الفهرس 
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Abstract
Pozzolana are materials which possess cementitious properties but do not react with water due to the formation of thin coating silica gel around the pozzolana grains when mixed with water which hinder the hydration reaction. However, when these pozzolana are mixed with any alkaline material, they undergo hydration reaction as a result of removal of silica gel coatings by alkali hydration.
The pozzolanic materials can be divided into two main groups, natural and artificial. The natural pozzolana was mostly materials of volcanic origin and certain diatomaceous earths. The artificial pozzolana are mainly products obtained by the heat treatment of clays, shale, certain siliceous rocks, silica fumes, rice husk ash and pulverized fuel ash. The subject of producing pozzolanic cements has been of considerable scientific and technological interest because such addition increases the chemical resistance to sulphate attack, lowers heat of hydration, and improves fire resistance. The use of pozzolanic cement is increasing world-wide because it needs less energy for production.
Condensed silica fume (SF) is a by-product of silicon or ferrosilicon alloys industries. Silica fume particles are spherical and have an average diameter of about 0.1 μm. It consists of ~ 99 % amorphous silica with a specific surface area of 20000 m2/kg. These characteristics account for the pozzolanic activity of silica fume in terms of both its capacity of binding lime and rate of hydration reaction.
Burnt clay is a good pozzolana. It can be used in the manufacture of pozzolanic cement or acts as filler in filled pozzolanic cement. It is expected to be resistant to sulphate and chlorides when blended with Portland or sulphate resisting cement. Fire is one of the major considerations in the design of building, it is extremely necessary to have a complete knowledge about the behavior of construction materials before using in the structural elements.
This investigation is divided in to two parts:
Part I
Part I aims to study the pozzolanic reactivity of burnt clay (BC) using cement kiln dust (CKD) and calcium hydroxide (CH) as activators this is in order to measure the most suitable temperature at which clay (Libyan clay) is fired to obtain burnt clay with high pozzolanic reactivity.
Three samples of artificial pozzolanas were prepared by burning of clay at three different temperatures of 600, 700 and 800°C for a soaking period of four hours. Six dry pozzolanic binder mixtures were prepared by mixing each sample of burnt clay with cement kiln dust (CKD) or calcium hydroxide (CH) as activators for the pozzolanic reaction.
Mixing was done using ball mill for six hours in order to attain the complete homogeneity of the mix and each dry mixture was mixed with water for paste preparation using water/cement ratio of 0.50 by weight. Mixing of the fresh paste was done for three minutes continuously.
The pastes were then cured at 100% relative humidity up to 24 hours, in order to attain a complete setting of the pastes, followed by curing under water for various time intervals of 3, 7, 28 and 90 days. At each curing time, compressive strength tests were done; then, the hydration reaction of the ground specimen was stopped using methanol/acetone mixture and the powdered specimens (after filtration) were dried at 90C. The dried specimens were then subjected to other physico-chemical studies.
The physico-chemical properties studied were: hydration kinetics, X-ray diffraction analysis (XRD), differential scanning calorimetry (DSC) as well as scanning electron microscopy (SEM). Hydration kinetics was studied by determining the chemically combined (non-evaporable) water and free lime contents of the dried pastes at the various ages of hydration.
The main conclusions derived from part I are summarized as follows:
1- The rate of hydration of BC – CKD blends is higher than that of BC – CH blends especially at the early ages of hydration.
2- Higher fractions of the remaining free calcium hydroxide are still present up to 90 days of hydration of BC – CH blends; while the free lime is almost consumed after 7 days of hydration of BC – CKD blends.
3- BC – CKD blends possess higher mechanical properties as compared to BC – CH blends at the early ages of hydration; meanwhile the paste made of BC – CH blends have higher strength characteristics than those of BC – CKD at the later ages of hydration.
4- The results of phase composition and microstructure of the formed hydrates could be related to the development of strength of the hardened pastes made of BC – CKD and BC – CH blends. A less dense structure of BC – CH pastes was obtained at the early ages of hydration which turns to a more dense structure at the later ages of hydration.
5- The temperature (700°C) can be considered to be the optimum temperature of firing clay to obtain burnt clay with suitable pozzolanic reactivity.
Part II
Part II aims to study the physicochemical and mechanical characteristics of some artificial pozzolanic cement pastes containing burnt clay (Libyan clay fired at 700°C) with and without silica fume.
Different pozzolanic cement pastes were prepared from different OPC-BC dry mixes made without and with silica fume. The OPC-BC blends were prepared by the partial substitution of OPC by BC (10, 20 and 30%) prepared by firing the clay at 700°C which represent the optimum temperature of pozzolanic activation of the clay. The OPC-BC-SF blends were prepared from OPC (70%) – BC (30%) blend by addition of 2.5 and 5% SF. Then each dry mixture mixed with water by using an initial water/solid mix ratio 0.30 and the pastes, thus obtained, were hydrated for various time intervals of 1, 3, 7, 28 and 90 days. At each time interval, the hydrated pastes were tested for compressive strength and the other physico-chemical properties were investigated using the ground dried samples.
On the basis of the results obtained in this investigation, the following conclusions could be derived:
1- Mix III, (70% OPC-30% BC), for economic reasons, can be considered as the reasonable composition which produces acceptable compressive strength results for the pozzolanic cement blends.
2- The physical action of pozzolanas (BC and SF) provides more denser, homogenous and uniform pastes having stronger hydraulic and strength-producing characters.
3- SF represents an efficient pozzolanic material, it activates the constituents of cement towards hydration and the air content has been reduced due to its microfilling effect which leads to an increase in the compressive strength of the hardened cement pastes containing SF.
4- The admixed pastes containing 5% SF (Mix III-S2) possess the lowest values of free lime contents indicating that the higher SF addition leads to higher pozzolanic action and lower values of free lime contents.
5- The variations of Wn-values with age of hydration show the same trends observed for the changes in values of compressive strength.
6- DSC thermograms and XRD difractograms obtained for the pastes indicate the formation of: nearly amorphous calcium silicate hydrates (mainly as CSH-I and CSH-II), calcium sulphoaluminate hydrates (ettringite and monosulphate hydrates), calcium aluminate hydrates (mainly as C4AH13), hydrogarnet (mainly as C3ASH4 and C2ASH8), CH and CaCO3.
7- After 90 days of hydration, the hardened OPC-B.C.-SF pastes possess a more denser microstructure as compared to the other investigated pastes.