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Juri Liiv, Speaker at Green Chemistry Conference
Tartu University, Estonia
Title : Ash and organic filler composite materials

Abstract:

Of all economic sectors, the construction sector is one with the largest ecological footprint. Manufacturing of Portland cement alone causes ca 8% of all anthropogenic carbon dioxide emissions. On the other hand, large amounts of calciferous and pozzolanic ashes and other alkaline wastes are land-deposited globally and vast amounts of organic wastes are landfilled or burned around the world annually. In addition to the emissions caused by construction activities, partially drained peatlands with lowered water table are an major source of greenhouse gases (GHG). Lower water table causes an increased penetration of atmospheric oxygen into the peat deposits exposed to air, which, in turn results in increased rate of degradation of organic matter and turning a peatland from a carbon sink to a source of GHG.

Our studies provide solutions to both main problems stated above. We have developed a novel peat-based composite building material suitable for three-dimensional (3D) printing of entire house boxes (walls, floors, ceilings, etc.). The 3D- printability of the novel material allows to decrease the waste generation in the entire construction process, to increase productivity and occupational safety, to cut costs and to increase the speed of construction works. The low price, availability and flexibility of the material in terms of construction technology allows rapid constructions works in disaster zones or regions affected by refugee crises. Peat is used as filler and combustion fly ash as binder to achieve excellent technical parameters - compressive and flexural strength, thermal and acoustic insulation capacity, light weight, fire integrity as well as resistance to weather conditions and pests. Peat has not been mentioned in literature in this role because of retarding properties of its humic fraction on the cementation process. This retarding effect of humic substances on hardening of pozzolanic mixtures were successfully overcome by using siliceous additives e.g. silica fume as additives, which also greatly improved the long setting time of kukersite oil shale ash used as a binder.

The mechanism of hardening of the peat-ash composite is hypothesized as the following: first, upon contact with water, the pH of pore water exceeds 13 due to the influence of oxides of alkali metals in the ash. This facilitates the oxidation of humates into oxalate. Then, alkali metal, aluminum and silicate ions (the latter mostly originating from siliceous additives) with alumina form insoluble feldspars leading to a drop in pH. At pH values lower than 12.45, Ca(OH)2 become soluble and Ca2+ ions are transferred to the pore water, allowing pozzolanic reactions to take place and calcium oxalate to precipitate. Finally, the residual Ca(OH)2 reacts with atmospheric CO2 to form CaCO3. The material was characterized by measuring its thermal resistance (<0.08?W/(m?K)),     compressive     and     bending     strengths (1.2?MPa    and    0.36?N/mm2,    respectively).    Using    peat composites in the construction industry allows reducing CO2 emissions from partially mined peatlands (currently the one of the largest contributors to the total CO2 emission from Estonia) and increasing the reuse of oil shale ash while providing a cheap and affordable raw material.

The same composite has several alternative uses. It can be used in vertical landscaping and gardening for making prefabricated solid planting blocks made of concrete composite. Vertical gardening is a rapidly growing trend in eco-friendly construction and urban planning. We have developed a construction technology that brings together CNC (computer numerical control) processing of supporting structures and peat-ash composite based planting blocks. The supporting timber structures are made of concentrically composited laminated timber (CCLT) and fixed together using traditional carpentry joints, thus they are easily assembled on the building site.

Another alternative application of peat-ash composite is mass-stabilization of road embankments constructed in peatlands, which commonly involves replacement of the peat with a fill-up soil of an adequate load-bearing capacity. This usually requires a lowering of the water level, turning a peatland from a carbon sink to a source of greenhouse gases. Thus, alternatives are sought that are less costly in both economic and ecological terms. Mass-stabilization technology substituting Portland cement with waste calcareous ashes, supplemented with pozzolanic and alkali additives to accelerate the setting and hardening processes, is an attractive alternative to soil replacement techniques. We have shown that the ashes of Estonian kukersite (oil shale) from both pulverized firing and circulating fluidized bed incineration process can be used as binding agents for peat stabilization without any addition of Portland cement. Hardened peat soil samples behave as a ductile material, and the cellulose fibers naturally present in peat give the peat–ash composite plasticity, acting mechanically similarely with the steel or glass fiber in ordinary reinforced concrete. The effect of peat fiber reinforcement is higher in cases of higher load and displacement of the composite, making the material usable in ecological constructions.

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