1 Introduction

Increasing the population in sedimentary environments with soft soils shows the necessity of escalating the load capacity of soil. Various soil improvement techniques have been studied such as chemical stabilization methods. The most used stabilizer for sand is cement. Cement is a soft roundness, water absorbent and binder. Neri (2015) reported the effect of cement and the effect of high pressure slurry injection on the hydro-mechanical properties of the soil before and after injection. Results showed that the increment of strength and the elasticity of hardness as well as decrease the permeability over time. However, soil density, particle size distribution and degree of cementation has significant correlation with the value of hardness and strength [2]. However, porosity and moisture content are key parameters to control the strength of the treated soil [3]. For example, the moisture content in samples with water to cement ratio up to 0.4 is not sufficient for hydration of cement particles [4]. Therefore, researches in the application of cement in slope stability, reconstruction of roads, foundation of road construction, refineries, reduction of permeability in reservoirs and protection of rivers was reported [5,6,7].

On the other hand, cement manufacturing requires a calcium source (usually limestone) and a source of silicon (such as clay or sand). Cement manufactured through the treatment of cooked calcium oxide with silicon oxide and iron oxide. Then, the material turns into almost black colored balls called clinger. In order to adjust the setting time, after cooling, the clinger is mixed and grounded with some gypsum. Therefore, the gray powder is produced. Under this process, lime and carbon dioxide (CO2) are produced as shown in reaction (1). The process is highly energy and emissions intensive because of the extreme heat requirement. Producing a ton of cement requires 4.7 million BTU of energy and generates CO2 which is a concern in environmental engineering.

$$ {\text{CaCO}}_{ 3} \to {\text{CaO}} + {\text{CO}}_{2} $$
(1)

Two sources are known for the release of CO2 in the cement production process. Firstly, combustion of fossil fuels for the activity of the rotary ovens, which are the largest sources for CO2 production. Secondly, the chemical process of converting limestone to lime. Thus, a total of 1.3 tons of CO2 per ton of cement is released into the atmosphere. Hence, it can conclude that soil improvement techniques using cement grouting have potential drawbacks such as high cost, high energy consumption and sometimes negative environmental impacts.

An alternative approach is to use biocement to improve the engineering properties of sand. Microbially induced calcite precipitation (MICP) has been introduced as a technique using reagents for modification of geotechnical properties of sand since 2005.

The process involves two main parts: (1) absorption of urease enzymes or urease cells on sand aggregates; (2) hydrolysis of the urea enzyme and formation of calcium carbonate crystals in the presence of calcium ions.

Biocement has been considered as an appropriate solution for replacement with cement grouting because of low viscosity and being environmentally friendly. However, among many studies concerning MICP technique, there are few studies considering the comparison of cost and environmental impacts of cement grouting and biocement.

The primary component of cement is limestone while calcium chloride also used as a crucial reagent in bio-cemented samples. Therefore, the present study discussed the comparison of conventional and innovative methods. In the grouting method, Portland cement was used as a chemical substance. Note that, the environmental concerns in the present study was focused on the calcium usage in Portland cement as well as biocement. Sariosseiri and Muhunthan (2009) reported that a compressive strength equal or greater than 345 kPa is required for an effective soil improvement. Therefore, it was chosen as a standard to compare the cement and bio treated methods of the present paper.

2 Materials and Methods

2.1 Materials

In the present study, sand from Garmsar region, Semnan province in Iran was provided. The size distribution curve according to ASTM 2487 has been carried out as shown in Fig. 1. Sand was categorized as SP based on its characteristics with the specific gravity of 1450 kg/m3. Shahrood Portland cement, Type 2, with a maximum setting time of 4 h (GS = 3.15 g/cm3) was selected as the stabilizer. Physical characteristic of Portland cement was reported in Table 1 [9].

Fig. 1.
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Size distribution curve

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Table 1. Physical characteristics of Portland cement (type II)
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2.2 Methods

Samples was prepared in a PVC mold of 50 * 100 mm. As shown in Table 2, the percentage of Portland cement (C) and water to cement (w/c) ratio has been chosen based on literature [1, 10,11,12]. Samples were treated with Portland cement using four procedures in room temperature. Procedures was chosen as close to the bio-treatment of sand. Detailed of the procedures has been reported in Table 2. The remarkable point after removing samples from the mold was that the method was fixed only at the points where the injection was performed. Therefore, the sand improvement was not uniform and the sample would be disrupted after the time elapsed. This was seen in samples of procedure II and III. Therefore, the UCS of samples of procedure I and IV was tested. In procedure I, samples were treated using mechanical mixing of sand, cement and water. While, in procedure IV, cement and sand was homogenously mixed and placed in the mold in a dry state. Then, a flow rate of water was injected. Among various setting time reported in literature [12, 13], 7 and 28-days was selected for samples for further investigation. Later on, selected samples were tested using unconfined compressive strength (UCS). Note that the code of selected samples were shown in Table 2.

Table 2. Testing program
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3 Results and Discussion

3.1 Comparison of Cement Treated Sand with Procedure I and IV

Energy consumption is one of the most important environmental issues related to cement production. Cement is one of the industries that consumes the most energy which includes direct fuel consumption for the extraction and transportation of raw materials. Therefore, less environmental degradation methods are in a favor.

Comparison of cement treated sand using procedure I and IV based on UCS, amount of Portland cement and calcium are presented in Fig. 2. The results show that the only method to prepare samples with less than 60 (kg/m3) is the injection method. Calcium is the most important component of cement that supplies it through the use of limestone. Also, it plays a significant role in increasing of compressive strength. In the same calcium content as shown in common part of the graphs of Fig. 2, it can conclude that the injection method produces higher UCS for the same calcium content.

Fig. 2.
figure 2

Compression of UCS and Calcium usage in the Portland cement treatment

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3.2 Comparison of Conventional and Microbial Soil Improvement Techniques

The use of microbial processes has been considered in recent years for in situ soil improvement. Biocementation has some advantages over existing technologies, such as less calcium usage in same UCS. The data obtained from previous studies and present study was gathered in Table 3. It can be stated that the calcium usage in biocement is half of cement treated samples. Therefore, it can surely state that one of the key benefits of bio-stabilization is to significantly reduce the loss of energy and eliminate carbon emission.

Table 3. Comparison of calcium (%) usage in biocement and Portland cement [9]
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Regarding to the cost of biogeochemical stabilization projects, it was reported that the cost was a factor of process and the specific details of each project. Despite the very limited field applications that have been done. The actual cost of various upgrading processes has remained largely unknown. To date, there have been widely differing studies and estimates due to revised and optimal designs that are ongoing to date. For MICP, the cost of materials (urea, calcium) and the total cost of consolidation (materials, equipment, and installation) in saturated soils were estimated. Cheng (2012) studied the economic solutions which led to a reduction in the cost of cultivating bacterial and the cost of chemicals [15]. Production of urease-producing bacteria is a major contributor to costs, including laboratory costs, equipment, implementation, chemicals, sterilization and transfer of the environment from the biotechnology company to the site. In research, they cultivated bacteria in a non-sterile environment, thus reducing the cost of 50% compared with sterilized culture media [15]. Comparison of UCS and cost in biocement (Cheng 2012) and Portland cement treatment (present study) were shown in Fig. 3.

Fig. 3.
figure 3

Comparison of UCS and cost in biocement (Cheng 2012) and Portland cement treatment (present study)

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As shown in Fig. 3, the modified biocementation, with non- sterile culture and the use of gravity flow, significantly reduces the costs. However, the results of treated samples with mechanical and injection methods with UCS equal to 700 kPa, are about $ 22 cheaper than the modified biocementation.

4 Conclusion

Cement is considered as an unhealthy substance because of its high energy consumption, extraction of large amounts of raw materials and land degradation. Therefore, the innovative bio-stabilization has been compared with the results of Portland cement. The results of samples treated by Portland cement indicated the direct correlation between C% and UCS. Samples treated with 6, 8, 10 and 12% after 28-days showed the increment of USC by 70, 108, 25 and 40% respectively in comparison with 7-days.

The results show the only method to prepare samples with cement less than 60 (kg/m3) is the injection method. In the same calcium content, it can conclude that samples treated with the injection method showed higher UCS. Regarding to the comparison of calcium usage, it can be stated that the calcium usage in biocementation is half of Portland cement grouting. Additionally, the results of samples treated with mechanical and injection methods with UCS of 700 kPa, are cheaper than the modified bio-treatment.