Abstract
Prepared metakaolin produced by calcination of a kaolin sample at 750°C after soaking for 5 hrs was tested as an active pozzolan for locally produced cement. Blended pastes of partially replaced ordinary Portland cement with different metakaolin amounts 5%, 10% and 15% were prepared, then they were hydrated with water for various time intervals of 1, 3, 7, 14 and 28 days. At each time interval, the hydrated paste specimens were tested for compressive strength, hydration kinetics and followed up using differential scanning calorimetry, X-ray diffractometry analyses and scanning electron microscopy techniques. Results showed that the blended pastes OPC-MK10 recorded the highest compressive strength values at all the hydration times reaching 33.10, 86.40, 101.20, 112, and 122 MPa with increasing the age of hydration as compared with the neat samples which recorded 31.70, 65.20, 72.6, 82 and 101.30 MPa respectively. Meanwhile, the OPC-MK10 blend showed a decrease in the free CaO content reaching 4.07%, 4.27%, 4.23%, 4.19%, and 4.11% CaO with increasing age of hydration compared with the neat samples which recorded 4.27%, 5.15%, 5.42%, 5.61%, and 5.96% CaO respectively. The DSC thermograms results for the hardened neat and OPC-Mk10 pastes at the 14 and 28 hydration days showed the formation of hydrated materials mainly calcium silicate hydrates, calcium sulphoaluminate hydrates and calcium aluminates hydrates. The X-ray diffractometry analyses of both hardened neat and OPC- MK10 showed that, the intensity of calcium hydroxide peaks of OPC-MK10 was lower than in case of neat OPC, while the peak of the hydration products of calcium silicate hydrates and calcium aluminates silicate hydrates of OPC- MK10 samples were of higher intensity than in case of neat OPC. Scanning electron microscopy micrographs indicated the formation of denser micro- structure for the hardened OPC-MK10 paste as compared to neat OPC pastes after the 28 days age of hydration.
Highlights
Since the first Portland cement with the present definition was produced in 1843 by William Aspdin, there has been a continuous process of evolution in cement process technology and cement itself
The results showed that the compressive strength measures for the neat Portland cement pastes demonstrates a remarkable increase during the early ages of hydration up to 3 days from 31.70 MPa at 1 day hydration to 65.20 MPa at 3 days hydration, reaching an increase by 105.68%
The results of the chemically combined water (Cw) content values for the neat ordinary Portland cement (OPC) and the blends containing different amounts of metakaolin versus the ages of hydrations indicated fast hydration reactions were occurred from the time of mixing up to 3 days of hydration; this was followed by an acceleration stage of hydration up to 7 days of hydration
Summary
Since the first Portland cement with the present definition was produced in 1843 by William Aspdin, there has been a continuous process of evolution in cement process technology and cement itself. The most common of these are fly ash (FA), silica fume (SF), aluminous scraps, iron and steel industry slag, and metakaolin (MK) These materials are used to replace part of cement or as a source of new cementless materials and help in reducing the released free lime that is produced during clinker hydration, [6] [7]. ASR can be explained as the situation where cement alkalis react with certain forms of silica in the aggregate component of a concrete, forming an alkali-silica gel at the aggregates surface This formation often referred to as “reaction rim” has a very strong affinity for water, and has a tendency to swell. Metakaolin offers good properties as supplementary cementing material It results to high performance, high strength and high weight concrete to be employed in structural and non-structural components of bridges or in pavement overlays. A thorough discussion of the role of metakaolin on locally produced cement performance in view of the formation of cementitious materials will be carried out in a “Step on the Road” for a better understanding of its possolanic activity
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