Cement Manufacturers Association (CMA)

18 with aluminium rich phases in cement such as calcium aluminate. Kaolinite is the most reactive form of clay mineral for cement applications. Clay with different amounts of kaolinite can be observed in different regions of the world, intermixed with impurities such as quartz, limestone, iron bearing phases and other rock forming minerals. Calcination of kaolinitic clay between 600- 800 0 C leads to the removal of OH- groups (dehydroxylation) from the crystalline structure to give a more structural disorder called metakaolin. When an external source of amorphous alumina adds to this metakaolin, limestone can further react with them. This so called synergic effect of limestone/metakaolin leads to the formation of an increased amount of hydrates which further fills in the porosity, increasing strength and reducing permeability of LC 3 . Calcination and Grinding of calcinedd clay Calcinedd clay can be produced using different thermal processes. This includes calcination in rotary kilns and flash calcination, which appear as the most promising alternatives on an industrial scale. Flash calcination exposes the material to much high temperature gradients (103-105 0 C/s) over a short period of time (usually 0.2-1s) leading to a higher specific surface area as compared to calcination in a rotary kiln. Thus flash calcination has been found to produce calcinedd clay with slightly higher reactivity when compared to static or rotary calcination. However, this difference is only significant at very early ages as afterwards the kaolinite content becomes predominant. In laboratory conditions, LC 3 constituents are normally ground separately in open-circuit grinding configuration. On the other hand, the most common grinding process in cement plants is the intergrinding of cement constituents in closed-circuit units. The main difference between separate and intergrinding units is that during intergrinding the components interact with one another. These interactions are mostly due to their differences in grindability. In the case of LC 3 , calcinedd clay and limestone have higher grindabilities (softer particles) compared to clinker (harder particles). Thus, with intergrinding, clinker tends to remain concentrated in the coarse fraction, (reducing its reactivity), while calcinedd clay and limestone become much finer, which may have a detrimental effect on workability. Workability and reactivity can be improved by separate grinding and optimization of the particle size distribution of the components. Compressive strength A benchmark test of compressive strength was carried out on standard mortar using LC 3 -50 with a clay to limestone ratio of 2:1 (50% clinker, 30% calcinedd clay, 15% limestone, and 5% gypsum). Avet and Scrivener (2018) have tested more than 50 clay samples with their kaolinite content Figure 1: Compressive strength evolution over time for LC 3 -50 mixes with clay of varying calcinedd kaolinite content. LC 3 -50 90 days 28 days 7 days 3 days 1 day R 2 =0.88 R 2 =0.88 R 2 =0.84 R 2 =0.62 R 2 =0.49 0 20 40 60 80 100 10 20 30 40 50 60 70 80 Calcined kaolinite content (%) Compressive strength (MPa)