c Pleiades Publishing, Ltd., 2016. ISSN 1063-7729, Astronomy Reports, 2016, Vol. 60, No. 2, pp. 306–312. 

Lunar Concrete: Prospects and Challenges* Anwar Khitab** , Waqas Anwar, Imran Mehmood, Syed Minhaj Saleem Kazmi, and Muhammad Junaid Munir Department of Civil Engineering, Mirpur University of Science and Technology, Mirpur, Azad Jammu & Kashmir, 10250 Pakistan Received April 20, 2015; in final form, June 26, 2015

Abstract—The possibility of using concrete as a construction material at the Moon surface is considered. Dissimilarities between the Earth and the Moon and their possible effects on concrete are also emphasized. Availability of constituent materials for concrete at lunar surface is addressed. An emphasis is given to two types of materials, namely, hydraulic concrete and sulfur concrete. Hydraulic concrete necessitates the use of water and sulfur concrete makes use of molten sulfur in lieu of cement and water. DOI: 10.1134/S1063772916020050

1. INTRODUCTION Concrete as a construction material has been used at least as far back as the Roman and Egyptian empires. Modern concrete is defined as a mixture of cement, water, aggregates, and in some cases, admixtures and additives [1]. Concrete is a versatile construction material and has strength, durability, versatility, and economy. It can be placed or molded into virtually any shape and reproduce any surface texture. With proper materials and techniques, concrete can withstand many hostile environments. Since concrete is a structural material, strength is a desirable property. Although the compressive strengths of ordinary concrete generally range from 15 to 40 megapascal (MPa), concrete can be made to withstand over 80 MPa for special tasks [2]. It is for these reasons that it is considered to be the first lunar construction material. 2. HISTORY OF SPACE EXPLORATION The first proposal to reach the outer space came through the Soviet scientist Konstantin Tsjolkoysky in his article “The Exploration of Cosmic Space by Means of Reaction Devices,” published in 1903 [3]. Then R. H. Goddard presented his paper, “A method of reaching extreme altitudes,” in 1919 [4]. A first rocket to reach the space was the V-2 rocket, launched by the Germans during the World War II [5]. Yuri Gagarin was the first cosmonaut to reach ∗ **

The text was submitted by the authors in English. E-mail: [email protected],chairman.ce@must. edu.pk

the upper space in 1961. Neil Armstrong was the first man, who has landed on the Moon surface along with Buzz Aldrin in 1969. Since 1969, some twelve landings have been made by the Soviets and US at the Moon surface along with plentiful scientific operations therein. Conquering the space, a man also had a vision to make colonies at some planetary body outside the Earth. Because of its proximity to the Earth, the Moon is considered to be the first choice of humankind. This factor is very important in the sense that lesser energy is be required for transport to and from the Moon. Its second advantage lies in the fact that the Moon possesses some gravity, approximately one sixth that of the Earth. Thirdly, the Moon has plenty of natural resources, which could be of great benefit to mankind [6]. Fourthly, it would cost less to send space vehicles to other planets from the Moon than from the Earth [7]. But not all is well with the Moon. There are certain consequences, too, which must be addressed. Although lunar gravity is an advantage, it is also a great disadvantage. Lunar day equals some twenty nine Earth days. Lunar atmosphere is much thinner than that of the Earth, providing almost no protection against radiation [8]. Last but not the least, there is a serious debatable question as how the humans will psychologically react to an alien environment of the Moon. Despite all ifs and buts, the truth is that mankind is advancing in its pursue to colonize the Moon as per famous saying of Frederick Douglass “without a struggle, there can be no progress.” 3. EARTH AND MOON: DISSIMILARITIES The Moon offers hard vacuum of free space and is also deprived of hydrolytic weakening process. This 306

LUNAR CONCRETE Chemical composition of cement, terrestrial fly ash, and moon dust Ingredient

OPC Terrestrial fly ash Moon dust

Lime

64

0.4–28

10

Silica

20

29–59.4

50

Alumina

6

5.2–34

15

Iron oxide

2.3

1.2–30

5–15

Agnesia

1.2

0.4–9

10

Sulfur trioxide

3.5

0.04–9

–

Sodium oxide

0.1

0.2–7

–

Potassium oxide

0.8

0.6–6.7

–

Titania

0.4

0.2–1.7

5

scenario leads to higher tensile strength of many materials as compared to the equivalent materials found on the Earth [9]. Extraction of these materials requires sufficient energy sources; however electric power from solar energy might ease the extraction process. As far as the construction equipment is concerned, there are two elementary differences from equipment used on the Earth [10]. The first is the dusty environment; lunar equipment should be designed to take into account abrasive lunar dust. The second problem is related to the reduced gravity which means that machinery weighs less on the Moon. So, in order to counter this setback, either very heavy machinery or some anchorage systems should be designed [11]. However due to the rapid advancement in the manufacturing and industry processes, all these concerns can be well tackled and construction materials may be derived from the Moon which offer better engineering properties as compared to those materials already available on the Earth. Some of the construction materials which can be extracted from the Moon are discussed below.

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5. HYDRAULIC CONCRETE Among all the construction materials, concrete is considered to be the primary choice, keeping in view its tremendous potential [13, 14]. An ordinary concrete is a mixture of cement, sand, gravels and water. In the table, the chemical composition of ordinary Portland cement (OPC), terrestrial fly ash and lunar dust (% by mass) are given for the reference [1, 15, 16]. The values given in table clearly demonstrate that chemical composition of lunar dust resembles terrestrial fly ash. Fly ash is an environmentally friendly material, being used as a partial replacement of cement since decades. Thus, lunar dust is a probable material for making concrete at the Moon. Lunar rock has been found to possess specific gravities of more than 2.6 [16], making it a perfect source of coarse aggregates for concrete. In addition, the lunar soil can be sieved to obtain fine aggregates. A wide variety of cements are used in the manufacturing of concrete. A typical Portland cement consists of 65% calcium oxide, 23% silica, 4% alumina, and small percentages of various inorganic compounds. Among these, calcium oxide is the most important one. It has been found that Apollo lunar soils and rocks have sufficient quantities of alumina, silicate and calcium oxide. Calcium rich plagioclase on the Moon has 19% by weight of calcium oxide [16]. This leads to the conclusion for production of cementitious materials by using lunar materials.

5.1. Lunar Aggregates Aggregates generally occupy 75% by weight of the concrete. Owing to very high percent volumetric proportion, a careful selection of aggregates becomes very important for all concrete job. A specific gravity of 2.6 for coarse aggregates is considered good for concrete [1]. The lunar aggregates are found to possess a specific gravity of more than 2.6, which is quite acceptable hence lunar rocks can be crushed to produce coarse aggregates while abundant lunar soils have the potential to produce well graded fine aggregates [17].

4. NATURE OF LUNAR SOIL Lunar regolith is formed by the continuous bombardment of lunar rocks by micrometeorites which cover the Moon surface. The soil has very good engineering properties including the density, porosity, adhesion etc. The bulk density of the regolith ranges from 0.9 to 1.1 g cm−3 near the surface and reaches a maximum of 1.9 g cm−3 at the depth of only 20 cm. Furthermore, the inter-particle adhesion in the soil is also very high. The regolith clumps together like damp beach sand [12]. ASTRONOMY REPORTS

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5.2. Water Water is an essential part of concrete. It chemically combines with cement, and together they compose binding products. The products bind together the filler components like fine and coarse aggregates. Water can either be supplied from the Earth or can be produced at the Moon by combining oxygen with hydrogen produced from the lunar soil [15]. Oxygen can be produced by (i) reduction of ilmenite (FeTiO3 ) with hydrogen (H2 ), carbon monoxide (CO) or methane (CH4 ) and (ii) molten silicate electrolysis [16].

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5.3. Production of Water at the Moon The Moon is not an anhydrous body. Several Apollo samples of lunar surface, which were thoroughly studied, have shown presence of water mostly as structurally bound OH- or as molecular H2 O absorbed in minerals. There is also a possibility that the interior of the Moon may be wetter than the surface and several distinct water reservoirs may also be present on the Moon [18]. The latest remote sensing results show that nearly the whole lunar surface is hydrated at least during some portions of the day [19, 20, 21]. Although the latest findings have challenged the 40 years old concept of anhydrous Moon, there are still controversies about the presence of sufficient water there. More research on Apollo samples and aircraft observations are required to completely fade away these controversies in coming years and agree on the “Hydrous Moon.” Anyhow, at the moment, there are numerous processes including the ones mentioned above, which can be used for the production of water on the Moon using the available lunar materials [22]. Two of these processes are briefly discussed below. First, we describe the production of lunar water from hydrogen. If lunar soil is heated up to 6000◦ C, it releases 80% of hydrogen and by further increasing the temperature to 9000◦ C, no hydrogen is left in the soil [23]. Released hydrogen is hot and can react with several oxides to produce water. Most of water produced might be contaminated with wet nitrogen (N2 ), however, some of water should be free from contamination and could be used for several purposes. The amount of produced uncontaminated water should be equivalent to about 10% of the mass of hydrogen produced [24]. For example, lunar soil contains considerable quantities of glass, and the main source may include volcanic action or melting produced by meteorite and micrometeorite [25]. This glass contains up to 20% by weight of FeO [24] which is decomposed by hydrogen into pure Fe and H2 O [26] following the chemical reaction indicated below: FeO(glass) + H2 → Fe0 + H2 O. The second method is the reduction with hydrogen sulfide. Soil found on the Moon can also produce water if it reacts with hydrogen sulfide gas. In particular, oxides of Fe, Ca, and Mg found in lunar soil are proposed to be reduced as follows [27]: MO + H2 S → MS + H2 O, where M stands for Fe, Ca or Mg. The process is not as simple as it seems to be because it requires great amount of heat energy and may take considerable time for completion.

Recent data about the presence of ice also need to be mentioned. On October 9, 2009, the impact plumes of the Lunar Crater Observation and Sensing Satellite (LCROSS) and its Centaur rocket stage in Cabeus crater near the south pole of the Moon showed the spectral signs of hydroxyl, which is an indicator that water ice is present in the floor of the crater [28]. Analysis of the results indicates that concentration of water in the impact area is about 6%, including nearly pure ice crystals at some spots. Furthermore, the Indian Chandrayaan-1 Moon Mineralogy Mapper experiment also showed low-concentration hydroxyl signatures over much of the lunar surface, not only in permanently shadowed craters but also in other regions [29]. The Mini-SAR experiment also indicated possible large deposits of water-ice in the northern lunar craters [30]. Apart from hydraulic concrete, there are also some other alternatives like using epoxies or sulfur as binder rather than cement and water [2]. Use of epoxies or sulfur in concrete eliminates the use of water for concrete. 6. SULFUR CONCRETE

6.1. Availability and Extraction on the Moon Sulfur is another potentially interesting lunar material. Sulfur contents in lunar rocks that were brought back, fluctuate significantly from sample to sample, having a maximum of about 0.3 wt % in high-Ti lavas from the Apollo 17 landing site at Taurus-Littrow [31]. Iron sulfide is the major carrier of sulfur at the Moon. Different samples that have been studied up to now are in the form of troilite (FeS), possibly of meteoritic origin. The existing knowledge of lunar chemistry indicates a significant deficit of volatile element abundances with respect to corresponding terrestrial materials. However, the prospect of finding sulfur-rich ores of lunar (nonmeteoritic) origin cannot be totally excluded. Blebs and veins of troilite were observed in at least one sample. The troilite appears as a result from reaction of sulfur-rich vapor with Fe from olivine, producing FeS and pyroxene. In any case, the process occurs on a small scale and offers the option of concentrated ores of sulfur to exist on the Moon [32]. Geochemical analysis of sulfur distribution in lunar materials shows a significant positive correlation with titanium content. In general the overall low sulfur content of known lunar materials may not seem, at first, attractive from an exploitation perspective. However, added advantages of sulfur must be considered. First, withdrawal of sulfur is a simple process, which gives a yield of about 90% at temperatures of the order of 1000◦ C ASTRONOMY REPORTS Vol. 60

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[33]. Sulfur is a by-product of oxygen extraction reactions; coproduction of sulfur during oxygen extraction from ilmenite-rich mare soils could yield sulfur in masses up to 10 percent of the mass of oxygen produced [34].

6.2. Advantage Over Hydraulic Concrete As mentioned above the conventional concrete comprises coarse aggregate, sand, and a calcium silicate based hydraulic binder. A chemical reaction of calcium silicate takes place when water is added and the whole mixture transforms into hardened mass known as hydraulic concrete. Silica exists on the Moon, but presence of water is debatable that whether or not water exists on the Moon [35]. Either water can be transported from the Earth or prepared on the lunar surface itself, but both options are costly [36]. Therefore, there is a need to find an alternative material or component that can bind solid ingredients of concrete and gain strength on solidification. Alternate of hydraulic concrete is sulfur concrete [37, 38]. Leonard and Johnson, for the first time suggested the use of sulfur concrete as a lunar material [39]. Sulfur based concrete is advantageous due to avoidance of high energy processes required for hydraulic cement production and also because water is not needed at all [40].

6.3. Preparation Preparation of sulfur concrete is quite different. Here sulfur (thermoplastic material) serves as an effective binder with a non-reactive aggregate. First, sulfur is melted; the crystallization of sulfur binder as monolithic sulfur takes place with 7% volume reduction [41]. Molten form of sulfur is then mixed with aggregate to make a mixture, which is then molded and allowed to harden [42, 43]. Sulfur concrete, particularly for use in acids and salts environments, has gained a lot of attention [34]. It has rapid setting time and low water permeability and has high fatigue life than conventional concrete [41]. As far as mechanical properties are concerned, sulfur concrete shows good results [33, 44–46]. Availability, high strength and durability properties make sulfur concrete very attractive option for the development of the first lunar construction activities [33]. Sulfur concrete generally has sulfur and aggregate contents (including minerals, rock sands and glasses) ranging from 12 to 22 and 78 to 88 wt %, respectively [47]. ASTRONOMY REPORTS

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6.4. Performance Sulfur in the form of the mineral troilite (FeS) has been found on the Moon and can be used for construction purposes [40]. From the Apollo 11 and 17 sites, these contents range from a few tens of ppm in ferroan anorthosites to over 2000 ppm sulfur in high-titanium lavas [48]. In the literature, the reduction of mineral to pure sulfur and successive use is well discussed [49, 50]. The lunar environment is contaminated with dust particles which cause serious problems [51]. Sulfur concrete can be used to form slab pavement to reduce the effect of dust [39]. Sulfur concrete is commonly used below freezing temperature where conventional concrete cannot be used [52]. Use of sulfur concrete is favorable with respect to the Earth environmental conditions. However, use of sulfur concrete on the Moon needs additional considerations [53]. Utilization of sulfur concrete may lead to relatively poor durability in response to repeated thermal cycles [39]. Lunar temperatures at the poles range from –60 to –220◦ C and +123 to –180◦ C with an average of –20◦ C at the equator. These are extreme temperature cycles. Sulfur melts at 120◦ C and then stiffens at a temperature above 148◦ C. So it cannot be used when temperature is above 120◦ C [40, 54]. At the Moon there is also a low pressure; the transition temperature keeps on decreasing with decreasing pressure [55]. Microscopic examination of sulfur concrete samples under cycling temperature revealed the clear de-bonding of sulfur from the aggregate in fracture surfaces whereas it was seen adhering in non-cycled samples [56]. It is true to assume that the sulfur in sulfur concrete would be likely to sublimate resulting in an unsound, deteriorated structure. Different ionizing radiations are present on the Moon [57] which is a serious hazard for material and humans. Sulfur concrete provides protection against these radiations [39]. Sometimes some compounds termed as plasticizers are added in sulfur concrete in small amounts to decrease cracking when sulfur goes through the transition state [40]. 7. EFFECT OF VACUUM The Moon is surrounded by a hard vacuum whose presence may affect the preparation of various construction materials, which are chemically or molecularly unstable in such environment [58]. As far as the behavior of concrete in vacuum is concerned, continuous interest has been seen on this topic over the last years [59], and it is considered to be an important matter for research as well as from construction point of view [58]. Pressure on the Moon is about 10−12 Torr (1.33 × −10 Pa). As discussed earlier, sulfur concrete is a 10

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favorable construction material on the Moon and it is also realistic to assume that sulfur in this concrete would be prone to sublimate resulting in a deteriorated, unsafe structure when exposed to such a low pressure. An experiment was carried out with some sulfur concrete samples. The as-cast surfaces were relatively smooth. However under vacuum, clear degradation of the surface was seen after 8 days and more so after 58 days. It was also found that the greater the amount of initial sulfur in the sample, the more sublimation it undergoes over a given period. However, the rate of sublimation decreases with time which is a good sign [53]. Furthermore, as far as hydraulic concrete is concerned, significant amount of water is present in it. Some water is chemically bonded in the hydration product while some water is adsorbed on the surface of gel particles. In addition to this, condensed water is also present in capillary pores. For the preparation of concrete, sufficient amount of additional water is added to a mixture of cement and aggregate and after its addition concrete becomes stronger. Thus, presence of water is very important for concrete in order to gain strength and evaporation of water from concrete may hinder this process of strength gaining. It has been found that moisture evaporation from concrete under vacuum is 3.97 × 10−8 g s−1 cm−2 as compared to moisture loss in controlled samples which is 0.92 × 10−8 g s−1 cm−2 . Cullingford et al. also confirmed that vacuum exposure releases free water from concrete at a much faster rate as compared to nonvacuumed atmosphere [58]. As far as quality of concrete is concerned, it is deteriorated to some extent when exposed to vacuum during construction [59]. Water loss from lunar concrete may seem an issue but it has also been established that when exposed to lunar environment, only free moisture in concrete may evaporate, but the chemically bonded water will not, which again is a good sign. Although there are findings regarding concrete quality deterioration on vacuum exposure, data provided by the “NASA Astrophysics Data System” show that the effects on concrete strength due to loss of free moisture, which generally takes place around 212◦ F (100◦ C) are not so adverse [60]. So, at the moment, it can be concluded that more research on this particular aspect is needed and vacuum effects on concrete over the period of several years must be evaluated rather than short time analysis. 8. EFFECT OF FREEZE AND THAW Sulfur concrete provides good but not ideal resistance towards freezing and thawing actions. In an experimental study, sulfur concrete specimens were divided into three groups; group I samples were kept

under room temperature, group II specimens were subjected to constant freezing temperature of –27◦ C and the last group was subjected to 50 cycles of freeze/thaw exposure. In this study, no significant reduction in strength was reported due to either constant freezing temperature or freeze thaw cycles [34]. Moreover, sulfur limestone aggregate-concrete not only maintained its integrity after long term testing under acid and salt solutions but also exhibited sufficient resistance to damage by freeze and thaw actions [61]. According to Gillott et al., conventional sulfur mortars and concretes show a freeze/thaw life of up to 80 cycles which is fine but less than the criteria set by ASTM C-666 [62]. In another experimental study, sulfur concrete samples were subjected to the freeze/thaw mechanism at the rate of 27 cycles per week between temperatures –12◦ C to 32◦ C in air, water and oil. Though the samples failed according to ASTM C-666 standards, analyses of the relative compressive strengths, surface conditions, and weight loss data of the water and air-stored samples indicated good freeze-thaw resistance with 107 and 140 cycles, respectively [63]. The decreased freeze/thaw durability of sulfur concrete is due to the contraction and expansion of sulfur crystals in the structure [62]. As temperature on the Moon may vary between 123◦ C to –220◦ C [64], durability of the sulfur concrete against frost actions may become a challenge there and some new techniques may be adopted to make the use of sulfur concrete feasible therein. According to Gillott et al. [62], sulfur concretes having a plurality of substantial spherical gas cells, uniformly dispersed throughout the matrix, display improved durability under cycles of freezing and thawing mechanism as compared to the conventional sulfur concrete. By using this mechanism the number of freeze/thaw cycles for sulfur concrete may be increased up to 300. As far as freezing and thawing mechanism in hydraulic concrete is concerned, air entraining mechanism improves the freeze/thaw resistance and concrete samples have been found durable even after 300 cycles [65] as compared to plain concrete which loses its elastic performance even after 100 cycles [66, 67]. However, artificial introduction of air voids in concrete is difficult to be achieved on the Moon as its atmosphere consists of an infinitesimal amount of gas when compared to the atmosphere of the Earth.1 This leads to the adoption of some other techniques. It is a general agreement that introduction of blast furnace cement with the granulated slag content of 1

http://www.nasa.gov/mission_pages/LADEE/news/lunaratmosphere.html ASTRONOMY REPORTS Vol. 60

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more than 60% shows high frost resistance but experiments have proved that it is still not as good as air entraining mechanism [68]. So, improvement of freeze thaw resistance of lunar concrete remains a challenge. 9. CONCLUSIONS Construction on the Moon requires dust proof machinery. Lesser gravity emphasizes the need of heavy machinery and proper anchorage system. It is estimated that very huge costs will be involved if the construction materials are transported from the Earth. Therefore, the lunar soil and rocks should be focused for construction materials. Physical and chemical properties of the lunar soil favor its utilization as building material. Concrete seems to be the first favorable material for buildings at lunar surface. Water for hydraulic concrete can be made at the surface of the Moon or the huge ice deposits can be utilized; Also, epoxies and sulfur can be used as binding materials. Effects of vacuum and freeze-thaw need further clarification. REFERENCES 1. A. M. Neville, Properties of Concrete (Pearson Education, London, 2011). 2. A. Khitab, Materials of Construction: Classical and Novel (Allied Books, Lahore, Pakistan, 2011). 3. J. A. Bleeker, J. Geiss, and M. Huber, The Century of Space Science (Springer, Netherlands, 2001). 4. R. H. Goddard, Astronautics 4, 24 (1959). 5. R. Ruggles and H. Brodie, J. Am. Statist. Assoc. 42, 72 (1947). 6. F. Tronchetti, The Exploitation of Natural Resources of the Moon and Other Celestial Bodies (Martinus Nijhoff, Netherlands, 2009). 7. G. K. O’Neill, G. Driggers, and B. O’Leary, Astronaut. Aeronaut. 18, 46 (1980). 8. S. A. Stern, Rev. Geograph. 37, 453 (1999). 9. J. D. Blacic, in Lunar Bases and Space Activities of the 21st Century, Ed. by W. W. Mendell (Lunar and Planet. Inst., Houston, 1985), Vol. 1, p. 487. 10. J. Brazell and A. Smith, USACERL Tech. Rep. M91/25 (USA, 1991). 11. J. C. Graf, Space (ASCE) 88, 190 (1988). 12. Y. C. Toklu, in Proceedings of the Space 2000, 7th International Conference and Exposition of Engineering, Construction, Operations, and Business, Albuquerque, New Mexico, February 27– March 2, 2000, Ed. by S. W. Johnson, Koon Meng Chua, R. G. Galloway, and Ph. I. Richter (Amer. Soc. Civil Engineers, 2000), Vol. 1, p. 822. 13. T. D. Lin, G. Ahmed, G. Hill, S. Robinson, T. Lin, C. Lindbergh, and J. O’Gallagher, Am. Concrete Inst. 125, 141 (1991). 14. S. Matsumoto, T. Yoshida, and K. Takagi, Am. Concrete Inst. 125, 29 (1991). ASTRONOMY REPORTS

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ASTRONOMY REPORTS Vol. 60

No. 2

2016