On 4th December 2006, NASA (National Aeronautics and Space Administration, U.S.A) unveiled their Global Exploration Strategy; "To return astronauts to the Moon no later than 2020". 13 other countries including Japan agreed on cooperative lunar exploration with USA, and have discussed the mission objectives for the exploration. The term "In-Situ Resource Utilization" means the production of materials from local resources on the target site, instead of delivering the materials. Employment of ISRU reduces launch mass/cost and mission risk, and increases usable material, power, margin of safety, and whole mission capabilities. Therefore, ISRU is considered to be essential technology for future space exploration, and identified as one of the mission objectives of the international lunar exploration.

Lunar concrete is one of possible materials obtained by ISRU, and expected to start demonstration on the Moon around at 2020. It is considered to be an essential material for future lunar base development due to;

Relatively cheap production cost compared to metals.

High performance under the severe lunar environment, such as extreme temperature, heavy space radiation, and meteorite impact

Versatile applications, such as precast panel production and assembly, onsite casting, grouting, shotcrete, etc

Assumed composition of lunar concrete is approximately 17% of cement, 78% of aggregate and 5% of water. All components of lunar concrete except 0.5 wt% of hydrogen can be produced by ISRU on the Moon. However, even though necessary elements for cement exist on the Moon, typical minerals of cement materials on the Earth, such as lime stone, iron ore, bauxite created by sedimentation or weathering, cannot be found on the Moon. In short, all minerals on the Moon are rather uniform and unnecessarily rich in SiO2 and poor in CaO to apply typical cement production method. Therefore, special technique called vacuum pyrolysis is proposed by T. D. Lin in 1981, as a promising method to produce cement from lunar materials. This cement production method utilizes difference in volatility of elements (FeO> MgO> SiO2> CaO> Al2O3), and evaporates highly-volatile elements, FeO, MgO, and SiO2, and condenses low volatility elements, CaO and Al2O3. Theoretically, as impurities evaporate, High Alumina Cement (HAC) can be manufactured as evaporation residues.

However, feasibility of this cement production method had never been experimentally proved. Actual chemical composition of evaporation residues should be investigated if evaporation of CaO initiates much before complete evaporation of SiO2 to deviate from the compositions of HAC. In addition to cement chemical composition, conversion efficiency from lunar soil to cement, evaporation rate of elements (cement production speed), and associated oxygen production by evaporation reaction had never been investigated by previous research. Lack of knowledge about associated oxygen generation in previous research resulted in wrong design of lunar concrete plant facilities and overestimation of mass and power budgets. Therefore, this research aimed to reveal production processes, facilities, and its mass and power budgets based on mechanism of cement and oxygen production processes and its experimental data.

Moreover, along with evaporation pass of SiO2, various HAC with different SiO2 content can be manufactured. From the energy and cost saving point of view, material processing rate should be minimized by employing either high SiO2 content cement (low SiO2 evaporation from lunar soil) or glassy highland soil as pozzolanic material. However, neither properties of high SiO2 content cement in the lunar cement chemical compositions nor pozzolanic reactivity of the glassy lunar soil has never been studied. Hence, hydration products, hydration reactivity and strength property of various cements were investigated, applying different hardening accelerators and curing temperature.

Thus, this research examined following three topics

Feasibility of cement and oxygen production by vacuum pyrolysis

Properties of cement with different SiO2 content and cooling conditions

All processes, facilities, mass and power budgets of a prototype lunar concrete plant based on vacuum pyrolysis and proper curing method of concrete

Firstly, to reveal the feasibility of cement and oxygen production by vacuum pyrolysis, lunar highland soil simulant was manufactured and processed in a vacuum furnace at 1937K, 1994K and 2045K. Following results were obtained by the experiment.

I.It was proved that vacuum pyrolysis technique enables to produce High Alumina Cement, containing CaO?Al2O3, a high hydration reactivity component, as a major component.

II.Practical chemical compositions of lunar cements were clarified and drawn by extrapolation lines

III.Evaporation rates of lunar soil simulant were measured in three different temperatures. Then, a versatile formula representing relationship between processing temperature, evaporation rate and evaporation surface area were acquired

In response to the proof of the feasibility of lunar cement production, lunar cement simulants were manufactured based on obtained chemical compositions in vacuum pyrolysis experiment. The lunar cements were CaO/Al2O3=0.66 and varied in SiO2 concentration (5%, 10%, and 15%) and crystal conditions (glass or crystal). Then, hydration products, hydration reactivity and strength property of the cements were investigated with various mixture compositions and curing temperature. For cost effective lunar concrete production, this study assumed to apply cement/concrete composition which requires low cement processing mass, by means of

Production of high SiO2 content cement

Replacement of cement with glassy highland soil

At the same time, the lunar concrete should have sufficient property to be applied in severe lunar environment

Marginal strength property to support load and avoid penetration by meteorite impact

Stability of structure by generating stable hydrates C3AH6 to prevent dehydration and structure failure of lunar concrete

From the experimental results, following conclusions were obtained.

I.Mixing with water at 20?C was found not effective for all the lunar cements to produce stable hydration products C3AH6 within short time period before vacuum exposure. Some activation is necessary.

II.For low SiO2 content cement, activation by mixing 0.1-1.0% of Li2CO3 or curing more than 40 ?C is sufficient to generate C3AH6 within 3days. Moreover, those mortar specimens developed 30MPa of compressive strength after conversion reaction.

III.In the case of high SiO2 content cements, hydration reaction was not observed with water at 20 ?C. Mixing Ca(OH)2 and Li2CO3 found to be effective to initiate hydration reaction. However, obtained hydration products were mainly unstable C2ASH8, and it was converted into C3AH6 at 100 ?C. Therefore, more than 100 ?C of high curing temperature was found to be necessary for low cost lunar concrete production which employs high SiO2 content compositions.

IV.As a conclusion, high temperature and high pressure curing method called Dry-Mix Steam Injection method (DMSI method) is considered to be rational for low cost concrete production. Since greater hydration reaction and more rapid conversion of low SiO2 cement were observed at 40 ?C of higher temperature, employment of high temperature curing is considered to be effective for all the lunar cements.

Then, necessary processes and facilities for lunar concrete production which employs vacuum pyrolysis technique and DSMI method were clarified. Then, mass and power budgets of the lunar concrete plant were estimated by sizing terrestrial facilities. Experimental results, lunar soil physical properties and lunar environmental factors were also taken into account. Estimated budgets declared that

I.The mass of concrete plant which has less than 300 tons of annual concrete production capacity is less than 20tons, and might be possible to be delivered to the Moon by one launch.

II.Employment of lunar In-Situ production reduces launch mass down to 9.5% to 5.5% in a case 100 tons to 500 tons of annual concrete production. Then, mass production of concrete is demonstrated to be more cost effective.

Consequently, suitable application of lunar concrete is discussed. To understand advantages and disadvantages of concrete, concrete was compared with three other In-Situ construction materials; regolith-bag, brick/cast regolith, and sulfur concrete. The comparison of launch mass saving effect showed that more cost effective materials in following order.

Rogolith-bag (1.5%)> Lunar brick (3.3%)> Hydraulic concrete (9.1%)>> Sulfur concrete (50.0%)

However, mass and power budgets of hydraulic concrete is considered to be competitive level, if versatile application of lunar concrete is taken into consideration.Through the comparison of launch mass and power budgets, application of each material, it was concluded that lunar base development needs to employ various different materials depending on the application. Hence, extensive effort to research and develop various materials should be exerted at the same time, considering advantages, disadvantages and application plan of each material for effective lunar base development.

提出された学位請求論文「Investigation on practical production methods of lunar concrete(ルナコンクリートの実用的製造手法に関する研究)」は、各種の実験によって、月面で実用可能な「ルナコンクリート」製造の工程と必要な設備を明らかにした論文であり、全5章からなっている。

第1章では、研究の背景、目的、既往の関連研究の成果等を明らかにしている。具体的には、月面での施設建設を想定した場合のISRU(:現地材料利用)の重要性を指摘し、月面材料からセメントを製造する手法として既に提案されている真空蒸発法等の未解決問題を明示した上で、真空蒸発法によりセメント・酸素製造が可能であることを実証すること、様々な養生方法を用いたルナコンクリートの性能を検証すること、真空蒸発法と適切な養生方法を取り入れた、ルナコンクリート工場の製造工程、設備、重量・エネルギー消費量を明確化すること、現地材料利用によって製造される他の建設材料との比較によってルナコンクリートの適切な使用方法を明らかにすることを、研究の目的としている。

第2章では、真空蒸発法によりセメントと酸素製造が可能であることを実証するため、月の高地部のレゴリスの模擬材料が作成され、真空蒸発炉において1937K、1994K、2045Kの3つの温度で加熱する真空蒸発の実験とその結果が示されている。具体的には、真空蒸発法を用いることで、高い水硬性を持つCaO-Al2O3を主体としたアルミナセメントを製造できること、実用的なルナセメントの化学組成、蒸発残留物の組成変化、3つの蒸発温度におけるレゴリス模擬材料の蒸発速度等が明らかにされている。そして、蒸発温度、蒸発速度、蒸発表面積の関係を表す普遍的な関係式を得ることに成功している。

第3章では真空蒸発実験で得られた化学組成を元にルナセメント模擬材料が製造されている。具体的には、SiO2含有量と結晶状態が異なるルナセメントを用いて、様々な配合と養生温度で水和生成物、水和反応性、強度発現性が検証されている。その結果、先ず、20Cで水と水和させた場合、全てのセメントにおいて、真空暴露前の短い期間で安定なC3AH6を生成させることは出来ないこと、28日後に100 Cで脱水すると大きな強度低下がおこることが明らかになり、安定なコンクリートを製造するためには、何らかの水和促進剤が必要であることが指摘されている。次いで、SiO2含有量の低いセメントでは、40 C以上の高温養生で、3日以内にC3AH6へのコンバージョンが見られること、また、養生28日後に100 Cで脱水すると大きな強度増進が見られることが明らかにされている。更に、SiO2含有量の高いセメントでは、20 Cの水とでは水和反応が起きないこと、ただし硬化促進剤の添加や高温養生によって水和反応が観察されること、100 C養生によってコンバージョンが起こることが明らかにされている。最後に、これらの結果を踏まえ、DMSI法という高温高圧養生法の有効性が指摘されている。

第4章では、真空蒸発法とDSMI法を用いたルナコンクリートの製造工程と設備が明らかにされている。具体的には、年間生産量350トン以下のコンクリート工場では、ロケットの一度の打ち上げで月面に輸送できると考えられること、年間100~500トンのコンクリートを製造すると仮定した場合、材料そのものを打ち上げるのに比べ、打ち上げ重量を大きく削減できること、マスプロダクションによる重量削減効果が高いことが明らかにされている。

第5章では、ルナコンクリートの適切な用途が明らかにされている。具体的には、コンクリートの長短所を理解するため、レゴリス・サンドバッグ、レンガ/キャスト、サルファーコンクリートの3つの材料との比較が行われ、水硬性コンクリートの重量削減効果はレゴリスバッグやレンガに比べて劣るが、このコンクリートが最も多様な用途が期待できることから、他の材料と十分に競争できる範囲の重量・エネルギー消費量だとされている。また、各材料の長短所を比較した結果、月面基地の効率的な発展のためには、用途によって様々な材料を適材適所で活用していくことが望ましいことが指摘されている。そして、最後に、前4章で新たに得られた知見を整理し、本論文の結論としている。

以上、本論文は、独創的な各種の実験と試算を通じて、月面での施設建設に適用可能なコンクリート製造工程とその設備を具体的に明らかにした論文であり、建築学の発展に寄与するところが大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。