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Tridymite has been produced trom a variety of fluxes. Most samples gave two first-order inversions at ,-~ 117 and ~ 163 ~ Calorimetric study shows that the heats of inversion exhibit considerable variation, and are thus not constants of tridymite. These differences are related to the polytypism and disorder. The 117 ~ inversion occurs with considerable hysteresis: a method for determining equilibrium transition temperatures from dynamic data is presented. Some of the tridymite samples also exhibited a second-order inversion in the range 230--250 ~ Preparation of tridymiteQuartz and silicic acid were used as starting materials. The quartz was supplied by Thermal Syndicate Ltd., and is crushed natural crystal selected for the manufacture of 'Specpure' silica glass. The silicic acid was supplied by Johns-Manville Co.: our analysis gave 84.70% SiO2 the balance being almost entirely water, with only 0.02% material non-volatile with HF. The tridymite phase of silica cannot be formed directly from either of these starting materials. Fenner [1 ] found that sodium tungstate was an efficient flux for the interconversion of the low density silica modifications; tridymite formed in the range 870 ~ 1470 ~ Sodium tungstate was used in the present study: this and other fluxes were obtained as reagent-grade chemicals. The general procedure used in producing tridymite was to make a mechanical mixture of silica and an appropriate quantity of flux and to heat the mixture, contained in a platinum crucible, for the desired length of time in an electrically heated muffle. At the conclusion of a run the outside of the crucible was water-quenched to facilitate removing the sample. Conversion of quartz or cristobalite (the latter is formed rapidly from silicic acid) to tridymite was judged by petrographic examination and X-ray powder diffraction. The tridymitization is slow; runs of several hundred hours' duration may be required, even at 1300-1400 ~ to ensure complete conversion. The tungstate-fluxed samples were extracted by refluxing with 5 % ammonium chloride solution, followed by two washings with 2 % NaOH solution and finally with a double wash of 1% HCI solution and distilled water. This sequence of washings prevents precipitation of tungstosilicic acids and yields a very pure preparation of the appropriate silica Y. Thermal Anal. 2, 1970

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The investigation of the effect of curing conditions on self-healing of pre-cracked concrete containing palm oil fuel ash (POFA)-concrete is presented. Concrete grade 30 was used and POFA cement replaced 10%, 20% and 30% of the total weight of ordinary Portland cement [1].The compressive strength was tested after 7, 28, 60 and 90 days cured in water in order to investigate the strength development of the POFA-concretes. In addition, the pre-crack (hairline crack) concrete was examined under compression load to determine the effectiveness of self-healing POFA-concretes after different curing conditions. Four types of curing condition, namely air, room temperature, wet and dry, and water curing were tested. An ultra-pulse velocity (UPV) test was performed after 7, 28, 60 and 90 days of curing to monitor the self-healing progress in the POFA-concrete. The results show that POFA-concrete witha 20% replacement level recorded the highest compressive strength. It was also revealed that the UPV readings increased significantly with increased curing age. The results showed that the selfhealing ability of pre-cracked POFA-concrete at 10% replacement level increases significantly for the water curing condition. From these findings, the optimum POFA replacement level was recorded at 20% for the strength, while inclusion of a 10% POFA replacement level was appropriate for self-healing concrete.