Silica Based Additives In The Packing Of Cement Mortars

Introduction

The construction industry utilizes concrete to a vast degree. Around 14 billion tons were used in 2007. Concrete is utilized as a part of the framework and in structures. It is made out of granular materials of various sizes and the size, scope of the composed solid mix covers wide interim. The general evaluating of the mix, containing particles from 300 nm to 32 mm decides the mix properties of the concrete. The properties in the fresh state (flow properties and workability) are for example represented by the particle size distribution (PSD), yet in addition the properties of the concrete in solidified state, for example, strength and durability, are influenced by the mix reviewing and coming about particle packing. One approach to additionally enhance the packing is to expand the solid size range, for instance by incorporating particles with sizes under 300 nm. Available materials which are right now accessible are silica fume (SF), fly ash and nano-silica (NS).

In general, Nano materials are characterized as materials of under 100 nm in size. At the point when the matter is lessened in size it changes its characteristics, for example, color and interaction with other matter such as chemical reactivity. The adjustment in the attributes is caused by the difference in the electronic properties. By the particle size, lessening, the surface area of the material is expanded. Because of this a higher percentage of the atoms can interact with other matter, for example, with the matrix of resins. Surface action is a key part of Nano materials. Agglomeration and aggregation blocks surface region from contact with other matter. Just all around scattered or single scattered particles permit using the full gainful capability of the matter. In result, great scattering lessens the amount of Nano materials expected to accomplish similar impacts. As most Nano materials are still genuinely costly, this angle is of high significance for the commercialization of item details containing Nano materials. Today, numerous Nano materials are created in a dry procedure. Accordingly, the particles should be blended into liquid formulations. This is where the most Nano particles shape agglomerates amid the wetting.Sand dunes are regular features of shoreline and desert conditions. As of now there is no national determination concerning the utilization of desert sand with very fine grain. To have the capacity to apply desert sand to mortar and cement in structural designing, mortar and concrete made of dune sand. Different looks into show that dune sand can be utilized as a fine aggregate in mortar and concrete for general structural engineering. Short discontinuous fibers are generally used to enhance the ductile and bending performance of fragile materials, for example, concrete. Fibers can enhance the properties of cement-based materials since they supply extra outlets for vitality absorption. If a crack forms in a fiber-reinforced cementitious composite due to tensile forces and fibers are available to bridge the gap, in order for the crack to propagate, additional energy must be supplied to break the fiber–matrix bond, allowing fiber pullout to occur, or fiber yielding or rupture must take place.

The hidden factor in deciding the method of disappointment is implanting length: there is a basic installation length above which the fiber will crack and beneath which pullout will happen. To additionally enhance the fortification impact, hybrid fibers of different types or different sizes have been picking up popularity in recent years.

In this PhD the aim of the proposed study is to enhance the packing of cement mortars, based on sand of dunes as a replacement of natural sand, by utilizing silica based additives at both; micro and nano scales. Using different types of fibers on cement mortar to improve its properties.

Particle size distribution

Optimum particle packing is a key for designing a dense, strong, and durable cement-basedmaterial. By optimizing cement and aggregate particle size distribution, the voids among the particles can be significantly minimized, thus increasing packing density, reducing the amount of binder required for filling pores, and improving the material strength, impermeability, and volume stability of the resulting products. Dense particle packing is generally formed by particles with varying particle size distributions, where voids can be successively filled up with smaller particles. One way to further improve the packing is by the use of more fine materials such as silica fume.

Various models have been developed for achieving maximum density, or optimal packing, of aggregate particles in concrete, among which is the Andreasen and Andersen (A&A) model. In concrete practice, groups of aggregate particles with a specific particle size distributions (PSD) are often combined in such a way that the PSD of the blended aggregate is getting as close as possible to a modeled PSD curve.

Some studies have examined the relationship between particle size distribution (PSD) in the raw materials and various physicochemico- mechanical properties of cement based mortars. However, all have focused on a single, specific component of the mortar formulation. Thus, many have examined the influence of cement PSD on hydration kinetics and hardened paste strength or its effect in admixtures (slag and fly ash) on their properties. The influence of particle size and shape in specific aggregates on the properties of hardened mortar has also been studied. To our knowledge, however none of the previous studies has dealt with the influence of the PSD of overall raw materials on the microstructure of hardened cement based mortar. Such influence is of a high interest since the internal microstructure of cement based mortar is a function of its porosity and pore size distribution (a result of voids between sand grains and those intrinsically present in cementitious products). Thus, a change in the size of sand grains (the major component by weight and volume) must have a dramatic effect on mortar microstructure. The internal microstructure of hardened mortar is known to affect its mechanical properties and, other interesting characteristics.

Mehta Mentioned in his research that the use of supplementary cementitious materials has increased from 10% in 1990 to about 15% in 2005, and it is viable to increase this number to about 50% in 2020. SCMs are beneficial not only in the sense that they contribute to sustainability and reduction in CO2 emissions, but also due to the potential ability of these materials to enhance the properties and performance of concrete.

Lee et al studied effect of fly ash and reached to Cement–fly ash blends generally show higher flowability than Portland cement paste. Some reasons that are assumed in the literatureto be responsible for this effect were summarised by. as follows: (a) the lower density of fly ash compared to cement causes a higher particle volume at constant mass ratio, so the paste volume of a concrete mixture increases; (b) fly ashreduces flocculation of the cement particles by a dilution effect; (c) reduced growth of hydrate products at early agedue to the slower reaction of the fly ash, particularly at high replacement levels; (d) the ‘ball bearing effect’ induced by the spherical shape of flyash particles that facilitates the movement of neighbouring particles. Also, increasing the density of particle packing by filling of smaller particles into the voids between larger particles, and the lower surface area/volume ratio of spherical particles compared to angular cement or slag grains, will result in lower water uptake in voids and on surfaces, and hence the excess water is made available to facilitate flow.

Geisenhanslueke et al said in his research that although many investigations exist on the function of silica fume in concrete, it is still of interest how silica fume or other forms of silica could be improved in terms of purity, particle size and particle size distribution to exhibit even better results. The optimization of silica addition may lead to further improvements of UHPC. But, as a by-product of an industrial production process silica fume is far from an optimized concrete additive.

Sextl et al reached to the nearer the dispersion of the silica comes to primary particle sizes, the higher is the compressive strength (at least for 7 d after mortar mixing). Therefore, it is clear that the dispersion of silicainto primary particle sizes or the smallest agglomerates possible is mandatory for further improvement of the compressive strength.

Dunes sand

In the Middle East and North Africa, dune sand is partially or wholly used as fine aggregate of concrete due to the scarcity of natural aggregate having satisfactory quality. The main purpose of the use of dune sand is the grading correction of crushed sand because crushed sand has mostly coarser particles in the fine aggregate size range. Although the particle size of dune sand is different depending on the region, it is very fine, mostly in the range of 0.15–0.6 mm, and its grading helps to correct poor grading of crushed sand. Also, the shape of dune sand particles is spherical and it can be beneficial to workability of concrete. But very fine particle size and large surface area of dune sand can make concrete cohesive and can lead to requirement of more water to meet suitable workability. Therefore, the important. point for the use of dune sand is how much the replacement of dune sand can extract the optimum properties of concrete. There are limited studies available in the literature about the use of dune sands in concrete and mortar. Courard et al study the properites of sand dunes. SEM investigations reveal the relatively rounded shape grains with some irregular and angular grains of dune sand. The EDX analysis demonstrates the essentially siliceous nature of dune sand. A decrease of workability was observed with increase of the percentage of dune sand. A higher amount of water was used for the concrete made with dune sand compared to the other mixtures, because of its very fine granulometry. A correction of the grading curve of dune sand by river sand having a finesses modulus of 2.78 was adopted. For the sands obtained from mixing DS and RS, an improvement of fineness modulus of dune sand, and the sand equivalent of river sand were observed, compared respectively to 100% DS and 100% RS.

Mezghiche] reached to sand of dune replacement of cement was found effective in improving the resistance of cement to sulfate attack. The sulfate resistance of sand of dune (SD) cement increased with increasing the sand of dune (SD) replacement level, cement containing (10%SD) replacements showed excellent durability to sulfate attack. The resistance of cement to chloride magnesium acid verified that the replacement of cement by sand of dune replacement level was better resistance to acid attack than the cement (without sand of dune replacement), cement containing sand dunes replacements showed excellent durability to acid attack. Also showed that the mortars containing cement with sand of dune (SD) to develop shrinkage of drying slightly higher than that of the mortar containing cement without additions, however, the replacement of cement by sand of dune showed decreased the expansion of the mortar. The concrete mixture using the dune sand as part of the fine aggregate will have a low slump and fluidity. However, the 10% fine aggregate replacement ratio of the dune sand was available for the in-filled concrete of steel tubes, when sufficient vibration and proper curing method is applied.

Guettalla et al. have compared strength properties of mortar mixes made with conventional sands and dune sand. Mixes made with dune sand only resulted in lower strengths. Kay et al made a comparison among concrete mixtures made with beach sand, wadi sand, dune sand, screened dune sand, and combinations of dune sand or screened dune sand with crushed rock fines. The results indicated that dune sand may provide a readily available alternative material for use as fine aggregate in concrete.

Kim et al reached to The highest values of compressive strength and tensile strength were shown in concrete mixtures with dune sand ratio 20% and the strength decreased with increase of dune sand ratio when dune sand ratio was over 20%. It could be due to the compactness of aggregate and the bond strength between aggregate surface and cement paste, which were affected by the change of dune sand ratio.

The typical failure modes of dune sand concrete-filled steel tubular stub columns were outward local buckling for square sections and outward global buckling for circular sections owing to the outward bulging of the crushed concrete. Similar to concrete-filled steel tubular stub columns using dune sand as 10% of fine aggregate and conventional concrete-filled steel tubular stub columns under axial compression, the dune sand concrete-filled steel tubular stub columns were able to develop a stable axial load versus axial strain response and exhibit a ductile behaviour since the steel tube could provide effective confinement to the core concrete. No obvious detrimental effect was found by using dune sand.

Effect of using mineral addition on properites of concrete and mortar

Silica fume

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.

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 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 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 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%).

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.

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:

1. 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.
2. The increase of compressive strength is most significant in concrete and that in paste is the least.
3. 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.
4. 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.
5. 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.