Final Report: Biostabilization of Rammed Earth for Reduction of Waste and CO2 EmissionsEPA Grant Number: SU835497
Title: Biostabilization of Rammed Earth for Reduction of Waste and CO2 Emissions
Investigators: Kraus, Chad , Bents, Alyson , Bents, Timothy , Boling, Joshua , Dawson, Zachary , Hirmas, Daniel , Johnson, Anna , Peek, Ben , Roberts, Jennifer A , Versteeg, David
Institution: University of Kansas Main Campus
EPA Project Officer: Levinson, Barbara
Project Period: August 15, 2013 through August 14, 2014
Project Amount: $14,980
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2013) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Challenge Area - Materials & Chemicals , P3 Awards , Sustainability
Rammed earth (RE) is an ancient building technique where soil constrained within formworks is pressed together in lifts. Rammed earth possesses low embodied energy, high recyclability, and low toxicity while having little impact on biodiversity and virtually no depletion of biological nutrients. This inherent sustainability has been compromised by modifications imported from site cast concrete construction in order to bring the strength of RE up to contemporary building standards—the most common stabilizer used being Portland cement.
The challenge we set out to address was to stabilize contemporary rammed earth to meet the expectations of building codes without the use of Portland cement or other stabilizers such as lime and asphaltic emulsions. The proposed work aimed to strengthen RE by a process known as microbially induced calcite precipitation. Natural soil microorganisms are known to biomineralize calcite in soil pore spaces, aggregating mineral grains and enhancing desirable properties of earthen materials. This process has been increasingly used in engineering applications (repairing existing concrete structures, stabilizing soils around foundations, etc.) over the past decade. Our goal was to significantly reduce the atmospheric CO2 emissions and waste associated with the production and application of Portland cement by exploring microbial biomineralization to enhance the material properties of RE in what we have termed ‘microbially indurated rammed earth’ (MIRE).
The specific objectives of this project were to: (1) Quantify the uncertainty in strength measurements of rammed earth testing cylinders in order to determine the number of samples needed to check for reasonable differences, (2) Determine an appropriate quantity of Sporosarcina pastuerii to enhance rammed earth compressive strength, (3) Assess the ability for Sporosarcina pastuerii to increase rammed earth compressive strength above a control, and (4) Investigate the use of alternative nitrogen fertilizers and carbon sources that could enhance the MIRE process and reduce waste of those sources.
We conducted a series of experiments to meet our objectives:
Experiment 1: Optimum bacterial concentration – Growth curves
Batch experiments were conducted on S. pasteurii, a bacterium known to precipitate calcite as a natural part of its urease metabolic pathway. The bacteria were prepared in 1 L of growth solution containing 37 g L‐1 brain heart infusion and 20 g L‐1 urea. A single culture was introduced into the growth solution. The growing bacteria were incubated at 35°C at 400 rpms for 1 week. At this point, 50 mL of bacteria solution was transferred to fresh media bottles containing growth solution to run optical density analysis to produce growth curves of the bacteria. Each batch culture of bacteria was analyzed on a mass spectrometer to determine the growth of the bacteria in solution at set intervals. Through these experiments it was determined that bacterial growth should be stopped after 70 hours to attain a bacterial cell concentration of 2.5 x 108. This bacterial cell count was used as the “high” concentration in determining the concentration of microorganisms to be used in MIRE experiments (described below).
Experiment 2: Comparative delivery study – Freeze-dried vs. Liquid
We compared the addition of microorganisms via a liquid suspension to a freeze-dried pellet in order to explore the potential to scale up this work in the field. The effect of the delivery method on a representative soil for MIRE experiments was investigated by setting up 4 containers: 2 had bacteria delivered from freeze dried pellets and 2 had fresh bacteria added in growth solution. The bacteria were allowed to grow for 2 and 7 days. After 2 days, 5 mg of soil was harvested from 6 points along each of the 2-day containers and diluted to 15 mL. Further dilutions of 1:10, 1:100 and 1:1000 were done to prepare DAPI stains of the grown bacteria. After 7 days, the process was repeated in the final 2 containers and DAPI stains were produced for both. These analyses suggest that there is no significant difference in bacterial activity when the bacteria are delivered in pellet form (freeze-dried) or in liquid form.
Experiment 3: Establishing test cylinder variability
In this experiment our goal was to understand the variability between cylinders in order to ensure statistical significance in our tests of three cylinders for each variation. To do this we rammed ten cylinders following the same procedure and crushed them after one week of curing. The results indicated a standard deviation of 4.6 psi that a sample size of three cylinders sufficiently represented the variability of the test specimen. Using this standard deviation, a power of 0.8, an α-level of 0.05, and a sample size of 3, our minimum detectable effect size was 15 psi.
Experiment 4: Determining concentration of microorganisms
In this experiment three concentrations of S. pasteurii were tested to determine if there was an ideal concentration to optimize growth, calcification, and strength. This experiment consisted of testing compressive strengths at each concentration against the control at 7 and 14 days. Our results indicated that the highest concentration began to outperform the control at 14 days. Using this result we decided that there was little risk of overpopulation and that maximizing concentration would yield the best results in future experiments.
Experiment 5: Microbially Indurated Rammed Earth (MIRE): Base Experiment
In the MIRE base experiment we set out to implement the combination of our previous experiments to optimize compressive strength in MIRE. The control cylinders included soil, artificial groundwater, urea and calcium chloride and the treatment consisted of soil, artificial groundwater, urea, calcium chloride and S. pasteurii. This experiment used standard intervals, testing compressive strength at 7, 14 and 28 days. We anticipated the MIRE to outperform the control starting at 14 days due to the behavior of the bacteria determined in the experiment described in Section d. However, our results did not follow our hypothesis. The control consistently outperformed the MIRE, and further research is necessary to understand what the growth deterrent is on S. pasteurii. Our working hypotheses include: (1) a possible biofilm effect that may either directly lubricate grain-to-grain contacts or retain moisture that lowers the compressive strength of the rammed samples, (2) the carbonate-rich quarry material used in this experiment may have buffered the pH and prevented precipitation of neoformed calcite, and (3) the nitrogenated organic carbon may have increased heterotrophic microbial processes acidifying the soil solution and possibly dissolving some of the carbonate in the original parent material weakening the overall compressive strength.
Experiment 6: MIRE: Blood experiment (2 sets of data, trial 1 and 2, replicate experiment)
For the MIRE blood experiment we investigated bovine blood, an alternative to urea as a nitrogen source for S.pasteurii. We followed the same procedure as the MIRE base experiment for testing compressive strength. Initially we substituted bovine blood for the urea directly, keeping the rest of the procedure the same and comparing it to a control without blood. The results were positive as the blood MIRE cylinders outperformed the control by more than three times at the 28 day test. In a second iteration of the same experiment, we wanted to explore the possibility of the blood itself having a strengthening effect on the soil. The replicated experiment used two controls, one a standard control and one using blood without S. pasteurii. Our variable was explored using both blood and S. pasteurii (Blood MIRE). The results of this test have opened a new path for the research as the blood itself outperformed both the control and the blood MIRE and nearly reached 1000 psi, the highest compressive strength attained in any MIRE experiments.
If given the opportunity, our future research path will include implementing blood and other nitrogen sources (such as molasses) as well as exploring alternative soil types in an effort to improve compressive strength of the soil.
Experiment 7: MIRE: Granite experiment
In the MIRE Granite experiment we set out to optimize particle size distribution and implement a different parent material due to possible buffering effects of the high pH limestone quarry material. To do this we crushed quarried granite into aggregate sizes of interest and supplemented the fine earth fraction using quartz sand. Unfortunately, we were unable to capture the smallest clay particles, which help glue the soil together initially, but we do believe that using a different parent material will improve our results over previous experiments. Our results indicate that the granite-sand mixture is not buffering the bathing solution since the pH is able to attain values close to a pH of 9 as expected. Calcium carbonate unexpectedly was detected in both the control and the MIRE samples indicating that further work is necessary.
Human prosperity is highly dependent on the built environment. As the human population continues to increase exponentially, our built environment will need to follow suit. Contemporary construction is typically expected to last less than 50 years with extensive upkeep required. Alternatively, rammed earth has stood the test of time and has lasted several thousands of years with minimal upkeep. Reducing upkeep and extending the lifespan of residential structures has great potential to reduce construction waste. It has the added benefit of reducing the need to rebuild structures, thus decreasing our dependence on extracting, transporting, and processing industrialized materials.
Microbially indurated rammed earth has the potential to be a sustainable alternative to low- intensity concrete construction applications (i.e., residential). Reductions in CO2emissions can be counted in the hundreds of millions of tons, meeting our needs to continue construction as the population increases while offsetting our environmental footprint. MIRE, if successful, would be a beautiful, non-toxic, pest and fire-resistant, low-maintenance, thermal and humidity balancing material, just like its nonstabilized ancestor.
Although the expense of contemporary rammed earth in North America is greater than some alternative construction methods, when viewed through the lens of life cycle analysis, it is economically appealing. As a monolithic construction material satisfying the roles of structure, thermal envelope, moisture barrier, interior finish, and exterior cladding, MIRE starts to narrow the economic gap. As with most emerging, or reemerging technologies, increased research, more efficient methods, and scaled up implementation, will lead to a reduction in costs.
In regards to the planet, rammed earth, stabilized through microbially induced carbonate mineral precipitation using high-protein industrial waste byproducts could likely reduce CO2emissions, lower embodied energy, reuse industrial byproducts, avoid extracting biological nutrients, endure for centuries or more, as opposed to the typical 50-year or less life of many building assemblies, increase reliance on local materials, thereby mitigating the environmental costs of transportation, and sequester carbon from the environment.