Effect Of Using Mineral Addition On Properites Of Concrete And Mortar

It is well known that supplementary cementitious materials in the form of natural Pozzolan or industrial by-products are able to modify the microstructure and interfacial zones of aggregate-paste or concrete (paste)-reinforcement, and improve durability of concrete. Silica fume has been recognized as one of the most effective supplementary cementitious materials which contribute to concrete durability improvement through pozzolanic reactions with free lime, pore size refinement and matrix densification, as well as cement paste–aggregate interfacial improvements. Silica fume is a very fine amorphous silica powder produced in electric arc furnaces as a by-product of the manufacture of alloys with silicon or elemental silicon (5). C akır et al (1) studied the effects of incorporating silica fume (SF) in the concrete mix design to improve the quality of recycled aggregates in concrete.

And reached to that the Concretes produced with natural and recycled aggregates incorporating silica fume underwent a reduction in early age compressive strength. The compressive strength decreased with increase in the silica fume content. However,Compressive strength loss of concretes containing recycled aggregate was less than the concretes containing natural aggregate at early age due to the SF usage. At 28 and 90 days, the strength of all the concrete mixtures with 5% and 10% of silica fume was increased, in comparison to the strength of the control concrete without silica fume. The pozzolanic effect of the SF that usually occurs after 7 days tends to increase the compressive strength of the concretes with this mineral admixture.

This effect is more significant in recycled aggregate concrete incorporating10% SF and containing 4/12 mm fraction recycled aggregates rather than the other concrete series. Karein et al (2) investigated the effects of silica fume granulation on durability and mechanical properties of concrete were tested. And reached to that the mechanical properties, durability, and permeability of concrete were analyzed by means of various tests on samples prepared with both granular silica fume and slurry silica fume. In addition, various water-to-binder ratios and cementing content were evaluated. Results indicated an increase in strength and surface electrical resistivity, and a decrease in permeability for both slurry silica fume and granule, compared to the control sample. The compressive strength of the mixes with granular silica fume and slurry silica fume were similar, and the differences in the results were less than 5%. This indicates that granular silica fume delivered a satisfactory performance. Using silica fume decreased the maximum and mean penetration depths in water absorption tests at the ages of 28 and 90 days. After analyzing the results, it was concluded that the averaged value of the penetration depth was a more reliable measure of permeability in compare of maximum value of the penetration depth.

In addition, samples with slurry silica fume and granular silica fume both exhibited similar values in terms of electrical surface resistance. At the age of 90 days, the electrical surface resistances of samples with granular silica fume were in the range of 94%–100% of those with slurry silica fume. The migration coefficient of chloride ion and the electrical charge passed in the rapid chloride migration test were less than 2% apart for samples having a water-to-cement ratio of 0. 45.

However, for samples having 0. 35, those with granular silica fume had a higher migration coefficient and charge flux. It seems that reducing the water-to-cement ratio and increasing viscosity hindered the disintegration and dispersion of granules during the mixing process. In turn, this reduced the reactivity of silica fume particles, which led to a poorer performance as compared to samples with slurry silica fume. Comparing the results of the compressive strength test with the results of the durability and permeability tests showed that 7. 5% replacement of cement with silica fume in the best case (i. e. , SSF35-400) increased the compressive strength by no more than 20% as compared to the control sample.

However, the electrical charge passed in the RCPT decreased to 25% and the electrical surface resistance increased four-fold as compared to the control sample. This could be explained by disconnect and obstruction of the pore structure with the products of pozzolanic reactions; however, this does not necessarily lead to a marked decrease in the volume of the pores. the results of this study indicated that granular silica fume could be a reliable substitute for slurry silica fume in various construction applications. This would remedy the problem of unwanted agglomeration, and would be superior to as produced silica fume and slurry silica fume due to the convenience of its transportation, maintenance, and application. Siddique (3) Studied the physical, chemical properties of silica fume, and its reaction mechanism. It deals with the effect of silica fume on the workability, porosity, compressive strength, splitting tensile strength, flexural strength, creep and shrinkage of concrete. Results of this study showed that

1. The addition of silica fume increases the 28-day compressive strength.
2. Silica fume does not have significant influence on the splitting tensile strength of concrete. Silica fume seemed to have a pronounced effect on flexural strength in comparison with splitting tensile strength. For flexural strengths, even very high percentages of silica fume significantly improved the strength. Also it was found that there was a steady increase in the flexural strength with increase in the silica fume replacement percentage.
3. Increasing the silica fume replacement level increased the secant modulus of concrete.
4. The inclusion of silica fume at high replacement levels significantly increased the autogenous shrinkage of concrete due to the refinement of pore size distribution that leads to a further increase in capillary tension and more contraction of the cement paste.
5. The plastic shrinkage strain increased with increasing dosage of silica fume.
6. Silica fume reduced the strain due to creep compared with Portland cement concrete.

Sanjuán et al (4) presented both a study on the effect of silica fume (SF) fineness on the pozzolanicity of blended cement and a method for improving coarse SF performance in making high-strength and high-performance concrete. The replacement of Portland cement with 25% of silica fume (45 lm sieve residue between 0. 98%, for SF I (C) and 4. 13%, for SF I (B)) produces high-strength mortar and such fineness gives the highest compressive strength. However, the coarser silica fume named SF I (A) does not exhibit a good performance when added in high amounts of 25% (45 lm sieve residue of 32. 11%).

1. Mortar made of silica fume that has a fineness of between 0. 98% and 4. 13% presented almost the same compressive strength at the curing ages after 28 days. A fineness of between 0. 98% and 4. 13% is advisable in producing a high-performance mortar or concrete. In summary, given that the pozzolanic performance of mixes with both silica fume are somewhat similar, it may be concluded that silica fume with a higher fineness does not have any peculiar influence in any way on the pozzolanicity performance of the mixes that involve this addition.
2. The silica fume with high fineness is a suitable pozzolanic material to be used in producing a better high-performance concrete. The addition of fine silica fume to Portland cements has been shown to give rise to physical (i. e. filler) and chemical (i. e. pozzolanic) effects on the microstructure of hardened pastes, leading to improved macro-properties of mortars and concretes, such as higher strength and lower permeability, among others. Zhang et al (5) studied the differences of effect of silica fume in paste, mortar and concrete by determining the non-evaporable water content of pastes, measuring the compressive strengths of pastes, mortars and concretes containing 5% and 10% raw silica fume or densified silica fume with water-to-binder ratios (W/B) of 0. 29 and 0. 24 and investigating the properties of interfacial transition zone between hardened paste and aggregate. It shows that:
3. The silica fume can significantly increase the non-evaporable water content of paste at later ages due to its pozzolanic activity. The addition of silica fume leads to a high hydration degree of ternary binder. The silica fume can make great contribution to the compressive strengths of paste, mortar and concrete at later ages.
4. The increase of compressive strength is most significant in concrete and that in paste is the least.
5. In the case of the same replacing level, the non-evaporable water content and compressive strength of paste containing densified silica fume are lower than that containing raw silica fume. This difference of the compressive strength becomes smaller in concrete, especially at later ages.
6. The silica fume agglomeration has been found in blended pastes, which cannot be broken down by normal mixing. The core of agglomeration does not take part in hydration and the agglomeration may be the weakness of binder pastes. Both densified and raw silica fume are well dispersed in concretes.
7. The interfacial transition zone is one of the main factors causing the different effect of silica fume in the paste, mortar and concrete. The silica fume can improve the interface bond strength between hardened paste and aggregate. The crystalline orientation degree, the crystalline size and the content of calcium hydroxide at the interface are obviously decreased by adding silica fume.