products. The blends used are listed in table 3. The grain size distribution of the blends is shown in figure 3. Mixtures of the individual blends with varying amounts of sulfur were made and 3- by 6-inch compressive samples cast of the sulfur-blend mixtures that produced workable mixes. The compressive strengths were determined in accordance with ASTM Method C-39-49 "Compressive Strength of Molded Concrete Cylinders" (1). The unmodified sulfur concentration giving the highest compressive strength mixture for each blend was established. After the optimum sulfuraggregate value was found, the amount of modified sulfur needed to give maximum compressive strength was determined.. The sulfur was modified by reaction with dicyclopentadiene or a mixture of dicyclopentadiene and dipentene before adding 3/4in to the aggregate blends and mixing. Mixtures of aggre gates and sulfur modified with dipentene alone were 3. 1:1 Desert sand and 3/8-inch volcanic rock? 4. 2:1 Desert sand and 3/8-inch volcanic rock..... 5. 1:1 Construction sand and 3/8-inch volcanic rock. 6. 2:1 Construction sand and 3/8-inch volcanic rock. 7. 1:1 Desert sand and 3/8-inch limestone rock...... 8. 2:1 Desert sand and 3/8-inch limestone rock...... 9. 1:1 Construction sand and 3/8-inch limestone rock 10. 2:1 Construction sand and 3/8-inch limestone rock VMA--Voids in the mineral aggregate. 1The particle size, in mm, below which is half of the sample weight. 2Weight ratio of sand and 3/8-inch rock. Metallographic Examination of Sulfur-Bonded Aggregates The role of sulfur in bonding aggregates in sulfur concretes and modified sulfur concretes was studied using scanning electron microscopy. Three types of sulfur-bonded aggregates were prepared. These were (1) sand and sulfur, (2) sand and sulfur modified with 5 pct dicyclopentadiene; and (3) sand and sulfur modified with 2 pct dipentene. Samples of each type were prepared using construction sand and desert blow sand. The samples were mounted in epoxy, polished and etched with 30 pct carbon disulfide (CS2) in ethanol (C2HOH). The etching was purposely severe to provide relief for scanning electron microscopy. Photomicrographs of three of the samples at X1,000 made with a Cambridge scanning electron microscope are shown in figure 4. The top photo is of desert sand and unmodified sulfur. The sulfur matrix consists of a stable phase of rather large equiaxed grains with an integranular Su phase which appear as stringers and which were virtually unattacked by the CS2 etch. The right photo is of construction sand bonded with sulfur modified by reaction with 5 pct dicyclopentadiene. It shows a noncrystalline continuous The continuous phase phase with small amounts of Su present as stringers. is presumed to be a sulfur-organic compound. The left photo is of desert sand bonded with sulfur which was modified by reaction with 2 pct dipentene. A mixture of noncrystalline and crystalline sulfur is present. The crystalline phase exhibits a lathlike matrix. Stringers of Su were also present. These studies indicated that dicyclopentadiene could be used to retard or eliminate the crystallization of sulfur used in sulfur concretes. EXPERIMENTAL RESULTS Compressive Strength The compressive strength of the 10 aggregate material blends listed in table 3 mixed with varying amounts of sulfur were determined on 3- by 6-inch test cylinders. After the amount of sulfur required to give maximum compressive strengths was determined, comparative values were determined for each blend using modified sulfur as the binder material. From the results given in table 4, it is apparent that less modified sulfur was required to obtain optimum mixtures. The 1-day strength of the modified sulfur concretes was generally not as high as those for sulfur concrete in blends 1 to 6 which are principally acidic rock aggregates. However, the strengths of both types of mixtures were similar for the blends 7 to 10 which were made with basic rock. All compressive strength values of the modified sulfur concretes increased more with aging than those of the sulfur concretes. This will be shown in the section that follows. Blend wt-pct psi Sulfur, Compressive Sulfur, Compressive Sulfur, Compressive strength, wt-pct strength, Compressive strengths of optimum sulfur concrete mixtures Unmodified sulfur Sulfur-2DCPD Sulfur-5DC PD1 wt-pct strength, psi psi 1..... 38 5,735 4,580 5..... 4,800 30 5,090 3,700 8..... 6,000 26 10.. 30 5,355 23 4,990 Sulfur-2DCPD-1/2DP Sulfur, Compressive Sulfur, Compressive strength, wt-pct strength, Sulfur-2DCPD-1DP wt-pct psi psi 1.... 26 4,540 4,100 7,340 5,940 6,030 21 5,380 4,970 7..... 8... 9.... 10... 1The modifier was either 2 or 5 wt-pct of the sulfur with DCPD and The differences in compressive strength measurements of small and large test cylinders were investigated. Before the standard procedure for preparing test samples was adopted, higher compressive strength values were obtained on 3- by 6-inch test cylinders than on 6- by 12-inch cylinders. The values were similar to those reported by Malhotra who obtained lower compressive strength values on the large cylinders (19). Where the standard procedure was used for preparing test specimens, no large differences in compressive strength values were found between the two sizes of cylinders. series of tests were made in which 300-lb batches of sulfur concrete were prepared in a heated mortar mixer. To confirm this, a cylinders 3 by 6 and 6 by 12 inches with 3-inch extension on top were cast Five each, compressive strength test Five sulfur concrete mixtures (aggregate blends 7, 8, 9, 13, and 14) were The excess 3 inches was cut from the top after solidification. investigated using both sulfur and modified sulfur as the binding agent. Mixtures using aggregate blends 13 and 14 are similar to the sulfur concrete mixtures prepared by Malhotra (19) and consisted of blends 9 and 10 (table 3) in which 6 wt-pet silica flour was substituted for an equivalent amount of from the mix. construction sand. The results obtained are summarized in table 5. The |