Portland CEM1 Cement/Silica Fume - Essay Example

?Introduction Cement is used in all building construction. It is a great advantage to lessen its cost. Silica fume are air-pollutants as a by-productof the production of ferrosilicon alloys. Due to its environmental impact, its proper disposal has been a concern. They are available with different silicon dioxide concentrations depending on the purity of the silicon metal being produced. It would be convenient if silica fumes are integrated to the cement mixtures. Studies have shown that silica fumes strengthen concrete structures. This experiment aimed to determine establish the effect of replacing percentages of cement with silica fume. Objectives The main objective of this experiment is to determine the effect of the utilization of silica fume in the production of Portland cement. This experiment specifically aims to: 1. produce three concrete mixes; Mix 1, Mix 2A and Mix 2B containing 0, 5, and 10% silica fume respectively; 2. and test the fabricated mixes in terms of slump test, maximum load, flexural strength, stress test, and compressive strength under European standards. Review of Related Literature Silica fumes are by-product of the manufacture of silicon metals or ferrosilicon alloys (Dunster 2009). It is called various names such as silica dust and microsilica (Kuennen 1996). This is because it consists of very fine spherical glassy particles. Its silicon dioxide content ranges from 61 to 98 percent depending on the silicon purity of the metal from which production the silica fumes are collected (ACI Committee 2000). Silica fume was first characterized in the 1950s. It was discovered to be harmful to the environment in the 1970s (Dunster 2009). Because of this, it has been a concern to use it instead of releasing it as an air-pollution. Silica fumes are lighter that cement, having density of about 2200 kg/m3 while that of the latter is 3100 kg/m3. Its surface area ranges from 13,000 to 30,000 m2/ kg while that of cement is 300 to 400 m2/ kg (ACI Committee 2000). Because of these properties, silica fume particles pack in between larger cement particle which enhances mechanical performance and chemical resistance of concrete. For example, silica fume decreases chloride penetration of concrete (Kuennen 1996). This property makes it suitable construction material for bridges, decks and other specialized structures since it reduces harmful effects of chloride in salt water, other chemicals and abrasions. In general, hardened silica fume concrete has lower permeability, improved durability, greater resistance to abrasion and impact, higher flexural strength, compressive strength and modulus of elasticity. Silica fume can also be added to concretes containing micro-fibers for explosive applications such as those involving exposure to fire (Dunster 2009). Studies show that the higher the silicon dioxide content of silica fumes, the more reactive it is with concrete (ACI Committee 2000). Materials and Equipment Raw Materials The raw materials used for this experiment were CEM 1 Portland cement, coarse aggregate, silica fume and water. Slump Test (British Standard Institution 2009) Most important equipment for the slump test was the mould. It was made of metal so it would not be readily attacked by cement paste. Its wall was at least 1.5mm thick. It was also smooth and free from projections and dents as required by the standard procedure. Its shape was a hollow frustum of a cone with base diameter of 200 mm, top diameter of 100 mm and height of 300 mm. The base and top were both right angles to the axis such that both were perfectly horizontal and parallel to each other. It had a base that it can be securely clamped to and two handles near the top. A compacting rod was also used in the experiment. It was straight and made of steel with a diameter of 16 mm and length of 600 mm. It was a straight rod with rounded ends. A square-mouthed shovel and a container were used for the remixing of sample before filling of mould. As required, the container was a flat tray made of a non-absorbent rigid material. A funnel was used in the filling of the mould. It was made of non-absorbent material to avoid being readily attacked by cement paste. A rule was used to measure the heights of the slump. Its measurements had intervals not exceeding 5mm as specified by standard procedure. A timer was also used. Testing Hard Concrete Cube moulds were prepared for the determination of compressive strength. Its dimension was 100mm by 100mm by 100mm. For the determination of flexural strength, moulds with the dimension of 100mm by 100mm by 500mm were made to create beams. A steel compacting rod was used for the tamping of concrete into the moulds. The rod has a diameter of 16mm and length of 600mm. A remixing container and shovel similar to that used for the slump test was also used. Methodology After preparing the materials and equipment for this experiment, the mixes with varying silica fume concentration were set. The appropriate amount of raw materials for Mix 1 (0% silica fume) were calculated and measured as recorded in Table 1. A water-over-cement ratio of 0.55 was considered. Table 1. Contents of Mix 1 with 0% silica fume. C30 C25 Cement CEM 1 3.50 kg Cement CEM 1 2.75 kg Sand 4.55 kg Sand 5.65 kg Coarse aggregate 10.60 kg Coarse aggregate 10.50 kg The dry aggregate were mixed together for 30 seconds. Half of the required amount of water was then mixed with the aggregate for 60 seconds. After 8 minutes, the cement was mixed in for another 60 seconds. The second half of the water was mixed in for 1 minute afterwards. Unlike Mix 1, Mix 2 and Mix 3 contained silica fume. The preparation of aggregate was same as that of mix 1. Five percent of the cement for Mix 2 was replaced with silica fume. Ten percent was replaced for Mix 3. The silica fume used was prepared by the technician due to its possible harm if not handled properly. It was provided as silica slurry which has 50% silica and 50% water. The component quantities are recorded in Table 2. Table 2. Contents of Mix 2 and Mix 3. Materials Mix 2A - 5% silica fume mix Mix 2B - 10% silica fume mix C30 C25 C30 C25 Cement (kg) 3.35 2.60 3.15 2.48 Silica slurry (kg) 0.35 0.28 0.70 0.55 Water (kg) 1.85 1.45 1.65 1.30 Fine Aggregates (kg) 4.55 5.65 4.55 5.65 Coarse Aggregates (kg) 10.60 10.50 10.60 10.50 water / (PC + Silica) Fume) 0.57 0.57 0.57 0.57 Just like in Mix 1, the dry aggregate were mixed together for 30 seconds before half of the water and silica was mixed in for 60 seconds. It was left for 8 minutes before the cement was added and mixed for another 60 seconds. Finally, the rest of the water was mixed in for 1 minute. A sample of each mix was then taken to undergo slump test. Slump Test (British Standard Institution 2009) First, the mould and base plate was prepared by dampening its walls and holding it horizontally in place. Then a sample of fresh concrete was taken and remixed n a container using a square mouthed shovel. The mould was then filled and compacted with the sample one-third at a time using the compacting rod. It was made sure that the tamping strokes were uniformly distributed over the cross-section of each layer. The mould was then raised vertically without any lateral motion after 2 to 5 seconds. The difference in height of the mould and height of the slump was recorded immediately after the removal of mould. The test was done continuously without interruptions within 150 seconds as required by standard procedure. All slumps were intact such that there was no occurrence of shear slump and therefore, no need for re-sampling. Preparation for Further Tests (British Standard Institution 2003) The hardened concrete from the various mixtures was to undergo tests to determine compressive and flexural strength. According to the standards, tests for compressive strength require the hardened concrete to be shaped as a cube. A prism, on the other hand, is the appropriate shape for the determination of flexural strength. The standard size for a concrete cube is 100 x 100 x 100 mm. Three cubes each were made for each Mix. The standard dimension for the prism or beam is 100mm by 100mm with a length of at least 350mm. This experiment formed beams that were 500mm long. For flexural strength test, the tolerance of ±0.2mm straightness of the surface that was in contact with the rollers was achieved. The samples were prepared and remixed in a steel tray using a square-mouthed shovel before being filled into the moulds. The fresh concrete was then compacted into the mold using the compacting rod as required by the standard. The mixtures were then left for 14 days to dry and harden. Testing of Hardened Concrete (British Standard Institution 2003) The concretes produced were taken from the moulds and labeled as shown in Figure 1. The products were then wiped dry before being positioned in the testing machine. It was also made sure that testing machine bearing surfaces were also wiped clean. The cubes were positioned at the center and in a way such that the load was applied perpendicularly to the direction of casting. Figure 1. Produced and labeled concrete beam. A constant rate of loading was continuously applied to the specimen without shock and increase. Two compression machines which conform to BS EB 12390 part 4 standards were used. One was for cubes and determination of compressive strength as shown in Figure 2a. The other one, as demonstrated in Figure 2b, was for obtainment of flexural strength of beams. This was done until the concrete broke as illustrated in Figure 3. The load at which the concrete broke was then recorded and analyzed. (a) (b) Figure 2. Compression machines for (a) compressive strength and (b) flexural strength determination. Figure 3. Breakage of concrete due to the attainment of maximum applied load. Results and Discussion The consistency of the cement mixtures were determined by slump test before hardening. The resulting values were either 14 or 15 mm. These values are within the range of 10 to 210 mm which implies that the mixtures were consistent enough and can proceed to hardening. The maximum load the prepared cubes were able to handle was recorded and statistically analyzed. Table 3 shows the one-way analysis of variance (ANOVA) which determined if the difference in maximum loads was significant. It can be observed that the F value of 26.64136 is greater than the critical value of 5.143253. Also, the P-value is in between 0.001 to 0.01. This simply implies that the difference in maximum load was very significant such that it can be concluded that the maximum load of concrete is highly affected by percent silica content. Figure 4 illustrates the trend of the effect of increasing silica fume content of concrete. Table 3. One-way Analysis of Variance for the Maximum Load of Cubes. Source of Variation SS Df MS F P-value F crit Between Groups 33156.9 2 16578.45 26.64136 0.001037 5.143253 Within Groups 3733.693 6 622.2822 Total 36890.59 8         Figure 4. Maximum allowable load for cubes versus silica fume concentration. It is clear that increase in silica fume concentration in turn increases maximum load of concrete. Figure 4 also shows that the results fit a polynomial trend such polynomial regression can predict other maximum load in terms of silica fume content with: where x is the percent silica fume content. The compressive strength of concrete was then derived from the maximum load. This was done using the formula: The values calculated were then analyzed statistically. The one-way ANOVA shown in Table 4 has the same conclusion as the first one. The compressive strength of concrete is very much affected by silica fume concentration. Table 4. One-Way Analysis of Variance on the Effect of Silica Fume Concentration to Compressive Strength. Source of Variation SS df MS F P-value F crit Between Groups 65.5256 2 32.7628 26.58527 0.001043 5.143253 Within Groups 7.3942 6 1.232367 Total 72.9198 8         Figure 5 illustrates the improvement of compressive strength due to the increase in silica fume concentration. The figure also shows the trend in the change of compressive strength due to silica fume content. Figure 5. Compressive strength of cubes versus silica fume concentration The maximum allowable load for the beams was also determined for the computation of flexural strength. Figure 6 shows the increase in maximum load due to increase in silica fume content. The value of R2 in the regression indicates that the relationship of silica fume content with maximum load for beams is highly linear. Figure 6. Maximum Load for Concrete Beams of Varying Silica Content Flexural strength of each mixture was calculated from maximum load using the formula: where f is flexural strength in MPa, P is the maximum load obtained from the experiment in N, and L,b, and d are beam length, width, and depth, respectively. Figure 7 demonstrate the linear increase of flexural strength of beams. Figure 7. Flexural Strength for Concrete Beams of Varying Silica Content Silica fumes can pack in between cement particles, making a finer pore structure (Kuennen 1996). This is because of its fine-sized particles. Based on the trends of the results, it can be observed that addition of silica fume improves flexural strength better than it improves compressive strength. Figure 6 shows that flexural strength increases steadily as silica fume is increased. Figure 5, on the other hand, illustrates that effect of silica fume content is greater at lower concentration. The change in compressive strength from 0 to 5% silica fume is greater than the change in compressive strength from 5 to 10% silica fume. Summary and Conclusion The experiment succeeded on creating cubes and beams of concrete with varying silica fume concentration. The experiment was also able to determine the maximum applied load that the produced concrete can carry. Compressive and flexural strength were obtained from the values of maximum load and the dimensions of the concrete blocks. Statistical analysis proved that the increase in maximum load due to increase in silica fume is highly significant. As supported by previous studies and the results of this experiment, silica fume greatly increases durability of concrete in terms of compressive and flexural strength. This is due to the density and particle size of silica fume. Its very fine particles have the ability to pack in between cement molecules. Because of this property, the concrete becomes more solid and compact such that it can endure greater compression. Also, concrete with silica fume are more flexible than regular concrete due to the finer particle size. Another advantage of the use of silica fume in concrete is that its fine particles provide a shiny, smooth finish to concrete. The compact structure of concrete with silica fume also increases chemical and abrasion resistance. This characteristic makes it ideal for special applications such as bridges and decks where silica fume can decrease chloride and abrasion attacks due to salt water. Combination of silica fume and micro-fiber in concrete has also been proven to be effective for application involving fire and explosives. Regression of results shows that flexural strength increases linearly as silica fume content increases. Compressive strength, on the other hand, has polynomial trend in increase of silica fume. As a conclusion, addition of silica fume to cement is very ideal. Not only does it solve the problem with silicon dioxide air pollution and decreases our dependence on cement. It also can improve cement performance. References ACI Committee, 2000. Guide for the Use of Silica Fume in Concrete. USA: American Concrete Institute. British Standard Institution, 2003. Testing Hardened Concrete Part 1: Shape, Dimensions, and Other Requirements for Specimen and Moulds. London:BSI British Standard Institution, 2003. Testing Hardened Concrete Part 2: Making and Curing specimens for strength tests. London:BSI British Standard Institution, 2009. Testing Hardened Concrete Part 2: Slump-Test. London:BSI British Standard Institution, 2003. Testing Hardened Concrete Part 3: Compressive Strength of Test Specimens. London:BSI Dunster, A., 2009. Silica Fume in Concrete, Information Paper. 5/09. pp. 1-11. Kuennen, T., 1996. Silica Fume Resurges, Concrete Products. March 1996 Issue. Appendix Table 5. Water and cement quantity based on 0.55 water-over-cement ratio. Table 6. Raw and computed values for Mix 1, 2, and 3. Mix Silica Fume Slump test (mm) Mould number Maximum load (kN) Stress (MPa) Flexural Strength (Mpa) Compressive Strength (Mpa) 1 0% 15 T18 793.7 35.27 35.28 Z53 743.2 33.03 33.03 AY 784.1 34.85 34.85 ZZ1 (beam) 14.41 4.324 7.21 2 5% 15 F5 877.9 39.02 39.02 UL 878.6 39.05 39.05 711 864.3 38.41 38.41 R47 (beam) 14.69 4.406 7.35 3 10% 14 TED 924.9 41.11 41.11 OU 883.5 39.27 39.27 T62 948.5 42.16 42.16 G42 (beam) 15.01 4.504 7.51 Read More