الفهرس 
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Abstract
Nano technology has become an important key in the field of construction and building materials. When using nano-matrials, three main advantages are considered. The first advantage is the production of high strength concrete for specific applications. The second advantage is the reduction of the amount of cement needed in concrete for similar strength and decreasing the cost of concrete and the environment impact of construction materials. The third one is the reduction of the construction period, because nano-materials can produce high strength concrete at short curing time. The reduction of the amount of cement in concrete can also cause significant national energy consumption and green house CO2 gas emission.
This work aims to study the hydration of pozzolanic cement pastes prepared from substitution of different percentages of OPC by fly ash (FA) up to 30 wt.% , silica fume (SF) up to 15 wt.% and nano- silica (NS) up to 6 wt.%. The effect of substitution of 2, 4 and 6 wt. % NS by equal amounts of FA as well as SF was also investigated.
The mixing of cement paste was made by using the water of consistency. The water of consistency, initial and final setting times (IST & FST) were measured for each cement paste. The hydration characteristics of the hydrated cement pastes such as combined water (Wn), free lime (F.L) contents and bulk density (dp) were determined up to 90-days. The compressive strength of cement mortars was measured as a function of curing time up to 90-days. Also, the hydration products of some selected samples were identified by using DTA, TG and XRD techniques.
The raw materials used in this work were OPC provided from Lafarge cement company, Egypt with blain surface area 3000 ± 50 cm2/g. Fly ash (FA) and silica fume (SF) were supplied from Sika chemical company, Egypt. The used fly ash is mainly composed of silica quartz in addition to detectable amount of mullite with glassy phase. On the other side, the silica fume is mainly amorphous material. Nano-silica (NS) used in this work with nano-size 15 nm. It was supplied from nano-technology lab, Faculty of science, Benisuef University, Egypt. It is prepared by mechanical grinding of very pure quartz.
The hydration characteristics of 10-30 wt, % FA as well as 5 - 15 wt.% SF was studied by the determination of water of consistency, setting times, chemically combined water, free lime, and bulk density of cement pastes up to 90-days. Also, the compressive strength of cement mortars was determined as a function of curing time up to 90-days. The effect of small amount of nano-silica (NS) up to 6 wt. % on the hydration characteristics of OPC, 30 wt. % FA and 15 wt. % SF pastes was also studied. The hydration products of some selected cement pastes were investigated using DTA, TG and XRD techniques.
from this study the following results were obtained:
1-The substitution of 10 wt. % OPC by FA accelerates the initial setting time, due to the nucleating effect without any detectable effect on the final setting time. On the other side, the increase of FA up to 30 wt.% elongates the initial and final setting time. The FA increases the water of consistency from 24.5 up to 28.4 % for 0 and 30 wt % FA substitutions. The substitution of OPC with FA up to 30 wt. % has no detectable effect on the values of chemically combined water of cement pastes up to 90-days of hydration. But, these values are lower than those of OPC pastes, due to the low pozzolanic activity of FA. The increase of FA content up to 20 wt. % increases the liberated Portlandite, especially at early ages of hydration up to 3-days. At 30 wt.% FA, the free lime contents of blended-cement pastes are higher than those of only OPC pastes up to 7-days then decreases due to its consumption by FA at later hydration ages. The replacement of OPC with FA decreases the bulk density as well as the compressive of cement mortars up to 90-days. As the amount of FA increases, the bulk density and compressive strength decrease gradually. It can be said that, 10 wt.% FA is the optimum substitution level.
2-The substitution of equal amounts of OPC by SF up to 15 wt.% increases the water of consistency and elongates the initial and final setting time, due to the high surface area of SF. It is an active pozzolana therefore; it promotes the hydration of Portland cement clinker phases and gives higher combined water contents than OPC. The presence of SF decreases the liberated protlandite with SF content and curing time up to 90-days. On the other side, the free lime of OPC pastes increases gradually with curing time, due to the continuous hydration of OPC phases such as C3S and β-C2S. It was found that, the blended cement containing 10 wt.% SF gives higher bulk density and compressive strength than OPC.
3-The replacement of OPC with 2, 4 and 6 wt. % NS, increases the amount of water of consistency and elongates the setting times with higher level than FA or SF. This is due to its higher surface area in comparison with FA and SF of micro-particles, whereas the nano-silica has nano-particle size of about 15 nm. The NS is completely amorphous. It was found that, the substitution of 4 wt. % of OPC with NS gives fast setting, due to its reaction with free protlandite. This value (4 wt. %) of NS gives the higher combined water and lower free lime contents. Also, 4 wt. % NS gives higher bulk density and compressive strength. The replacement of OPC with 6 wt. % NS has a negative effect on the hydration characteristics of OPC pastes. It can be said that, the substitution of OPC with (2-4) wt. % NS is the suitable amount, which gives good properties.
4- In the preparation of nano-blended cements containing 2, 4 and 6 wt. % instead of FA or SF, it was found that, 2-4 wt. % of nano-silica gives the suitable chemical and physico-mechanical properties of FA or SF blended cements.
The results of combined water and free lime contents of the blended cement pastes prepared from FA or SF in the presence of small amount of NS as well as OPC with NS up to 6 wt. % are in a good agreement with each other and with those of DTA, TG and XRD techniques.
The main conclusions can be derived below:
A- The substitution of OPC with 5-10 wt. % FA as well as SF gives pozzolanic cements with reasonable characteristics. Also, the substitution of 30 wt. % FA or 15 wt.% SF by 2-4 wt. % NS shows the suitable mix compositions for the preparation of composite cements from OPC with FA or SF and NS.
B- The substitution of OPC with equal amount of NS from 2 – 4 wt. % gives good influence on the physico-chemical and mechanical properties of cement pastes. This is mainly due to the activation of OPC by NS particles.
C- Generally, composite cement containing FA or SF with NS gives higher bulk density and compressive strength than pozzolanic cement with FA or SF.