Earth buildings are still a common type of residence for one-third of the world’s population. However, these buildings are not durable or resistant against earthquakes and floods, and this amplifies their potential harm to humans. Earthen construction without soil binders (e.g., cement) is known to result in poor strength and durability performance of earth buildings. Failure to use construction binders is related to the imbalance in binder prices in different countries. In particular, the price of cement in Africa, Middle East, and Southwest Asia countries is extremely high relative to the global trend of consumer goods and accounts for the limited usage of cement in those regions. Moreover, environmental concerns regarding cement usage have recently risen due to high CO2 emissions. Meanwhile, biopolymers have been introduced as an alternative binder for soil strengthening. Previous studies and feasibility attempts in this area show that the mechanical properties (i.e., compressive strength) of biopolymer mixed soil blocks (i.e, both 1% xanthan gum and 1% gellan gum) satisfied the international criteria for binders used in earthen structures. Economic and market analyses have demonstrated that the biopolymer binder has high potential as a self-sufficient local construction binder for earth buildings where the usage of ordinary cement is restricted.
Earth has been the most commonly used material for building and construction since the beginning of human civilization. Since the Industrial Revolution, diverse building and construction materials such as cement and steel have become the basis of modern civilization and have replaced the use of conventional building materials (i.e., earth and wood). However, it was reported that about 30% of residential buildings were still made of earth as of 1994 (Figure
Global distribution of earth buildings [
Earth house types can be categorized by the usage of construction binders (e.g., cement) and the main formation method [
Diverse types of binders made for construction have been used widely for soil buildings, but the production of binder also entails the generation of carbon dioxide. The production process for cement (which is the most universal binder for construction) has specifically been noted as the source of about 5% of global greenhouse gases (CO2) and accordingly the necessity of restraining the use of cement has been raised [
Market price of cement1 in each country.
Continents | Countries | Cement price (USD/ton) | Continents | Countries | Cement price (USD/ton) |
---|---|---|---|---|---|
Africa | Niger | 280 | Asia | South Korea | 68 |
Kenya | 190 | China | 57 | ||
Mali | 203 | Japan | 125 | ||
Mozambique | 160 | India | 98 | ||
Nigeria | 223 | Pakistan | 106 | ||
Cameroon | 200 | Bangladesh | 112 | ||
Rwanda | 200 | Indonesia | 125 | ||
Morocco | 150 | America | Peru | 202 | |
Egypt | 65 | United States | 91 | ||
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Middle East | Yemen | 214 | Europe | Russia | 89 |
Afghanistan | 91 | Germany | 93 | ||
Iraq | 120 | France | 132 | ||
Kuwait | 74 | UK | 102 |
Generally, the market price of consumer goods tends to increase along with an increase of GDP per capita [
Global cement and Big Mac price trends with GDP per capita.
Biopolymers are normally composed of biodegradable polysaccharides and are generated by organisms such as algae, bacteria, and fungi by consuming carbon during cultivation. Diverse kinds of biopolymers have been discovered and developed in many fields for respective applications. In particular, with rising oil prices and the threat of increasing CO2 emissions, the market for biopolymer based plastic products has been expanding, as a replacement for high CO2 emitting products.
Several attempts to introduce biogenic biopolymers as an additive or supplement in construction engineering have been reported. Matsuoka et al. [
Xanthan gum is an anionic polysaccharide composed of D-glucuronic acid, D-mannose, pyruvylated mannose, 6-O-acetyl D-mannose, and 1,4-linked glucan [
Gellan gum is a water-soluble polysaccharide fermented from
To investigate the soil strengthening effect of biopolymer treatment, we used Korean residual soil (KRS) as the soil material in this study. KRS is well known as “
Like other adobe or soil buildings, KRS buildings have weaknesses in strength and durability. Thus, we tested the use of biopolymers as a mixing binder to reinforce the strength of natural KRS. Natural KRS from Gochang, Korea, was air-dried at room temperature (18°C) and pulverized (i.e., detachment of agglomerated soil particles) to be suitable for proper mixing.
In the laboratory, we mixed KRS with xanthan gum and gellan gum to prepare biopolymer-treated KRS cube specimens. Ordinary Portland Cement (OPC) mixed and untreated (i.e., natural) KRS samples were prepared simultaneously, to compare the strengthening behavior of biopolymer treatment with preexisting soil construction (i.e., strengthening) methods.
For biopolymer (i.e., xanthan gum and gellan gum) mixing, 1000 g of dried and ground KRS was first mixed with 10 g (i.e.,
Experimental program. (a) Biopolymer-soil mixing. (b) Mixture molding. (c) UTM after 1 day of curing (ductile). (d) UTM at 28 days of curing (brittle and strong).
For OPC mixing, the cement to soil ratio in mass units (
The details of each mixing condition are summarized in Table
Mixing conditions of biopolymer-KRS mixtures.
Specimen | Mixing condition [g] | Initial mass ratio [%] | |||
---|---|---|---|---|---|
Dried soil | Binder | Water | Binder/soil | Water content | |
Xanthan gum | 1,000 | 10 | 600 | 1.0 | 60 |
Gellan gum | 1,000 | 10 | 600 | 1.0 | 60 |
Ordinary cement | 1,000 | 100 (cement) | 600 | 10.0 | 60 |
Natural (untreated) soil | 1,000 | — | 600 | — | 60 |
In general, the compressive strength of soil-cement mixtures (i.e.,
Maximum compressive strength values of KRS specimens and design criteria for earthen structures.
Both 1% xanthan gum and 1% gellan gum treated soils show higher compressive strength values than the 10% cement mixed KRS. The strength of the soil mixed with 1% of xanthan gum was 6.31 MPa, which is more than 2.3 times higher than that of the soil mixed with 10% of Ordinary Portland Cement (i.e., 2.65 MPa). A previous study shows that 0.5% of xanthan gum in the soil mixture could increase its strength above the level of soil mixed with 10% cement [
In the initial mixing stage, biopolymers tend to adsorb water immediately and form hydrogels, which enlarge the pore space between soil particles at molding. During curing and dehydration, water evaporates from the hydrogels, rendering firmer and stronger matrices between the biopolymers and soil particles. As a result, the final dried biopolymer-soil mixture can have high strength even under relatively low dry density (i.e., 1% gellan gum = 1.35 g/cm3, 1% xanthan gum = 1.38 g/cm3, and 10% OPC = 1.44 g/cm3 in this study).
Several design criteria are set for bricks used for construction and building engineering (Table
Design criteria for earthen structures.
Design criteria | BS EN 1996-31 | BS EN 771-12 | IBC 20123 |
---|---|---|---|
Minimum compressive strength [MPa] | 2 MPa (for 10% cement : soil) |
5 MPa (soil brick) | 2 MPa (rammed earth brick) |
2Specification for compacted clay masonry units (compressive strength of 337.5 mm × 112.5 mm × 112.5 mm brick).
3International Building Code (IBC), International Code Council (ICC) 2012.
The average maximum compressive strength values of KRS specimens at 28 days are compared with typical design criteria of masonry structures (i.e., EN 1996-3, BS EN 771-1, and IBC 2012) in Figure
Meanwhile, BS EN 1996-3 establishes the minimum strength of a wall element for a low-rise building to be higher than 5.2 MPa. In this aspect, 10% cement mixed KRS and 1% gellan gum mixed KRS are insufficient for use for single-story buildings, while 1% xanthan gum mixing is applicable for low-rise soil building construction. Moreover, the high strength of 1% xanthan gum treatment (i.e., 6.3 MPa) is a compressive strength level almost equivalent to the minimum strength of 20% ordinary cement mixing (i.e., 6 MPa), indicating the high strengthening efficiency of xanthan gum treatment, even with 1/20th (i.e., 1% versus 20%) the amount of material quantity compared to cement mixing.
Given the mechanical performance of biopolymer treatment, biopolymers are highly feasible for use as soil binders. However, the strength and stability of soil structures become critical with the presence of excess water conditions (e.g., wet or submerged). A previous study shows that the wet strength of biopolymer-treated soils is reduced to approximately 1/10th that of the dry strength when fully saturated under water [
Petrochemical polymers have been applied diversely in modern civilization due to their demonstrated excellent performance. However, their prices are sensitive to fluctuations in oil prices, and they come with the added disadvantages of environmental damage, due to their retarded degradability, and the creation of carbon dioxide in their production process. Consequently, the need for more environment-friendly polymers has emerged, and accordingly studies to develop diverse bio-based plastics or polymers have been actively conducted [
Bio-based plastics or polymers can have diverse molecular structures depending on their respective polymerization processes, and this has enabled the production of customized biopolymers that have desirable strength or plasticity, with inherent biodegradability, along with low or limited carbon dioxide generation during production. On this basis, they have been regarded as a promising alternative to petrochemical products [
Major global companies in the areas of chemical engineering and product manufacture are leading the development and production of biopolymers and bioplastics. Recently, several leading companies concluded an agreement together to produce environment-friendly biopolymers, and they introduced specifications and a certification system for biodegradable polymers [
The economic feasibility of biopolymers has been growing due to the expansion of biopolymer markets and the development of technologies associated with biopolymers (Figure
Expected trend and growth of the global biopolymer market.
The price competitiveness of biopolymers, which were 35–100 times more expensive in the early 2000s relative to conventional petrochemical polymers, has also been improving. The price gap had dropped by 2.5–7.5 times in 2007 by virtue of consistent development in technology and increased environmental regulations [
Market price trend of xanthan gum over 30 years (1985–2014).
In general, the major factors determining the price level of biopolymers such as xanthan gum are (1) the source of carbon, (2) the fermentation process, and (3) the recovery ratio. In particular, the level of biopolymer recovery from the fermentation medium (i.e., carbon source, e.g., sugar water and glucose) is an essential component affecting cost.
Regarding the importance of improving the recovery ratio, it was reported that a 20% increase in the recovery of biopolymers (from 60% to 80%) could reduce the price of the biopolymer by 10% [
In addition, there have been many efforts to diversify the biopolymer carbon source, which is the major constituent of the macromolecular polysaccharide, as well as studies aimed at optimizing the production conditions of biopolymers [
For instance, the price of starch varies from 240 to 500 USD/ton, with a global average of 390 USD/ton in 2014 [
Estimated cost comparison for 1-ton soil treatment using cement (10%) and XG (0.5% and 1%).
Figure
Moreover, most biopolymers sold in the current global market are food-grade, and up to 50% of the production costs of food-grade biopolymers are related to downstream purification steps, many of which would not be necessary for nonfood applications such as construction [
About 30%–40% of the world’s population are still dwelling in buildings made of soil despite massive urbanization using modern construction technology. Such a high portion of the population is dwelling in soil buildings due to interrelated economic and environmental factors, including the availability of soil as a local and inexpensive construction material. This is problematic, since traditional soil buildings (made of soils without binders) are typically vulnerable to water and seismic loads. To cope with such problems, binders are required for soil strengthening. However, the most representative construction binder for soils (i.e., cement) accounts for more than 5% of the global annual CO2 emissions [
Thus, in this study, the use of microbially produced biopolymers as an economic and environment-friendly alternative binder for the construction of soil buildings is introduced. Feasibility studies conducted to test the comparative strength of soils treated with biopolymers confirmed that a very small amount (i.e., 0.5% of the whole contents) of biopolymers mixed with soil resulted in a higher unconfined compression strength than that of soil mixed with a large amount of cement (i.e., 10% of the whole content).
The economic feasibility of biopolymers relative to cement has yet to be improved; however with the trend of technological developments in this field it is highly likely that a market of biopolymers for construction purposes will develop. Further cost reductions are expected with the improved recovery ratio of biopolymers, together with the diversification and exploration of low priced carbon sources, and the commercialization and mass production of biopolymers specifically for construction purposes. These advances will enable countries with higher cement prices to obtain comparatively cheaper local construction binders. Furthermore, since the prices of carbon sources primarily used for the cultivation of biopolymers are lower in less developed countries, where the cost of cement is highest, the local commercialization of such biopolymers could contribute to the improvement of the strength and durability of soil buildings in countries that rely on them the most.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research described in this paper was financially supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (no. 2015R1A2A2A03006268), by a grant from the Strategic Research Project (Development of Key Excavation Solutions for Expandable Urban Underground Space) funded by the Korea Institute of Civil Engineering and Building Technology (KICT), and by the KAIST End-Run Program (no. N01150661) supported by the Korea Ministry of Science, ICT and Future Planning (MISP).