Developing supplementary cementitious materials from waste London clay
Creators: name: Zhou, Ding
Publisher: name: Imperial College London
Year: 2016

description: Major tunnelling projects in London have generated enormous amounts of excavated clay, and there will be even larger production of excavated London clay in the next few years. This research focuses on investigating the technical feasibility of processing excavated London clay into a supplementary cementitious material (SCM) suitable for the use in concrete. Excavated London clay was calcined at a range of temperatures between 600 and 1000 °C for 2 hours. The as-received and calcined London clay samples were characterized using techniques including XRF, XRD, FTIR, TGA/DTG, ICP, SEM, nitrogen adsorption, laser diffraction, isothermal conduction calorimetry and pycnometry. London clay is a complex mix of various types of clay and non-clay minerals, such as kaolinite (30.2 wt.%), illite (11.9 wt.%), montmorillonite (41.3 wt.%), chlorite, pyrite, goethite, feldspar and quartz (16.6 wt.%). Calcining excavated London clay resulted in oxidation, dehydration, dehydroxylation, amorphization and recrystallization, causing significant compositional and structural changes to clay and non-clay minerals. The degree of change depended on the calcining temperature. At 600 °C, kaolinite was entirely dehydroxylated, and the removal of octahedral hydroxyls led to a collapse of the 1:1 layered structure. As a result, metakaolin was formed. In contrast, the dehydroxylation of illite and montmorillonite started below 600 °C but finished at around 800 °C. Additionally, the two clay minerals did not suffer significant loss in crystallinity from complete dehydroxylation. The collapse of the 2:1 layered structure of illite and montmorillonite took place only when the calcining temperature was 900 °C and above. It was also observed that the recrystallization of spinel occurred above 950 °C. The assessment of pozzolanic reactivity for calcined London clays was performed using the strength activity index (SAI) test, Frattini test, portlandite consumption test and the Chapelle test. The results showed that excavated London clay can be transformed into a SCM by calcining, and the optimum calcining temperature is 900 °C. The decrease at 950 °C can be attributed to the occurrence of spinel recrystallization. London clay calcined at 900 °C was used to produce concrete at replacement levels up to 30 wt.% and three water-to-binder ratios (0.3, 0.4, 0.5). A CEM-I replacement of up to 30 wt.% showed no detrimental effect on workability or the compressive strength of concrete. In addition, the concrete with 30 wt.% of CEM-I substituted by calcined London clay and a w/b ratio of 0.3 had greater strength than control concrete after 28 days curing. At a replacement of 20 wt.% and a w/b ratio of 0.4, the concrete containing calcined London clay had similar 90-day compressive strength to those incorporating pulverised fuel ash, ground granulated blastfurnace slag and silica fume. Carbon emission estimation showed that a 30 wt.% substitution of CEM-I by calcined London clay in concrete produces 27% less CO2 emission compared to 100 wt.% CEM-I. This study has demonstrated that it is technically feasible to use calcined London clay as a supplementary cementitious material for use in concrete.