Tag Archives: Microstructure

Features of microstructure development in the inner product of reacting slag in blended cement

The morphology of inner product region derived from alite or belite is identical to those present in neat OPC paste. In mature pastes, fully hydrated small grains of both alite and GGBFS often display a coarse morphology.

Generally, inner product in larger grains has a dense homogeneous morphology with very fine porosity, and large slag grains often have a rim of fine textured C-S-H which can exist for many years and merge into outer product C-S-H gel. Mg, Al-rich laths or possibly platelets present within the inner product C-S-H gel, which are oriented either towards the outer boundary of the inner product or randomly even at the outer boundary.

The development of the plates occurs to different extents in different slag grains, possibly depending on composition and also appearing to depend on particle size considering platelets or laths typically appear very early in small grains. When the plates are very well developed they predominate over the C-S-H gel; however, in many inner product regions, even in 3 year old pastes, the plates are less well developed and a high proportion of C-S-H gel is still present. It is not obvious from observations in the early stages that the morphology in the inner product regions is plate-like, thus it is better to be described as laths or needles.

Another microstructural feature of the inner products is the presence of small, round and poorly crystalline particles, which are rich in iron and aluminum and also often contain significant amounts titanium.

Features of microstructure development in the outer product of reacting slag in blended cement

Similar to Portland cement, microstructure of slag hydrate could also be classified as inner product, which formed within the boundaries of the original anhydrous grains, and outer product that formed in the originally water-filled spaces.

The major phases present in the outer product region of neat OPC pastes are C-S-H gel, Ca(OH)2, AFm and AFt. The addition of slag affects the morphology of the microstructure of slag blended cement. The morphology of the outer product C-S-H varies with chemical composition, viz. at high Ca/Si ratio it has a fibrillar morphology which gradually changes to foil-like with a reduction in Ca/Si ratio. In pure OPC or with low slag loading, amorphous C-S-H has a strongly linear directional characteristic being fibrillar in its appearance in the TEM. At approximate 75% GGBFS regions of foil-like C-S-H form without these linear characteristics.

As the slag fraction is increased, the foil-like morphology gradually replaces the fibrillar morphology. However, the fibrillar feature of C-S-H gels occurs in all blends except neat GGBFS system. Foil-like morphology may be coarser or finer, which depends on the space constraints upon the development of C-S-H gel. Although the linear and fibrillar morphology has fine porosity, its inefficient filling of space appears to leave some fairly coarse, interconnected pores, while the more evenly distributed pores of the foil-like C-S-H are probably less well interconnected, which may account for the beneficial effects of slag in reducing diffusion rates in blended pastes, thus being largely responsible for the improved durability performance.

The aluminate hydrates AFt and AFm can occur in all the OPC-bearing blends. These phases are found to be identical in morphology to those present in neat OPC pastes, which has a needle-like morphology. Special attention should be paid when preparing the specimen because AFt phase may suffer dehydration.

Microstructure development of limestone blended cement

Though the chemical reaction of the fine particles of limestone in cement paste is limited to a low extent due to its inert property, the addition of limestone particles has significant effect on the microstructure development. Research conducted by Sellevold et al. using mercury intrusion method showed that specimens containing 12% CaCO3, which have the main body of the particles smaller than 0.1 µm had finer pore structure and somewhat reduced total pore volume.

They attribute this observation to the nucleation effect: The introduction of a large number of nucleation sites could result in a more homogeneous distribution of calcium silicate hydrate and thus a less open pore structure. The effect appears to be in the capillary pore structure rather than in the gel pores. The size and distribution of CH are also influenced by the presence of limestone.

In a limestone-filled paste which used 25% limestone larger regions of calcium hydroxide were unevenly distributed throughout paste while in the case of Portland cement small regions calcium hydroxide were evenly distributed. Further investigation found that limestone added in amounts of 5% or 25% enhance the formation of hydration rims of calcium silicate hydrate surrounding C3S particles due to the accelerated hydration rate of C3S.

Results from the image analysis showed that the interface between limestone and hydrates in limestone-filled self-compacting concrete (SCC) is quite porous, while the interface around the unhydrated cement in high performance cement paste is much denser.
Figures show a comparison of microstructure using BSE between limestone-filled cement paste and Portland cement paste. Two possible reasons could be responsible for this distinct difference in mcrostruture.

On the one hand, as the hydration process of cement proceeds, cement particles expand and get connect with each other to form a network, which result in strength gain; However, limestone particles do not expand during the whole hydration process due to its low reactivity, thus making the porosity of the former becomes lower than the latter one.

On the other hand, the addition of limestone may play a negative role in the microstructure development process, because it could block the expansion of hydrating cement, which makes the microstructure of limestone-filled cement paste more porous as showed in figure.

limestone-HPClimestone
Comparison of BSE images of limestone cement (up) and Portland cement (down) at age of 7 days

Research by Pipilikaki et al. showed that limestone used in blended cements changes hardened cements’ pore structure. The capillaries pore size is increased from 20 nm to 40 nm when the maximum amount (35%) of limestone that is allowed by EN 197-1. At the same time, the addition of limestone reduces the threshold diameter of the paste and presents a more uniform pore size distribution.

The author concluded that this behavior can be attributed to the better packing of particles due to the filling effect of the limestone powder, resulting in that chemical species enter the hardened cement more difficult but transport easier inside the cement paste due to the pores have approximately the same size. As gel pores are concerned, addition of limestone in cement produces smaller gel pores probably due to the accelerated hydration rates.