UTILIZATION OF SOME NANO-MATERIALS IN THE IMPROVEMENT OF CEMENT QUALITY

Abstract

The subject of producing pozzolanic cements with the partial substitution of ordinary Portland cement by pozzolanic materials, natural or artificial, has been of considerable scientific and technological interest because such addition increases the chemical resistance to sulphate and chloride attack, lowers heat of hydration and improves both of hydraulic properties and fire resistance of these blended cement pastes and mortars. The use of pozzolanic cement is increasing worldwide because it needs less energy for production. Metakaolin is a white fine powdered material in the form of anhydrous alumino-silicate derived from the calcination of raw kaolin at a specific temperature range. This investigation is divided into four parts: Part (1) Part (1) aims to study the pozzolanic activity of nano-metakaolin (NMK) at different activation temperatures. The pozzolanic activity of NMK samples, prepared by firing of Al2O3 – rich nano-kaolin (NK) at different temperatures of 750, 775, 800 and 825oC for 2 hours, was studied on 80% NMK – 20% hydrated lime pastes (water/solid ratio =1 by weight) at different ages of hydration from 2 hours up to 7 days. The main conclusion derived from the results of hydration kinetics and differential scanning calorimetry (DSC) thermograms is the high reactivity of NMK prepared by firing of Al2O3 – rich nano-kaolin (NK) at 750oC and this type of NMK is recommended for the improvement of physico-mechanical properties of the hardened NMK-OPC pastes and mortars as will be discussed later in this investigation. Part (2) Part (2) aims to study the physicochemical and mechanical characteristics of some artificial pozzolanic cement pastes containing calcined nano-clay in the form nano-metakaolin (NMK) fired at 750oC. The OPC-NMK blends were prepared by the partial substitution of OPC by 4, 6 and 8 % of NMK. Then each dry mixture was mixed with water by using an initial water/solid ratio of 0.28. The pastes were hydrated for various time intervals of 1, 3, 7, 28 and 90 days. At each time interval, the specimens of hydrated pastes were tested for their compressive strength and the crushed specimens were used for the other physicochemical properties on the ground dried samples. On the basis of the results obtained in this investigation, the following conclusions could be derived: 1. The hardened specimens paste made of 96% OPC – 4% NMK (mix M4) possess the highest strength values at most hydration ages as compared to the other pastes containing 0, 6 and 8% nano-metakaolin (mixes M0, M6 and M8). 2. The variations of the combined water contents (Wn, %) with the age of hydration are agreed with the variations of the compressive strength values. 3. The free lime contents obtained for all admixed OPC-NMK pastes are lower than those of the neat OPC paste (M0). 4. The XRD diffractograms and DSC thermograms obtained for the hardened 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, calcium hydroxide (CH) and CaCO3. 5. The scanning electron microscopy micrographs indicate the formation of more dense microstructure for the hardened OPC- NMK pastes as compared to those of the neat OPC paste after 90 days of hydration. Part (3) Part (3) aims to study mechanical properties, thermal analysis and microstructure of pozzolanic cement mortars containing nano-metakaolin (NMK) obtained by firing NK at 750oC. The hardened cement mortar were prepared by the partial substitution of OPC by 0, 4, 6 and 8% NMK with sand as fine aggregate by the sand/(OPC+NMK) weight ratio 2.5:1 with water/solid ratio of 0.48 by weight. All cement mortars were cured under water for 1, 3, 7 and 28 days. On the basis of the results obtained in this investigation, the following conclusions could be derived 1. At the early stages of hydration (1 and 3 days), the hardened cement mortar made of OPC/sand with a weight ratio of 1:2.5 possess higher strength values as compared to the other mortars containing NMK (S4, S6 and S8). 2. At the intermediate stage of hydration (7 days); the compressive strength values of all mixes are close together (comparable values). 3. At final stage of OPC-NMK hydration (28 days of curing); the compressive strength values of hardened OPC-NMK mortars (S4, S6 and S8) have higher values as compared to the hardened neat cement mortar free from NMK (S0). 4. The mortar specimens made of OPC (94%) – NMK (6%) blend (S6) possess the highest strength values after 28 days of hydration and this OPC-NMK blend is recommended for further studies of microstructure of the hardened mortars. 5. The results of SEM micrographs obtained for the hardened mortar specimens made of the neat OPC (S0) and OPC-NMK blend (S6), after 28 days of hydration indicate the more dense structure of the hardened mortar specimens made of OPC-NMK blend (S6) as compared to those of the neat OPC mortar specimens. This is mainly due to the formation of excessive amounts of the hydration products, mainly as calcium silicate hydrates (CSH) and calcium aluminosilicate hydrates (CASH), with nearly amorphous and microcrystalline characters.