Effect of Calcium Carbide Residue on Strength ...

The purpose of this research is to explore the feasibility of using calcium carbide residue (CCR), a by-product from acetylene gas production, as a solid alkaline activator on the strength development in CCR&#;Portland cement-stabilized dredged sludge (CPDS). The effects of cement content, CCR content and curing time on the strength development of CPDS were investigated using a series of unconfined compressive strength (UCS), pH and electric conductivity (EC) tests. Scanning electron microscopy and X-ray diffraction analyses were performed to gain additional insight into the mechanism of strength development. Meanwhile, the carbon footprints of CPDS were calculated. Following the results, it was found that CCR can significantly improve the strength of cemented dredged sludge. On the basis of the strength difference (ΔUCS) and strength growth rate (UCS gr ), it was recommended that utilizing 20% cement with the addition of 20% CCR is the most effective way to develop the long-term strength of CPDS. In addition, the microstructural analysis verified that the optimum proportion of CCR benefits the formation of hydration products in CPDS, particularly needle-like gel ettringite, resulting in a less-porous and dense inter-locked structure. Furthermore, the solidification mechanism of CPDS was discussed and revealed. Finally, it was confirmed that CCR can be a sustainable alternative and effective green alkaline activator for the aim of improving cemented dredged sludge.

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1. Introduction

With the development of engineering constructions, river dredging, lake dredging and the construction of port channels result in the inevitable large production of dredged sludge [1,2,3], which causes both environmental and ecological damage. Traditional disposal methods such as dumping offshore or burying in the garbage dump would cause unavoidable environmental and ecological hazards to the surrounding environment. Dredged sludge has the natural characteristics of high compressibility, high water content, low bearing capacity and rich organic matter [4,5,6]. That is, the untreated dredged sludge is difficult to be used directly in engineering due to its poor engineering properties. In this context, soil solidification is one of the effective methods for dealing with the gradually increasing dredged sludge [7]. Through a series of chemical reactions, the cured dredged sludge not only solves the environmental problems caused by stacked sludge but also can be transformed into construction materials (e.g., road embankments, structural backfill, etc.).

Portland cement (PC) is the most common curing agent and is widely used for soft soil improvement and solidification of dredged sludge, since it is readily available with a significant curing effect at a low price [8,9,10,11,12,13]. The main curing mechanism of cemented soil is through the hydration, which results in the formation of a variety of hydration products, including calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH), thus increasing the strength [14,15,16,17,18]. However, the application of cement as a stabilizer has raised serious concerns due to its environmental issues:(i) quarries need to be expanded for cement production; (ii) significant CO2 emissions (approximately 0.95 ton) from manufacturing and other associated industrial processes, which contributes 5&#;7% of global carbon dioxide emissions; (iii) the production of non-beneficial by-products, which can cause cement&#;soil to usually exhibit high alkalinity, is detrimental to groundwater quality [19,20,21,22]. Thus, based on the aim of reducing the consumption of cement while improving the strength of solidified soil, the addition of small amounts of the activator may be a promising approach.

Typical activators include quicklime, hydroxides of alkali metals or alkaline earth metals, carbonates, sulfates and silicates. Among them, the most commonly used materials are quicklime, sodium silicate and sodium hydroxide. Quicklime has long been used as a curing agent for the solidification of soft soil because of the remarkable curing effect [19,20]. It is usually used as an activator agent together with cement for curing soft soils [23,24]. However, the production of quicklime, same as cement, consumes large amounts of natural resources, leading to negative environmental impacts. In addition, the preparation of sodium silicate and sodium hydroxide also consumes significant amounts of electrical energy [25]. These traditional alkaline activators are not only expensive but also wasteful on natural resources in the generation process, and they even are poisonous in some cases. Due to the disadvantage of traditional alkaline activators, it is crucial to find a substitute for them.

Calcium carbide residue (CCR) is a by-product of the acetylene industry, with calcium hydroxide as the primary component. According to statistics, there are many acetylene gas production devices all over the world, resulting in a large amount of CCR production. Among them, China is the largest manufacturer and consumer of calcium carbide around the world, accounting for 90&#;95% of global supply and demand, and the annual output of dry CCR in China is 900,000&#;1,140,000 t [26]. In recent years, CCR has been widely utilized for cement manufacturing [26,27,28], but its total utilization rate remains still low (no more than 10%) [29]. Moreover, most of them are landfilled in waste dumps, which not only results in valuable land occupation but also causes huge contamination to the surrounding environment including the pollution of ground water and soil due to its high alkalinity [30]. Because CCR is chemically and mineralogically similar to hydrated lime, CCR has the potential to replace lime in civil engineering [31]. Previous studies proved that soft clays can be successfully treated with CCR. Horpibulsuk et al. [32] investigated the use of CCR as a binder for the treatment of over-wet clay soils used to fill embankment materials and showed that CCR-stabilized soils outperform lime-stabilized soils. Latifi et al. [33] used CCR to improve mechanical properties of green bentonite, demonstrating the potential of CCR as a sustainable alternative to conventional stabilizers. Moreover, the effectiveness of CCR has also been studied in conjunction with other environmentally friendly industrial by-products, including blast furnace refining slag [34] and biomass ash [35]. For example, Yi et al. [34] used CCR to excite GGBS for the improvement of soft soils and found that the mechanical properties of CCR-GGBS stabilized soils outperform those of Portland cement stabilized soils.

So far, CCR has been used itself alone or in conjunction with other industrial by-products as a soil stabilizer. However, there is very limited information on CCR used as a solid alkaline activator to substitute traditional activators for cement binder. The application of CCR as an eco-friendly activator in cement-stabilized dredged sludge is thus novel and crucial among the geotechnical fraternity. Therefore, the aim of this study is to investigate the effect of CCR as a solid alkaline activator, an environmentally friendly alternative to traditional activators, to improve mechanical properties of cement-stabilized dredged sludge. A variety of tests including unconfined compressive strength (UCS), pH and electric conductivity (EC) were conducted to investigate the physical&#;chemical and mechanical properties of CCR-Portland cement-stabilized dredged sludge (CPDS). An array of microstructure tests were conducted to fully reveal the possible curing mechanisms. Furthermore, the carbon footprints of CPDS were calculated and compared with those of cemented dredged sludge. Finally, the solidification mechanism of CPDS was discussed and revealed in this research. The outcome of this study would utilize large amounts of CCR from landfill sites and reduce carbon emissions from Portland cement production significantly.

Key takeaways:

• Some fruit growers use calcium carbide to ripen fruit artificially. This is more common in regions outside the U.S. 

• Calcium carbide is known to have dangerous side effects. But this is a bigger concern for farm workers who come into direct contact with it. 

• If you&#;re buying fruit in the U.S., you probably don&#;t have to worry about side effects of calcium carbide. But there are still a few things you can do to avoid any potential harmful exposures. 

Calcium carbide (CaC2) is a chemical that&#;s used to ripen fruit. Ripening chemicals are used to keep fresh fruit available, even when it&#;s not in season. But calcium carbide is risky to use because it&#;s hazardous to human health. And reports of the use of calcium carbide have increased in recent years, mostly in developing countries. This article explains how calcium carbide is used to ripen fruit and how to avoid the risks linked to it.

How is calcium carbide used to ripen fruit?

When calcium carbide is mixed with water, it releases acetylene gas. Acetylene gas acts as a fruit ripener. It has a similar activity to ethylene, a natural hormone that fruits produce. Ripe fruits release this gas &#; this is the reason why some fruits ripen quicker when you store them with other fruits.

Ethylene gas is also applied to fruit as an artificial ripener. It&#;s approved by the U.S. National Organic Standards Board, which makes guidelines for organic fruit standards. Ethylene is much safer than acetylene and calcium carbide.   

Are there health risks associated with calcium carbide?

There are several health risks associated with calcium carbide and acetylene gas.

One risk of eating fruit ripened with calcium carbide is due to contamination. Calcium carbide is made to process metal and industrial chemicals. It&#;s not intended to be used in food preparation. Because of this, it includes other chemicals that are toxic to humans, especially arsenic and phosphine. 

Exposure to these chemicals can cause side effects like:

• Mouth and nose irritation 

• Nausea and vomiting

• Thirst 

• Dizziness 

• Fatigue

• Skin problems 

• Kidney failure

Workers in fruit production who have heavy exposure to calcium carbide have had serious health problems. These include:

• Pulmonary edema, a condition where fluid builds in the lungs 

• Cardiac arrest, when the heart suddenly stops

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Other risks are with acetylene gas itself. Acetylene gas is highly flammable and explosive. It also can cause breathing problems in those who work around it. This poses more of a risk to fruit workers than people who eat fruit. 

Acetylene gas can accumulate in fruits when large amounts are used to ripen it. It may be possible to have negative health effects from eating massive quantities of fruits that have stored acetylene gas. This could include nervous system problems like dizziness, fatigue, and confusion. But this doesn&#;t seem to be a common issue.

Should you avoid fruits ripened with calcium carbide? 

If you know that a fruit was ripened with calcium carbide, it&#;s best to avoid it. But if you live in the U.S., you may not have to worry about eating fruit ripened with calcium carbide. Ethylene is often used when fruits are artificially ripened in the U.S.

Many countries have banned the use of calcium carbide in fruit ripening. But some fruit producers still use it illegally to keep costs down. Reports of the use of calcium carbide are mostly from countries in South Asia and Africa.

How can you avoid calcium carbide?

You would probably need to eat a lot of fruit imported from certain locations to experience any harmful side effects. But there are certain things you can do to avoid eating fruit ripened with calcium carbide:

• Wash your fruit well. This is to remove any chemicals that could remain on your fruit. 

• Eat local fruit when you can. Fruit that doesn&#;t need to be shipped can be picked when it&#;s riper.

• Eat fruit that&#;s in season. Seasonal fruits don&#;t need any ripening agents. 

• Notice how the fruit looks and tastes. Fruit that&#;s artificially ripened is more likely to be ripe only on the outside. And it may be less sweet and flavorful than naturally ripened fruit. 

• Check online if you&#;re eating imported fruit. The country where the fruit is from should be on the label. You can do an online search to see if calcium carbide use has been reported in that country.

Another way you can avoid calcium carbide is to buy organic fruit &#;  if you can. Organic fruit has been grown with certain guidelines. Ethylene can be used as a ripening agent but not calcium carbide. 

But organic food is more expensive, and can be harder to find at your nearest grocery store. So don&#;t worry if this isn&#;t an option for you. A good wash will do the trick. And buying local produce (at a farm stand or farmers market) is a great way to lower potential exposure to harmful chemicals. Many times, it&#;s also less expensive than regular fruit at the grocery store.

The bottom line

Calcium carbide and acetylene gas are used by some growers to ripen fruit. These chemicals are unsafe for humans. In the U.S., fruit producers usually use ethylene gas, which is a ripening hormone made by plants. If you&#;re concerned about calcium carbide, it&#;s a good idea to buy local, in-season fruit. If you buy imported fruit, wash it well to avoid chemicals.

Why trust our experts?

Written by:

Katherine Krive, DO, is a freelance medical writer and editor. She is also a psychiatrist who has practiced in hospital, academic, and community settings.

Edited by:

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

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