Clay Microstructure


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The MIP samples were obtained by carefully breaking the stabilized samples with a chisel. The representative samples of mm pieces weighing between 1. Hydration of the samples was stopped by freezing and drying, as prepared in the SEM examination.

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Mercury porosimetry is expressed by the Washburn equation Washburn, Due to the heat, the water is lost, leading to the decrease in overall weight. The amount of Ca OH 2 can be approximated from this lost water by Equation 2 , which is 4. The change of the cementitious products can be expressed by the change of Ca OH 2 since they are the hydration products. Compaction characteristics of cement stabilized clay are shown in Figure 1. Its specific gravity is 2. It is found that the maximum dry unit weight of the stabilized samples is higher than that of the unstabilized samples whereas their optimum water content is practically the same.

This characteristic is the same as that of cement stabilized coarse-grained soils as reported by Horpibulsuk et al. Thus, clay particles flocculate into larger clusters, which increases in the plastic limit with an insignificant change in the liquid limit vide Table 1. As such, the plasticity index of the mixture decreases due to the significant increase in the plastic limit. Because the OWC of low swelling clays is mainly controlled by the liquid limit Horpibulsuk et al. Figure 2 shows the compaction curve of the fly ash FA blended cement stabilized clay for different replacement ratios ratios of cement to fly ash, C:F compared with that of the unstabilized clay.

It is noted that the compaction curve of the stabilized clay is insignificantly dependent upon replacement ratio and fly ash particles. Maximum dry unit weight of the stabilized clay is higher than that of the unstabilized clay whereas their optimum water content is practically the same.

Plots of dry unit weight versus water content of the uncemented and the cemented samples compacted under standard and modified Proctor energies Horpibulsuk et al. Typical strength-water content relationships for different curing times and compacton energies of the stabilized samples are shown in Figure 3. The strength of the stabilized samples increases with water content up to 1. At a particular curing time, the strength curve depends on the compaction energy. As the compaction energy increases, the maximum strength increases and the water content at maximum strength decreases.

For the same compaction energy, the strength curves follow the same pattern for all curing times, which are almost symmetrical around 1. Figure 4 shows the strength versus water content relationship of the CFA blended cement stabilized clay at different replacement ratios after 60 days of curing compared with that of the unstabilized clay. The maximum strengths of the stabilized clay are at about 1. This is because engineering properties of unstabilized clay are mainly dependent upon the densification packing.

Strength versus water content relationship of the CFA blended cement stabilized clay at different replacement ratios and 60 days of curing Horpibulsuk et al. Figure 5 shows the strength development with cement content varied over a wide range of the stabilized samples compacted under the modified Proctor energy at 1. The strength increase can be classified into three zones.

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As the cement content increases, the cement per grain contact point increases and, upon hardening, imparts a commensurate amount of bonding at the contact points. This zone is designated as the active zone. Beyond this zone, the strength development slows down while still gradually increasing. The incremental gradient becomes nearly zero and does not make any further significant improvement.

This zone is identified as the deterioration zone. Influence of replacement ratio on the strength development of the blended cement stabilized clay compacted at water content w of 1. The sudden strength development with time is not found for all replacement ratios. This finding is different from concrete technology where the role of fly ash as a pozzolanic material comes into play after a long curing time generally after 60 days.

In other words, the strength of concrete mixed with fly ash is higher than that without fly ash after about 60 days of curing. Relationship between strength development and replacement ratio of the CFA blended cement stabilized clay at different curing times Horpibulsuk et al. For compacted fine-grained soils, the soil structure mainly controls the strength and resistance to deformation, which is governed by compaction energy and water content.

Compaction breaks down the large clay clusters into smaller clusters and reduces the pore space. Figure 7 shows SEM photos of the unstabilized samples compacted under the modified Proctor energy at water contents in the range of 0. On the wet side of optimum vide Figure 7c , a dispersed structure is likely to develop because the quantity of pore water is enough to develop a complete double layer of the ions that are attracted to the clay particles. As such, the clay particles and clay clusters easily slide over each other when sheared, which causes low strength and stiffness.

On the dry side of optimum vide Figure 7a , there is not sufficient water to develop a complete double-layer; thus, the distance between two clay platelets is small enough for van der Waals type attraction to dominate. Such an attraction leads to flocculation with more surface to edge bonds; thus, more aggregates of platelets lead to compressible flocs, which make up the overall structure.

At the OWC, the structure results from a combination of these two characteristics. Under this condition, the compacted sample exhibits the highest strength and stiffness. SEM photos of the uncemented samples compacted at different molding water contents under modified Proctor energy Horpibulsuk et al.

After 4 hours of curing, the soil clusters and the pores are covered and filled by the cement gel hydrated cement vide Figure 8a. Over time, the hydration products in the pores are clearly seen and the soil-cement clusters tend to be larger vide Figures 8b through d because of the growth of cementitious products over time vide Table 2. The effect of curing time on the pore size distribution of the stabilized samples is illustrated in Figure 9. It is found that, during the early stage of hydration fewer than 7 days of curing , the volume of pores smaller than 0.

This result shows that during 7 days of curing, the cementitious products fill pores smaller than 0. After 7 days of curing, the volume of pores larger than 0. As a result, the volume of small pores smaller than 0. Figures 10 a-c, 10d-g, and 10h-j show SEM photos of the cemented samples in the active, inert, and deterioration zones, respectively. As the cement content increases in the active zone, hydration products are clearly seen in the pores vide Figures 10b and c and the cementitious products significantly increase Table 3.

The cementitious products not only enhance the inter-cluster bonding strength but also fill the pore space, as shown in Figure 11 : the volume of pores smaller than 0. As a result, the strength significantly increases with cement. This results in an insignificant change in the pore size distribution and, thus, the strength. For the deterioration zone Figure 10 h-j , few hydration products are detected.

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Both the volumes of the highest pore size interval 1. This is because the increase in cement content significantly reduces the water content, which decreases the degree of hydration and, thus, cementitious products Table 3.

General concepts of cement - Hydration and microstructure, K. Scrivener, EPFL

It is noted that the hydration products growing from the cement grains connect fly ash particles and clay-cement clusters together. Some of the surfaces of fly ash particles are coated with layers of amounts of hydration products. However, they are still smooth with different curing times.

This finding is different from concrete technology where the precipitation in the pozzolanic reaction is indicated by the etching on fly ash surface Fraay et al. This is because the input of cement in concrete is high enough to produce a relatively high amount of Ca OH 2 to be consumed for pozzolanic reaction.

From this observation, it is thus possible to conclude that the pozzolanic reaction is minimal for strength development in the blended cement stabilized clay. SEM photos of the cemented samples compacted at different cement contents under modified Proctor energy after 7 days of curing Horpibulsuk et al. Pore size distribution of the cemented samples compacted at different water contents and under modified Proctor energy after 7 days of curing time Horpibulsuk et al. Ca OH 2 of the cemented samples compacted at different cement contents under modified Proctor energy after 7 days of curing.

SEM photos of the blended cement stabilized clay at different replacement ratios after 28 days of curing Horpibulsuk et al. SEM photos of the blended cement stabilized clay at different replacement ratios after 60 days of curing Horpibulsuk et al. Figure 14 shows the pore size distribution of the CFA blended cement stabilized clay at different curing times and replacement ratios.

The pore size distribution for all replacement ratios is almost identical since the grain size distribution and D 50 of PC and CFA are practically the same. This implies that the strength of blended cement stabilized clay is not directly dependent upon only pore size distribution.

Microstructure based simulation of clay composites thermal properties - IEEE Conference Publication

However, it might control permeability and durability. This is due to the growth of cementitious products filling up pores. For a particular water content and curing time, Ca OH 2 for the CFA blended cement stabilized clay decreases with replacement ratio only when the replacement ratio is in excess of a certain value.

This finding is different from concrete technology in which Ca OH 2 decreases significantly with the increase in fineness and replacement ratio Berry et al.

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It is thus concluded that cementitious products mainly control the strength development. In other words, the strengths of the blended cement stabilized clay having different mixing condition binder content, replacement ratios, and curing time could be identical as long as cementitious products are the same. Pore size distribution of the blended cement stabilized clay at different replacement ratios and curing times Horpibulsuk et al.

Table 4. Ca OH 2 of the blended cement stabilized clay at different replacement ratios and curing times. This implies that a number of large clay-cement clusters possessing large pore space reduce when fly ashes are utilized.


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