Research On Some Aspects Of Metakaolin-modified Concrete For Marine Engineering

Concrete materials and structures are applied and constructed in variousenvironments, such as cities, marine, low temperature and salt corrosion conditions.Along with the wide application of concrete, the durability of concrete attracted moreand more attentions from civil engineers and materials scientists. Recently thedemand of marine concrete increases greatly with the continuous development andutilization of marine resources and the durability of marine concrete are taken moreseriously than ever. The structure of marine concrete permanently is emerged in theseawater and is corroded by harmful seawater substances. The marine structuredegrades gradually, with damages on the surface, such as cracks, expansion, strippingand flaking. Research shows that addition of metakaolin in the concrete improves itsmechanical properties and durability. Considering the difficulties of transportation offresh water and fine aggregate during the construction of marine structures, researchon the relationship between structure and properties of concrete under marineenvironment will improve the development of marine engineering.In this study, based on the addition of small amount of metakaolin, together withvarious raw materials, three systems of marine concrete were developed, namelycoarse aggregate-river sand-fresh water-metakaolin modified (CA-RS-FW-MK)concrete, coarse aggregate-river sand-artificial seawater-metakaolin modified(CA-RS-AS-MK) concrete and coarse aggregate-marine sand-fresh water-metakaolinmodified (CA-MS-FW-MK) concrete. The microstructure, mechanical properties,durability and hydration mechanisms of the three systems under standard curing,sodium chloride curing, sodium sulphate curing and magnesium sulphate curingconditions were characterised by a range of analytical techniques. The main findingsare as follows.(1) Research on the mechanical properties of the above systems under standardcuring conditions shows that addition of5wt%MK achieves the highest strengthactivation index (SAI) in the pastes, mortar and concrete of all three systems.(2) Calorimetry results of fresh water-MK and seawater-MK systems shows thatthe addition of MK lowered the total heat evolved in the first72hours in the freshwater system, while the addition of seawater increased the total heat evolved. Theaddition of3wt%MK increased the heat evolution rate in both systems comparing to neat OPC paste, but the addition of5wt%and6wt%MK decreased the heat evolutionrate.(3) In the fresh water system, the hydration products are C-S-H, CH, ettringite,AFm and hydrotalcite. In the seawater system, hydrocalumite was identified togetherwith above minerals.(4) Quantitative analysis shows that CH content in the neat OPC paste increasedwith hydration process, while for the MK-modified pastes, CH content increased from3days to7days but decreased from7days to28days, indicating pozzolanic reactionof MK with CH. For the same amount of addition of MK, CH content was higher inthe pastes of seawater system comparing to fresh water system at the same age,indicating improvement of hydration by seawater.(5) Research on the chloride ingress shows that the addition of5wt%MKeffectively improved the compressive strength and chloride corrosion properties inboth fresh water and seawater systems due to the improvement of density andformation of Friedel’s salt. Calcite, CH and Friedel’s salt were identified in thehydration products after chloride corrosion. Ettringite, hydrotalcite and hydrocalumitewere transformed to calcite and Friedel’s salt, which immobilized the chloride.(6) Research on the sulphate corrosion shows that the compressive strength of thesamples with5wt%MK addition was the highest after sulphate corrosion in eachsystem. The compressive strength of the sample with5wt%MK addition in theseawater system was the highest before and after the corrosion comparing any samplein all systems, indicating that seawater and addition of5wt%MK improved theproperties of sulphate corrosion of concrete. The analysis of the depth of corrosionindicated that the addition of MK especially improved the sulphate corrosionproperties of concrete comparing to paste and mortar.