectrometer (Mattson Galaxy Series FTIR 3020) that was used in a diffusive reflectance mode. In the second type of specimen, the washed and dried aggregate was embedded in the cement slurries. After 28 d, the material was sectioned, and pieces were immersed in a low viscosity epoxy resin (Epon 815/Epon V-40 (Shell Chemical Company) mixed in the ratio of 100/43) to allow the specimens to become impregnated with the resin. The resin was hardened at 25"C for 4 days. The impregnated aggregate-cement surfaces were then polished with 12 pgm alumina paper and 1 gm alumina powder. The non-polar solvent kerosene was used as a coolant for the final polishing step and to wash the specimens in an ultrasonic cleaner between polishing steps. After coating the dried specimens surfaces with carbon, the specimens were examined with a scanning electron microscope (SEM) (Hitachi S-570) equipped with detectors for secondary and backscattered electrons and energy-dispersive X-ray spectroscopy (EDX). RESULTS Cement-Aggregate Bond To test the bond strength between the cement and aggregate, large limestone rocks was sectioned and ground, and the cement pastes were applied to these surfaces. Cements both with and without
(b)
(a) Figure 1.
A graphic showing the two types of bond failures between cement paste and a flat aggregate (limestone) surface. (a) Cement paste without PVA: adhesive failure over the whole surface. (b) Cement paste with PVA: adhesive failure over less than 50% of the surface; cohesive failure in the cement phase over the rest. 348
PVA were examined. The flat aggregate surface removed essentially all of the mechanical locking contribution to bond strength. After allowing the paste to harden in a moisture-saturated atmosphere for 28 d, the specimens were tested. Cement-aggregate interfaces were cleaved by inserting a razor blade into the aggregate-cement bond at the interface. The bond for specimens without PVA separated very easily without any debris from the opposite surface being le
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