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Nano-Silica Particles in Cement Industry

Utilization of nano materials in construction industry has attracted many researches attention in recent years [1, 2, 3] Investigation of using NS is one of the active research themes due to successful results of using silica fume as a desirable source of pozzolan for high performance concrete (HPC) and high-strength concrete (HSC) production and accessibility of pure NS particles compared to silica fume. Micro silica is a by-product of ferrosilicon industry with specific surface area (SSA) of 20 m2/gr and 90% purity. However, NSs are industrially produced with a purity near to 100%. They can be manufactured with various desired specific surfaces areas ranging from 60 to over 600 m2/gr. NS was found to be much more efficient than micro silica for improving durability, strength and permeability of cement based materials in literature. Widely varying results have been reported by various researchers ranging from considerable strength enhancement to strength reductions. Several studies that investigated the effect of NSs on compression strength of cement paste, mortar and concrete are listed as follows.

Cement Paste Colloidal NS with a 40% solid content and 20 nm average particles size was used by Shih et al.4 in 0.2, 0.4, 0.6 and 0.8% of binder (total cementitious material) weight. In this study optimum dosage of NS was reported as 0.6%. Maximum strength increment with optimum dosage occurred at 14 days and it was reported 60% compared to the control mix.4 However, at further ages this extent was decreased and it reported 43% at 56 days.4 Effect of NS powder with 15 nm average particles size and 160 m2/gr SSA on properties of cement paste was investigated by Qing et al.[5] In this study NS was used as a cement replacement in 2, 3 and 5 of binder weight. Results showed that compressive strength increases as increasing the NS percentage. Strength enhancement with 5% dosage at 3 and 28 days was reported 98 and 120% with respect to the control, respectively. Additionally, NS changed setting time by reducing the initial and final set of time. Increasing NS dosage reduced final setting time of specimens considerably.6 Lin et al.7 used 1, 2 and 3% NS powder with 10 nm average particles size and 670 m2/gr SSA. Optimum dosage of NS 3% was reported and strength enhancement with respect to control was 6 and 4.5% at 7 and 28 days, respectively.

Mortar Powdery NS with 40 nm average particles size and 60 m2/gr SSA was used by Jo et al.8 in 3, 6, 10 and 12% of binder weight. The NS was used as a cement replacement and results were compared to micro silica. The optimum dosage reported 12% and maximum amount of strength enhancement at 7 days was 177% with respect to the control. However, maximum effect for silica fume occurred at 15% and was 42%. Sneff et al.9 used 30 % solid colloidal NS called Levasil300 with 9 nm average particles size and 300 m2/gr SSA in mortar. It was conducted that using NS decreases workability and simultaneously increases compressive strength of specimens. In other investigation10 about influence of NS on mechanical properties of cement mortar, a NS powder with an average diameter of 15 nm and 160 m2/gr SSA was introduced as a 3, 5 and 10% of binder weight. In this report, strength enhancement with 10% dosage at 7 and 28 days was reported 20% and 26% with respect to the control, respectively. Current results were different with other reports by using the same optimum dosage. Khanzadi et al.11 studied effect of colloidal NS called Cembinder8® with 80 m2/gr SSA used in  2.5, 5, 7.5 and 10 percent in cement mortar. In this study, compressive strength increased for 10% NS. In Sadrmomtazi et al.12, 13 research, the effect of colloidal NS with 50nm average particles size used in 1, 3, 5, 7 and 9% of cement replacement on cement mortar was studied. The optimum dosage of NS in this study was 7% and more dosages had no effect on compressive strength enhancement.

Concrete Ramezanianpour et al.14 studied influence of NS called Cembinder8® in concrete. The maximum amount of strength enhancement with 7.5% dosage of NS was reported 38% at 3 days. This enhancement decreased at further ages and at 90 days was reported 14%. Influence of NS powder with 10 nm (3.94e-7 in.) average particles size and 640 m2/gr SSA was studied by Li et al.15 by adding 1 and 3% NS in ratio of cement weight replacement. The optimum dosage reported 1% and compressive strength increased 12% at 28 days with respect to the control. Effect of NS on strength and freeze-thaw durability of concrete was studied by Baomin et al.16 In this research NS powder with average particles size of 16 nm and 158 m2/gr SSA used as a 3%. Results showed that compressive strength at 7 and 28 days increases 9 and 8%, respectively. Above statements show significant differences between reported results about NS performance on cement based materials strength. Results in literature showed that the optimum dosages varied from 0.6 to 10% and effect of different dosages on strength is also varied between 170% increase to decrease in some cases. Furthermore, unlike to silica fume that have permanent characteristics, NSs are developed with different particles sizes and different SSA from 60 to 600 m2/gr. This research aims to study the effect of NS particles size and SSA on mechanical properties of Portland cement pastes and mortars. Therefore, powdery and colloidal NSs with different SSA were employed and their influence in different dosages on mechanical properties of cement past and mortar were investigated. Additionally, to enhance our understanding about the overall behaviour of NS, mechanical properties of specimens containing NS has been compared to silica fume as a well-known pozzolan.