Sonjoy Deb, B.Tech, Civil
“With depleting quantity of key construction resource “River Sand” and with increased construction requirement, the need for alternative material to River Sand is immense. Stabilized Excavation Soil offers an alternative to this scarce “River Sand”. However, the research support is needed to establish a formal understanding for usage of the same. Various research findings from different types of soils with low, medium and high plasticity (with different proportion and type of clay) which were used as fine aggregates to produce cement mortar are presented here. The same soil samples were treated by dry sieving and/or stabilized with calcium bearing stabilizers to improve the mortar properties. Treated plastic soils were then used as fine aggregate in cement mortar. Mortar properties such as strength and shrinkage were determined and compared with control mortar made of river sand. The key benefits obtained is lower carbon footprint and embodied energy of the resultant mortar mix ”
River sand is used as a common and convenient fine aggregate source in concrete for several decades. However, the rate of depletion of river sand has led to search for alternatives. Many studies have been made on the use of industrial by-products or recycled materials as a partial or full replacement of fine aggregate in concrete applications. Crushed stone aggregate is a widely accepted fine aggregate alternative. Other naturally available materials investigated in the literature are dune sand, laterite soil, excavation soil (raw earth) and marine sediments. Each of these alternatives affects the concrete properties in different ways when used as fine aggregate (Table 1). For example, angular shape of crushed stone aggregate affects the workability, whereas the presence of excessive fines is reported to be a common problem in most other alternatives. On the other side, excavation soil generated from mines over-burden and large-scale construction work contains clay. These soils cannot be directly used in cement mortar/concrete as the presence of clay causes high shrinkage strains. Alternative systems like geopolymerisation facilitate the use of clay bearing materials by positively exploiting the clay as Si-Al precursors. However, treating the raw material becomes inevitable to reduce/eliminate the problems of fines, clay and other unwanted inclusions.
Sieving with water or combination of washing and sieving is commonly adopted in crushed stone to remove fines and in off shores and to remove chlorides. Wet sieving in excavation soil with 75 mm size sieve removed up to 40% of fines and clay and observed to be an effective method, provided the wash water and residual clay are properly managed. Usage of wash water obtained from washing treatment of foundry sand was studied for its use in concrete. However, the quality of wash water depends on the type of material processed. Wet sieving requires adequate water and a proper facility for processing the wash water for reuse. Under these circumstances, there is a need to identify alternative method of treatment of excavation soil. In the present study, the effectiveness of simpler and traditional soil treatment techniques viz., i) granulometric adjustment by removing the fines and ii) stabilization to reduce the clay reactivity, on the properties of processed soil and its performance as fine aggregate in mortar have been evaluated.
Stabilization is widely used in geotechnical applications for earth-based materials, embankments, road sub-base and foundations. Materials possessing calcium ions, primarily lime is used as the stabilizer. Lime is known for its effectiveness in stabilization, as the calcium ions in lime diffuses through cluster of clay particles. Natural pozzolans and industrial wastes which have calcium ions are also adopted as stabilizers in recent times to make the process environmental friendly. Such stabilizers are reported to provide both pozzolanic and stabilizing effect with reactive silica-alumina and calcium respectively. Though a large number of studies are available on geotechnical application of soil stabilization, there is a need for under standing the behaviour of stabilized soil when used as fine aggregate in cement mortar.
In this study, soils of different plasticity and clay mineralogy were chosen and treated by dry sieving, stabilization and combination of both. Lime (with 97% Ca(OH)2) and slag (Ground Granulated Blast furnace Slag, GGBS) were used as stabilizers. Lime was preferred due to the availability of high calcium content. Slag was chosen as cementitious stabilizer to compare the effectiveness with that of lime. The treated soils were then used as fine aggregate in cement mortar. The mortar properties such as dry density, compressive strength, water absorption, and shrinkage strain were compared with control mortar made of river sand.
Materials and methodology Adopted In The Research Studies
Dry sieving process
Out of various soil types, three types of soils from local excavation works can be chosen based on their plasticity index viz., Low (LP), medium (MP) and high (HP) soils. Combination of kaolinite, illite and montmorillonite clay minerals were present in these soils, making them vary in their plasticity characteristics. Dry sieving can be done with 600 mm size sieve to remove clay and fines. Percentage distribution of sand, silt and clay size particles in low medium and high plastic soils before and after dry sieving process are determined by hydrometer analysis and a typical range is presented in Figure 1. Dry sieving of plastic soil removed 5, 12, 16% of clay (<2 mm) and 5, 8, 8%of silt (2–75 mm) in low plastic, medium and high plastic soils, respectively. Even after dry sieving, the percentage particles <75 mm (silt + clay) present in the low, medium and high plastic soil were 19, 28 and 34%, respectively. This can be attributed to the adhesive nature of clay on to sand particle and hence did not get removed completely with dry sieving. Hence, there is a need to stabilize the sieved soil to treat the clay present in them.
Soil samples passing 4.75 mm can be used for stabilization studies and are listed in Table 1. Two stabilizers viz., cementitious stabilizer (commercially available processed GGBS) containing reactive silica–alumina with Ca(OH)2 and a hydraulic stabilizer(lime) with 97% of Ca(OH)2 can be chosen. The chemical composition of slag (GGBS) is given in Table 2. As a trial measure, lime up to 10% and slag up to 20%effectively improves the compressive strength and reduces the shrinkage strain of cement mortar. Hence, the respective maximum dosages were limited at these levels.
It is reported that for proper hydration of the soil with stabilizer and to form non-reactive crystalline products, 20% additional water content to that of Optimum Moisture Content (OMC) and a curing age of 28 days should be adopted for stabilization. Hence, OMC was first determined for each soil to fix the water content needed for the stabilization process. After dry mixing the soil sample with predefined dosage of lime/slag in a Hobart mixer, water content of OMC + 20%was used to make the slurry of the soil-stabilizer mix. The slurry was allowed for curing and stabilization for 28 days in a closed container to avoid loss of moisture. Before proceeding to study the mortar properties with stabilized soils, the index properties of stabilized soils were determined which can be used to assess the relative effectiveness of the stabilizers and its dosages on different types of soil.
Effect of dry sieving of soil in cement mortar properties
Research has shown that for maintaining constant workability, cement mortar with unprocessed low, medium and high plastic soil requires 44, 62 and 72.5% higher water content than that of mortar with river sand. Affinity of clay particles towards water is the main reason behind this behaviour. Kaolinite mineral in the low plastic soil is charge balanced, whereas montmorillonite in medium and high plastic soil has affinity towards H+ ions present in water molecules and adsorbs them in their interlayer spaces. This results in demand for excess water to maintain the workability. Such higher water content is undesirable as it adversely affect the properties of hardened mortar. Though it is only the clay particles that have affinity towards water, silt particles also contribute to the increases in water demand due to the increase in surface area to be wetted. Removal of some of these particles by dry sieving reduced the water demand by 8, 22, and19% for mortar with low, medium and high plastic soils, respectively.
“To maintain constant workability, cement mortar with unprocessed low, medium and high plastic soil requires 44, 62 and 72.5% higher water content than that of mortar
with river sand”
Compressive strength of cement mortar with unprocessed soil varies from 5 MPa to 21 MPa while control mortar with river sand shows a compressive strength of 46 MPa with a corresponding dry density of 2105 kg/m3. In mortar with unprocessed soil, the clay remains in un-stabilized layers (Fig. 2a) which agglomerates, shrinks and forms weak, porous reaction products in cementitious composites. Those reaction products near the interfacial transition zone (ITZ) of aggregates results in poor bonding and crack formation, reducing the density and strength drastically (Fig. 2b). With increasing soil plasticity, water demand and invariably the porosity and permeability of the mortar mix increases, resulting in reduced density and strength properties. The reduction in water demand with dry sieving of soils led to an increase in the dry density of mortar by 100–200 kg/m3 and a corresponding improvement in compressive strength of 23,37, and 80% for mortar with low, medium and high plastic soil, respectively. However, high plastic soil with almost 60% of clay and fines did not result in significant reduction in drying shrinkage after dry sieving as considerable amount of highly reactive clay particles were retained in the sieved high plastic soil. Though the increase in shrinkage strains can be related to the increase in water demand of the mortar with clayey aggregates, it has been proved that the addition of extra water does not affect the overall shrinkage in cement system with clay content. This is attributed to the fact that this extra water helps in the improved compaction of such mixture containing clay and hence reduces the shrinkage rate. Hence, the shrinkage caused is mainly due to the action of clay mineral to the exposed environment rather than the amount of water content. As the presence of bound clay in dry sieved soils affects the mortar properties at varying degree, as a next step, stabilization was adopted for both unprocessed as well as dry sieved soils to assess the performance of these soils in cement mortar.
Effect of stabilization of unprocessed and sieved soil in cement mortar properties
Water demand for constant workability
Research has also shown that for a given dosage of stabilizer, mortar with lime or slag stabilized LP soil demands almost the same water content, even though the amount of calcium available for the reaction is higher in lime compared to slag. Similar behaviour is observed in cement mortar with sieved low plastic soil with an average reduction in water demand of 34%. Low plastic soil with charge balanced kaolinite clay mineralis comparatively not an active participant of stabilization reactions. However, at higher stabilizer dosage, mortar with sieved low plastic soil could achieve a desired workability at lower water content comparable to cement mortar with river sand. This improvement in workability must be due to the dilution effect in the clay particles with lime/slag that reduces the water demand.
It has been reported that as the water requirement for constant workability reduces with stabilizer dosage, the dry density increases irrespective of type of soil and stabilizer. For a given stabilizer dosage, mortar with slag stabilized LP soil shows higher density increment compared to mortar with lime stabilized LP soil. The reactive silica and alumina together with Ca(OH)2in slag contributes to the formation of dense hydration products during the stabilization process. This seals the non-reactive clay and fine particles resulting in increase in the dry density. At a dosage of 20% slag, mortar with sieved and stabilized low plastic soil could reach a dry density of 2050 kg/m3 which is almost equal to that of control mortar.
The compressive strength of mortar with stabilized soil increases with stabilizer dosage. In mortar with LP soil, lime stabilization does not contribute to significant increase in strength and density improvement beyond 6% dosage whereas, slag stabilization shows linear improvement up to a dosage of 20%. With slag, there is a dense reaction product formation resulting in reduction in pore size and increase in strength. For a given stabilizer dosage, mortar with medium and high plastic soil shows higher compressive strength with slag stabilization compared to that of lime stabilization.
“For a given stabilizer dosage, mortar with
medium and highplastic soil shows higher
compressive strength with slag stabilization compared
to that of lime stabilization.”
Water absorption of cement mortars with stabilized soils for a constant stabilizer dosage, shows reduction in water absorption is higher for slag compared to lime as stabilizer in mortar with stabilized low plastic soil. This can also be related to the increased strength and density of the mortar with slag stabilization. With the combination of sieving and stabilization, the mortar with LP soil could reach a water absorption value equal to control mortar at 20% dosage level of slag.
Drying shrinkage strains
As per research findings, irrespective of type of stabilizer, an increase in stabilizer dosage reduces the drying shrinkage strain. Mortar with stabilized low plastic soil does not exhibit significant reduction in shrinkage strains beyond the dosage of 4% lime and 10% slag. For a constant dosage of stabilizer, mortar with low plastic soil shows a 10%higher reduction in shrinkage strain with slag stabilization compared to lime. This is attributed to the physicochemical phenomenon exhibited by the slag stabilization of soil. Mortar with slag stabilized LP soil shows a dry density value of2050 kg/m3 and a corresponding drying shrinkage of 1300 microstrains. Mortar with lime stabilized low plastic soil with the dry density of 1800 kg/m3 could reach a shrinkage value of 2300 micro strains without sieving process.
The cost of the construction material is location specific and large variation exists all over the world. Here, to analyse the economic feasibility of the stabilization process for excavation soil in fine aggregate production, the embodied energy for a typical scenario is calculated which can be related to the economic viability. The non-renewable energy used in the acquisition, processing and transportation of any material is termed as embodied energy. The energy consumption and carbon footprint involved in various treatments of excavated soil are compared with conventional fine aggregate i.e., river sand in Table 3.
Though naturally occurring river sand does not add to the embodied energy, it is normally transported from the remote area to the site at an average distance of 50–100 km. This raises its embodied energy to 0.2 MJ/kg. It is possible to restrict the transportation distance of excavated soil and using it in local construction sites which could reduce the transportation energy of these materials. However, processing of excavation soil adds to the energy consumption which is very less compared to that of energy involved in river sand procurement. Hence, the transportation of river sand from remote places adds to the embodied energy which also increases the cost of the material. This is also reasonable in the point of waste reutilization and environmental conservation by using excavated soil waste which will be otherwise dumped in landfills. This helps in reducing, recycling and reusing of waste, resulting in sustainable circular economy.
Dry sieving of soil removes some of the fines and clay particles, thereby improves soil-based mortar properties. However, this can be an effective method to improve cement mortar properties with non-plastic or low plastic soils. Water demand of mortar increased with increasing soil plasticity which could be reduced by stabilization. Combination of sieving and stabilization of soil helps in controlling the water demand to maintain a constant workability. Dry density and compressive strength improved with slag stabilized plastic soil mortars compared to lime stabilized soil mortar. Water absorption and shrinkage are influenced by factors such as clay mineralogy, stabilizer type and dosage. Soil with nonreactive clay minerals, stabilized with slag gives lowest water absorption and shrinkage strains. Whereas, lime performed as a better stabilizer in soil with high plasticity. The significant improvement observed in Very lower embodied energy and carbon footprint content of the resultant mortar mix. The research and Academic institutions needs to further deep dive on this and establish a common guidelines for industries to follow and also propose codal provisions for the same because unless there is a codal provision, the construction industry adaptability of this will be a major challenge.