1 Introduction

River sand, as one of natural resources used by human beings for thousands of years, plays a key role in the development of multiple industries, especially in construction industries [1]. The use of sand has stood the test of time. From the ancient Great Wall to modern skyscrapers, sand and gravel are needed as raw materials due to its unique particle shape and irreplaceability as fine aggregates in concrete and mortar.

In the past two decades, the shifts in construction patterns, population growth, urbanization and infrastructure development have tripled river sand demand. About 50 billion tons of sands are consumed per year, with an average of 18 kg per person per day [2]. Alternative building materials have not alleviated the sand shortage. As for existing buildings in China, it is estimated that steel structure roughly accounts for 10% and timber structure accounts for 5%. Approximately 85% existing buildings consume natural sand. Every year over 10.5 billion tons of river sand are used in concrete and mortar all over the world [3, 4]. After decades of rapid development of the construction industry, the total amount of construction industry in China has gradually stabilized. Data of total completion area of all kinds of architectures and the cement consumption in 2003–2019 is summarized and shown in Fig. 1 [3]. The construction industry in China remained stable at a high level between 2014 and 2019 after the rapid development in 2003–2014. Judging from the current development trend of data, the high construction volume and the consumption of nature resource will still last for a long period in China, which makes the problem of sand resource shortage more urgent to be resolved. Compared with other industries, the shortage of sand resources in the construction industry is particularly acute worldwide.

Fig. 1
figure 1

Data of construction area and cement production in China (2003–2019) [3]. a Development of construction area and cement production in China; b Construction area for different functions in China

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River sand is the best choice of fine aggregate for the civil engineering industry, which is related to the unique formation process of river sand. River sand is erosional product of geotechnical body over a long-term. Shattered geotechnical body and original sand enter rivers that are like conveyor belts [5]. The shape and particle size of river sand are appropriate and the content of impurities is low after the scouring of the river water, which makes river sand much suitable for constructions [6]. It is self-evident that the human demand for sand and gravel is as great as the demand for houses.

Therefore, river sand has become the largest natural resource used in construction and the most extracted materials worldwide, exceeding fossil fuels and biomass [5]. However, unlike other resources, river sand is generally regarded as a non-rare natural resource, and it is generally taken locally without strict regulation. So, mining river sand can quickly bring considerable economic benefits, which in turn leads to excessive exploitation of river sand. This scenario directly leads to a much faster consumption than nature production in river sand resource. [7].

The hazards of over-exploitation are manifold. First of all, the large-scale mechanical sand mining will change the direction of the river, destroy the stability of the river bank and bed, increase soil erosion, and flood [7, 8]. In coastal areas, excessive sand mining can also cause problems such as rising sea levels and narrowing of beaches [9]. Secondly, large-scale sand mining will also exacerbate water scarcity [5]. River sand is part of the aquifer and maintains the connection between the river and groundwater. Excessive sand mining will cause the groundwater level to drop. River sand contains water and is mined blindly. A large amount of water is lost during mining and transportation [10]. In addition, destroying surface vegetation will weaken its water purification function, causing drinking water sources to be polluted.

The shortage of river sand resource and suffered hazards are regarded as looming crisis [7], and many studies have focused on the consequences of river sand shortage and how to alleviate this gap. As the supply of previously available river sand became scare, various countries and regions gradually began to restrict the arbitrary mining of river sand. The emphasis on sand resources has received widespread attention, and stricter resource conservation and utilization measures and trade restrictions have been proposed. However, the effect is not satisfactory. For example, the Yangtze River, the longest river in Asia, drainage area accounts for about one fifth of China's land area. For centuries, the Yangtze River has been irrigating this land and accumulating a lot of sand. According to “Yangtze Mud and Sand Bulletin” [7], the natural transportation amount of river sand in Yangtze River has been dropped significantly. The data of sediment transport from six hydrological stations along the Yangtze River in four provinces are summarized and illustrated in Fig. 2 [11]. From 2006 to 2017, the sediment transport in the main stream of the Yangtze River showed a significant downward trend. To cope with the excessive consumption of river sand, China began to implement the Yangtze River Sand Mining Management Regulations in 2002. But an inadequate regulatory system made this policy futile. Countless sand mining ships still stole river sand at night. According to the statistics, since 2002, the Yangtze River has contributed 840 million tons of sand, but only 80.46 million tons of those got permit. For decades, various human activities have almost used the accumulation up [11].

Fig. 2
figure 2

Sediment transport in Yangtze River [11]

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At the same time, analogy with petroleum resources and biomass energy, the international disputes faced by river sand resources after the shortage should also be paid attention [12]. With the uneven increase in demand for river sand, the natural distribution of river sand resources has also been studied [13]. The distribution around the world of river sand resources and their natural replenishment are unable to obtain directly. In order to quantify and highlight the uneven distribution of river sand, the amount of sand transport can be roughly used to characterize the distribution of river sand resources in various regions. Based on the study of 957 rivers around the world, Asian rivers have the largest annual sand transport volume, accounting for more than half of the world [13]. However, Asian countries have relatively few management measures on the river sand. Whether this uneven distribution and utilization will exacerbate the shortage of river sand is still uncertain.

The importance of river sand and the impact of sand mining have been gradually realized, and a variety of solutions to the shortage problem of river sand have been proposed in most countries. The local and global impacts of river sand collection are deeply summarized to prove its damage to the environment. Existing and conceivable measures are classified and analyzed, and five scenarios are proposed to discuss the effects of these measures. Technical measures, policies and bans, which can be summarized as “5R” (Reduce, Recycle, Reuse, Replace, and Recover), could partially alleviate the negative influence caused by excessive mining of river sand and disorderly trade. Despite those efforts, a global river sand strategy for solving the shortage and using resources sustainably does not yet exist.

2 Excessive river sand mining and its impact

River sand resources are mined from freshwater lakes and rivers. During the process of mining, transportation, and trading, they have had an important impact on the environment, economy and society, both locally and globally.

2.1 Environmental impact

River sand attached to the riverbed is defense of geotechnical body. Disorderly mining leads to irregular pits where the slope angles increase and mechanical strength of geotechnical body has decreased significantly. This can lead to local collapse or cracks, and the balance between water and sand is broken. Apart from the adjustment of riverbed, capillary water in the ground surface is destroyed, which in turn causes land desertification. The impact on topography is bigger than we ever thought [14].

Sand mining exposes groundwater and changes the condition of groundwater circle. The ability of riverbeds to store underground subsurface flow will disappear, the hydraulic connection between underground subsurface flow and shallow pipelines changes, and pipelines cannot be replenished, resulting in drying up of shallow wells, the death of trees, and the reduction of agricultural production [3]. Bridges and embankments were built according to long-term hydrological situation, and large-scale sand mining result in water level drop. Pile foundations will be exposed to harsh environment and easily damaged.

Besides, water bodies and biodiversity are also affected. Sand mining stirs up sediment and reduces water transparency of rivers and lakes, which weakens the underwater light condition. And then the photosynthesis of flora can be impaired which leading to its delay growth or even destruction [1].

2.2 Social impact

Scarce river sand resources are given the value of trade, and further through international trade caused a lot of social impact. Countries that cannot meet their own demand for river sand shifted their focus to other countries. According to China's natural sand import and export trade data (see Fig. 3) [15], such a large volume of trade activities will inevitably lead to energy consumption and environmental pollution. The uneven distribution of river sand resources between regions has caused a lot of expenditure to be consumed in the process of transportation and trading, which intensifies the economic impact of river sand resources.

Fig. 3
figure 3

The natural sand import and export data in China [15]

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After the gradual expansion of regional influence, the shortage of river sand resources may trigger predatory development like other mineral resources that have been valued, which may gradually develop into international resource problems and even lead to international disputes. Cross-regional rivers will directly induce conflicts between countries in the basin, such as the Mekong River, while trade conflicts between regions will indirectly induce trade conflicts among countries [9, 12]. But so far, there are no relevant policies and regulations to prevent or resolve such disputes, which should be paid attention seriously.

3 5Rs principle and solution

The sand shortage is becoming a worldwide problem, to solve this problem, new “5Rs” principles was taken up with in this paper, and the strategies for river sand shortage solutions are summarized as “Reduce, Recycle, Reuse, Replace, and Recover”, which are explained in details in the following subchapters.

3.1 Reduce

This principle means reducing river sand consumption, and is the most basic measure to solve the river sand shortage. This principle requires that the amount of river sand should be reduced as much as possible while meeting the needs and requirements of humans for construction.

On the one hand, the demand for new buildings and repair the buildings should be reduced. It is estimated that the safety of about 30% ~ 50% of buildings has declined or entered a period of functional degradation [16]. However, the pursuit of economic development requires developing countries to adopt simple and cheap demolition methods in the short term [17]. Waste building materials cannot be used, and a brand-new building consumes a lot of river sand, which makes the gap between supply and demand of river sand increasingly widening. Therefore, in the long run, the profit of this demolition method is not worth the loss. In view of this situation, some research has been conducted on the renovation of the building. If new structures are not built and old ones are not demolished, building maintenance will consume fewer natural resources.

On the other hand, more consideration should be given to the structure with fewer consumables, such as the timber structures, bamboo, and rattan structures, etc. Timber structure is a simple structure that has good seismic behavior and excellent thermal insulation performance [18]. Due to forest degradation and the pursuit of high-rise buildings, the occupation rate of timber structures in some countries is now very low, but in some countries, the occupation rate of timber structure houses is as high as 90% [19]. Therefore, with the increase of plantation forests and the technology improvement of timber structure, it is also a very potential alternative in China. Similarly, bamboo and rattan have been used as building materials in various areas of residence and life since ancient times: pavilions, viewing platforms, etc., are not only resting places, but also unique ornament [20]. Bamboo and vine structure are easy to maintain, safe and comfortable. The most important reason is that the bamboos and vines grow rapidly, which proves that bamboo and vine structures are sustainable building types. In recent years, the International Network for Bamboo and Rattan (INBAR) has cooperated with the International Organization for Standardization (ISO) to formulate relevant standards and specifications for bamboo buildings. With the active promotion of the development of bamboo and rattan structures by the government and international organizations, bamboo and rattan have been applied as green building materials in the construction industry. The application will have broad development prospects.

3.2 Recycle

The recycling utilization of river sand is the most sustainable way to solve the shortage of nature resources [21]. It is estimated that the total construction and demolition waste (C&D waste) generation was over 1.5 billion tons in 2017 (excluding construction spoil) and waste concrete accounted for 30% to 40% of the total C&D waste [22]. Over 40% of recycling production from waste concrete is suitable to use as recycled sand in preparing concrete or mortar.

Recycling of concrete waste has the potential to reduce the amount of building waste requiring to be disposed of at landfills and to preserve natural resources and energy. The Chinese government has already taken big steps to promote the recycling of waste concrete as secondary resources. China has carried out reclamation of waste concrete in more than 30 provinces and cities, and about 100 recycling plants with a capacity of more than 1 million tons per year have been built [23]. There have been many recycled concrete buildings rising in China successfully, such as the Shanghai Ecological House located in the 2010 Shanghai World Expo Park, residential buildings in Du Jiangyan City constructed in the post-earthquake reconstruction area and a 12-storey high-rise building in Shanghai [24]. It should be known that these successfully implemented cases do not represent the end of construction waste recycling. These examples are only a good start in terms of current construction waste utilization rate.

3.3 Reuse

To reduce the demand for river sand and dispose of old buildings, new ideas and technologies are proposed. Nowadays a new way dealing with overage buildings, called deconstruction, has been proposed [25]. Like building blocks, old buildings are deconstructed into components and regroup new buildings with building blocks. With this solution, the continuing service of buildings rarely requires natural materials. An old building could be reassembled again and again, with little or no natural materials. Buildings suitable for deconstruction must undergo a reasonable design for disassembly and assembly during the entire construction. The implementation of such new technologies can enable building components to be reused in the future, which will save the consumption of natural sand and other resources [26].

3.4 Replace

This principle means replacing natural river sand with substitutions, which could be divided into three main categories. The first category is natural sand and gravel that is less developed and will not have much impact on the environment. The second category is artificially manufactured or modified sand and gravel, while the third category is solid waste after crushed, sieved, and pre-treated.

The first category of natural sand and gravel includes gravel from excavated soil waste and marine aggregates. Sand and gravel in the excavated soil waste produced by engineering construction replace natural river sand. Studies have shown that the similarity of chemical composition between the excavated soil waste and natural river sand is 60% ~ 85%. The available utilization rate of the treated excavated soil waste is as high as 98.4% [27]. When the replacement rate of this kind of sand and gravel is lower than 50%, the mechanical properties of the. concrete have not been significantly affected [28].

The application of marine aggregates in the construction industry is not uncommon, but because sea sand contains chloride ions, it will accelerate the corrosion of steel bars in the concrete building, which may cause damage to the building in severe cases. It is necessary to clean and desalinate sea sand before applying it in the concrete building [26]. Therefore, a large amount of sea sand is often used as a fill project. If the scale and technology are expanded and the cost is reduced, sea sand will be a very potential substitute [29, 30].

With huge reserves and low cost, desert sand is widely concerned and it is expected to replace river sand in concrete in the future. The area of desert around the world has exceeded 36 million square kilometers, accounting for about 20% of the global land area. The average thickness of the world desert is about 3.5 m. It is roughly estimated that the desert sand volume is 1.26 × 1014 m3 [31]. Large area of desert has caused various environmental problems, including sandstorms and climate deterioration. The total amount of construction sand is limited compared to the total amount of desert sand. Therefore, the use of desert sand can not only solve the problem of the shortage of construction sand, but also solve the environmental problem to a certain extent [32].

Despite these advantages, desert sand cannot completely replace river sand in construction because of its physical and chemical properties [33]. Compared with river sand, desert sand has a finer particle size with more than 80% of the grains smaller than 0.16 mm, which does not meet the size range for fine aggregate in the concrete standard in China, Europe and the USA. The extra fine particle size of desert sand brings higher water requirement, higher air content, lower workability, and lower strength of concrete [34]. In terms of chemical properties, corrosive elements in the desert sand can cause corrosion of reinforcing steel in building components, which in turn reduces the durability of concrete structure and cannot be widely promoted. Studies show that desert sand can replace part of the river sand in construction with measures, including sieving to acceptable grades, using efficient admixture and finding suitable applications in construction materials [35]. Applications with lower strength requirements for cement-based materials, such as masonry materials, are expected to use desert sand instead of river sand [36].

The second category of sand and gravel includes machine-made sand, waste plastic modified lightweight aggregate and so on. Manufactured sand refers to sand processed by machines and other auxiliary equipment. The raw material can be natural stone or tailings. Compared with natural river sand, the manufactured sand has less impact on the environment.

The third category is using solid waste to replace sand and fillers in concrete, which has been suggested by some researchers [37, 38]. For instance, waste glass can be used as a replacement for river sand in concrete after crushing and sieving [37], while it can also be used as a high value-added supplementary cementitious material after grounded to fine powder [39]. On another note, the marine clay may also be used as filler substitution in concrete [38]. After calcined at 700 °C, the crushed marine clay can replace the filler in ultra-high performance concrete.

In addition, some industrial waste can also be used as substitutes for sand and gravel. In recent years, some scholars have processed waste plastics into aggregates and used them to prepare concrete or mortar [40]. Relevant studies have shown that using waste plastic particles to modify ordinary concrete can not only realize the renewable utilization of waste plastics, but also improve the performance of ordinary concrete. Studies have shown that plastic particles can significantly improve the flexural toughness and fatigue properties of concrete, and replacing lightweight aggregates with plastic particles with a replacement rate of less than 8% can increase the compressive strength, splitting tensile strength and flexural strength of concrete [41]. The volume of waste plastics is huge and difficult to degrade. When used to prepare sand and gravel, it not only solves the problem of sand shortage, but also solves the problem of environmental pollution caused by waste plastics.

3.5 Recover

Alleviating the shortage of river sand requires not only reducing consumption, but also increasing natural replenishment. Excessive sand mining has induced great impact and harm to the natural environment. In turn, the deterioration of nature has also caused a negative impact on human beings. Therefore, to solve the problem of the shortage of sand and gravel, it is important to recover the natural environment. River sand is erosional product of geotechnical body over a long-term. Shattered geotechnical body and original sand enter rivers that are like conveyor belts. Therefore, river sand can be divided into replenishing sand and geological deposits. Excessive sand mining not only exhausts replenishing sand, and consume a lot of deposits in just a few decades [42]. In order to solve the environmental degradation caused by excessive sand mining, in principle, natural restoration should be the main method and artificial restoration is supplemented. Therefore, the most basic and most important measure is to at least stop the increase in mining, so that the natural environment can regain river sand replenishment. In addition, some artificial restoration measures should be taken into account. For example, the restoration of the coastline is mainly through the methods of beach berm sand replenishment, artificial dune integration, underwater sand dams and sand dike filling [43]. Through these methods, it is possible to restore natural river sand and achieve sustainable development of sand and gravel mining. At the same time, the impact of restricting sand mining is multifaceted. For example, China is enforcing a ten-year fishing ban in the Yangtze River [11]. Sand mining will destroy the living environment of organisms of the river, which will greatly reduce the effect of the ban on fishing. If the ten-year policy of exhausting sand can be implemented at the same time, at least the effect of the fishing ban will be significantly improved.

4 Policy scenario and system dynamic modelling

4.1 Local policy

It is a prerequisite for maintaining available river sand resource to balance the sand collection and natural replenishment of sand. Compared with China, developed countries such as the USA and UK have paid more attention to river sand resources. Sand mining generally happens in aquatic ecosystems, due to excessively disordered mining. It will bring serious ecological and environmental problems, in addition to destroying sand resources.

In response to the negative impact of sand mining, the Chinese government issued regulations on river management in 1988 [44]. Recently, the relevant departments have been strengthening rectification throughout the country. The sand mining industry ushered in a storm of rectification across the country, and many local governments have issued documents to prohibit random sand mining activities. According to incomplete statistics, there are more than 20 municipalities, nearly 200 prefecture-level cities, and more than 1,000 county-level cities, which prohibit and restrict sand mining [45]. However, as it can be seen, these measures have had limited effect. Probably we should improve management strategies.

Sand mining vessels are easier to supervise, and control vessels often regulate sand mining behavior indirectly [46]. In the past, these measures may not be effectively implemented. However, QR code and base station positioning technology may be able to solve this problem well. As we have seen, in the fight against the epidemic, these technologies can not only indicate the place of departure and destination, but also can roughly track the itinerary, which can better manage the sand mining ship.

Massive demand makes the value of sand and stone rise rapidly and remain high, and high profits give birth to disorderly mining, and it is infeasible to blindly prohibition, which has been confirmed in many other countries like Nepal, India, etc. [47]. China is positively looking for alternatives. Manufactured sand has recently been vigorously promoted in China. Besides, with the support of policies, the production scale has rapidly changed from a simple dispersed artificial or semi-mechanical workshop to a large-scale intensive mechanized automated factory, and the industry has made great progress. Manufactured sand gradually replaces natural sand to make up for the market demand. At present, it accounts for nearly 70% of construction sand. In addition to manufactured sand, recycled sand and other alternatives got promoted and related technical specifications have been issued and applied in production.

Although regulations including “Water Law of the People's Republic of China” have been enacted in China, the status of river sand resources has not been elevated to the level of national strategic. The collection of river sand has been prohibited by law and the penalties for pirate sand mining have been increased. But due to the lack of effective supervision, there are still many unreasonable phenomena existing from river sand mining to application in the construction industry. More stringent supervision and attention to river sand resource are expected [48]. In addition to maintaining sand available curve through law and regulations, encouraging policies for new technologies are expected to reduce the local demand for sand resources.

4.2 Global policy around the world

In addition to local policy restricting the unsustainable development of river sands, international policies are also crucial. After local policy restricts domestic river sand mining, the market will greatly increase imports, thus implementing excessive sand mining in countries that do not restrict river sand mining, called predatory sand mining. Predatory mining will have a major impact on the environment, energy and international trade in regions where sand resources are currently abundant, further exacerbating the global unsustainable use and shortage of sand resources [1]. Obviously, the shortage of sand resources and predatory sand mining will further trigger international disputes. But unlike oil, natural gas and rare metals resources that have been highly valued, river sand is not restricted by international conventions. Restricted mining and export bans in some countries cannot solve the global shortage of sand resources. In the sand resource international disputes, regional-scale sand resource mediation agreements were adopted.

The USA has made a good demonstration about this. The House of Representatives in USA passed The National Strategic and Critical Minerals Production Act of 2015 [49]. The bill stipulates that gravel aggregates and related minerals used in infrastructure construction are recognized as key strategic resources in the United States. This bill aims to establish a market monitoring mechanism to avoid under-capacity or over-capacity in the strategic mineral resources industry, which will greatly protect the domestic gravel resources. Therefore, international sand resource contracts are suggested to be formed. Referring to the Kyoto Protocol on Carbon Emissions Limitation, the International Union of River Sand Resources should restrict sand mining in all countries and make recommendations on river sand trade from a macro perspective.

4.3 System dynamic modelling

The system dynamics (SD) model is commonly used to study changes of related variables in dynamic changes [50]. Seven different kinds of data have been considered to predict the demand of river sand, including economic, population, structure, policy, recycled and replaced subsystem (Fig. 4). In each subsystem, the original facts of impacts, the transmission factors of impacts and the main impacts that directly affect river sand demand are summarized in green, blue and brown box diagrams, respectively. Besides, the relationships between different factors are represented by arrows and serve as the basis for building the system dynamic model.

Fig. 4
figure 4

Subsystem of system dynamics model for river sand demand

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First, the demand of river sand without any solutions taken has been predicted by the system dynamics model as demonstrated in Fig. 5. In this model, the river sand price, concrete and masonry structure ratio, replacement ratio of recycled sand and regeneration speed of river sand are considered unchanged, thus simulating a situation where no river sand restriction measures are taken. GDP growth rate, birth rate and death rate are set to change over time, and its change trend is based on changes in previous years and government plan. The relationship between building demand and its factors is obtained by linear regression.

Fig. 5
figure 5

System dynamics modeling for basic river sand demand

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Another system dynamics model was set to explore the impact of various strategies for solving the problem of river sand shortage on the demand of river sand, as shown in Fig. 6. In this model, the data of building demand was originated in the original model. The price of river sand is considered to be influenced by the proportion of imports and mining policies, and affects the proportion of sand for structural use, the proportion of river sand substitutes used, and the recycled rate.

Fig. 6
figure 6

System dynamics modeling for changed river sand demand

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All data were collected from database of China National Bureau of Statistics (data.stats.gov.cn), database of China Aggregates Association (www.zgss.org.cn/index.html), and database of Yangtze River Hydrological Station (www.cjh.com.cn).

4.4 Policy scenarios

The introduction demonstrated the strategic significance of sand resources and the urgency of solving the problem of river sand shortage. To obtain a feasible solution and analyze the effects, some related data was collected and prediction model was established. Using the method in the Sect. 4.3, the curve of sustainable sand resources supplies and the demand of sand resource have been predicted. The data including natural replenishment of river sand in Yangtze River, building area of completed constructions, birth and death rate, GDP per capita, etc. are used to establish the model.

Based on the above discussions, five scenarios with different measures were developed to estimate the change in river sand demand over the period 2020–2035. The compositions of the policy scenarios are showed in Fig. 7. Scenarios were settled by three technical measures (recycle, reduce, and replace) and market policy to promote technology applications. The details of five policy scenarios are as follows:

  1. 1)

    WoPS: Without policy scenarios, business as usual;

  2. 2)

    HreuPS: Promote building construction reduction, with low recycled and replaced rate;

  3. 3)

    HrecPS; High recycled rate, with high urban renewal rate and low replaced rate;

  4. 4)

    HrepPS, High replaced rate, with normal urban renewal rate and low recycled rate;

  5. 5)

    WAPS, With all policy scenarios, minimize the gap between supply and demand.

Fig. 7
figure 7

The relationship between policy scenarios. Note: H and L represent high and low intensity of policy scenario

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5 Results and discussions

The results of the SD model are summarized in Fig. 8. WoPS highlights the future development of river sand shortage and provides a baseline for comparing the effectiveness of policy scenarios. Three scenarios (HreuPS, HrecPS, HrepPS) compare the effects of policy scenario with single measure. WAPS combines all the measures to reduce the demand for river sand and considers the interaction between the individual measures to estimate the extent to which the shortage of river sand could be alleviated by the setting measures.

Fig. 8
figure 8

Results of the SD model for different policy scenarios

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It has been shown in the results that the four types of technology and policy measures can partly serve the purpose of reducing the demand for river sand. In the case of a single policy scenario, the gap can be reduced to 52%, 43%, and 35% respectively in 2035. In the case of WAPS, the gap will be reduced to 4%, which is almost equivalent to the contradiction between conventional supply and demand of river sand resources.

It is noted that the effect of HRecPS in the early stage is not as good as that of HRepPS. This is because the application of river sand substitutes has been accepted by the market, while recycled sand has not been completely recognized and accepted. However, in the later stage, the river sand substitutes are still natural non-renewable resources, which also has contradiction between supply and demand. On the contrary, the recycled sand products from construction waste will have a wider source in the future. The gap between supply and demand will further expand (up to 62.70%) if no measures are taken into account. The implementation of single measure can fill up to 16% of the gap, while for compound measures, it can fill up to 27.20% of the gap. The model tested the situation where all measures were implemented vigorously, and proved that the demand for river sand could be maintained at the current level and slightly decreased.

5.1 Potential solutions

5.1.1 Decrease the ratio of concrete structure consuming less sand

Compared to the concrete structure, the timber structure and steel structure more in line with renewable requirements. Production process without sand and reusability greatly reduces the demand for sand.

5.1.2 Complete industry specification and solve technical problems

As mentioned, some European countries consumed lots of marine aggregates without obvious adverse effects. However, due to the lack of technical support and regulatory system, marine aggregates are used only as filling materials in China. With the innovation of technology, desalinated sea sand can not only be used as building aggregate, but the performance of the building is almost the same as that of ordinary buildings. What should governments do is issuing technical guidance and providing policy support. Thus, more alternatives could be used at acceptable cost, such as FRP-reinforced concrete with sea sand.

5.1.3 Establish an international river sand sustainable development alliance

Predatory exploitation of sand resources, long-distance transportation, and international trade are exacerbating the environmental impact and triggering international disputes. In order to curb the global shortage of sand resources, a global organization of sand resources like the Organization of the Petroleum Exporting Countries (OPEC) need be established. The alliance can reach a unified import and export agreement, stipulate rules for mediating disputes, and restrict excessive sand mining caused by international trade.

5.2 Impact and contribution of the above solutions to the construction industry

In addition to studying the impact of the above solutions on the supply and demand of river sand, the impact on the construction industry needs to be discussed. The impact can be divided into the following three parts:

Development of sustainable structural systems. The promotion of new technologies will gradually move the construction industry towards sustainability and environmental protection. More sustainable materials will be used in construction, gradually changing from the people-oriented development concept to the idea of harmonious coexistence between human and nature, which is beneficial to the sustainable development of construction industry.

Development of construction retrofit and deconstruction. The method of urban renewal has gradually changed from building reconstruction to building retrofit after disassembly, which greatly reduced the amount of initial construction work. With the economic progress, the proportion of building repair and strengthening in China has been greatly increased, and the architectural deconstruction is gradually being accepted as a new sustainable building renewal way.

Usage of replacement and recycled production of river sand. The further research will bring more suitable and sustainable substitutes for river sand and more efficient recycling technology, which will further reduce the cost of raw materials in the construction industry. The industry has more profits to support the practice of new technologies and promote comprehensive progress in the construction industry in materials, design, and construction.

6 Concluding remarks

River sand has been widely used in variety industries, and has been proven to be in short supply currently and during the coming decades. The status of river sand has been evaluated as a strategic resource by many countries. Moreover, local and global problems, including geological, environmental, and economic issues, were caused in the process of mining, transportation, trading and use of river sand.

The supply of river sand affects the development of economy, modernization, and urbanization while the mining of river sand influences the local geology and the global environment. Therefore, it is necessary to balance the production and utilization of river sand resources. 5Rs, including reduce, recycle, reuse, replace, and recover, were summarized as strategies to obviously alleviate the shortage of river sand resources.

Through the prediction of multiple-linear-regression and system dynamics model, the demand for river sand will keep increasing in the next 15 years with a decreasing growth rate. System dynamics model was settled with seven subsystems to predict the impact of policy measures on the demand of river sand. Technical solutions and policy measures have been sorted out and summarized in five situations. The model evaluated the impact of different scenarios on river sand demand, which was proved that can remain unchanged under high policy strategies.

Three suggestions are put forward, from the perspectives of structural material selection, industry technical specifications, and the International Sustainable Development Alliance, to jointly solve the local and global river sand shortages problem. The problem has been particularly prominent in China in the past decades, but it will become a worldwide problem in the future. Achieving sustainable development of the construction industry requires global collaboration, the promotion of new technologies, and agreements on comprehensive solutions for river sand resources.