Paper sacks provided equivalent shelf life as plastic sacks20 January 2020
A recent study found that paper sacks and form-fill-seal (FFS) polyethylene sacks provided equivalent protection for cement when stored for 18 months under the same conditions. The study was conducted by the Norwegian research organisation SINTEF on behalf of the European Paper Sack Research Group (ESG), a collaboration between CEPI Eurokraft and EUROSAC
“Since cement hardens when reacting with water, perfect product protection of the packaging is vital when storing cement for a longer period of time in order to conserve its quality and properties,” explains Catherine Kerninon, General Delegate of EUROSAC. “To ensure that our industry meets the requirements of cement producers and can compete within the packaging market for cement, we investigated the shelf life of the two sack solutions that are used most widely on the market: cement paper sacks and cement plastic sacks.” For the study, typical sack solutions of each type were chosen: a standard cement paper valve sack made of two paper layers of 80 g and 70 g with a 12 μm high density polyethylene (HDPE) free film barrier, and a standard plastic sack made of three layers of COEX PE film (LDPE, HDPE and LLDPE) with a total thickness of 120 µm.
Three sacks of each type were filled with the same type of cement and stored in an outside storage house in Norway where they were exposed to ambient weather conditions throughout the test period. The sacks were stacked on a wooden pallet and covered by a plastic stretch hood – a typical storage situation. “Although the average storage time for bagged cement in Europe is estimated to be no longer than 2 to 3 months, in our study we wanted to investigate how the sacks perform within longer storage periods of up to 18 months,” says Catherine Kerninon. “Therefore, after three different time periods, one sack of each type was collected.” After sampling and homogenisation of the cement, the samples were subjected to different standardised test methods for measuring the quality of the cement after each storage period.
Results demonstrated long shelf life for both sack types
The outcome of the tests revealed: Whether total weight, level of hydration, mortar strength, initial flow behaviour or 28-day compressive strength – the quality and performance of the periodically sampled cement from both types of sacks was well within the requirements of the cement industry. “The study shows: If you want to store cement for a period of at least 18 months, paper sacks provide a good shelf life performance. They perfectly protect the quality and properties of your product,” summarises Catherine Kerninon. A short video explains the design of the study and its most important results.
At the same time, paper sacks have strong environmental credentials: their carbon footprint is 2.5 times smaller than that of plastic sacks. This is the result of another recent study by the Swedish research institute RISE. Read more details here.
Best practices for storing paper sacks
“Paper sacks are an efficient and sustainable packaging solution for powdered and loose materials like cement,” concludes Catherine Kerninon. “In order to take full advantage of these properties, we have published recommendations on how to handle industrial paper sacks properly along the supply chain.” Part 2 provides information about risks and best practices during distribution and storage to distributors and retailers. Download the handling recommendations here.
Paper and plastic sacks provide equivalent shelf life
The European Paper Sack Research Group (ESG) commissioned the Norwegian research organisation SINTEF to investigate the shelf life of the packaging solutions used most widely throughout Europe for the packaging of cement: paper sacks and polyethylene form-fill-seal (FFS) sacks. Stakeholders have claimed that plastic bags provide a better shelf life than paper bags. Although the typical storage time for bagged cement in Europe is estimated to be no longer than 2 to 3 months, the study set out to determine how the sacks perform throughout longer storage periods of up to 18 months.
For the investigation, a standard 25 kg European cement paper valve sack made of two paper layers of 80 g and 70 g with a 12 μm high density polyethylene (HDPE) free film barrier and a standard plastic sack made of three layers of COEX PE film (LDPE, HDPE and LLDPE) with a total thickness of 120 µm were used. All sacks were filled with Portland cement CEM I 52.5 R according to European standard EN 197-1 cement. Due to their smaller capacity, two plastic sacks were employed for each batch of 25 kg cement.
Stored in an outside storage house in Norway, a total of three sacks per type were tested. Representing the typical secondary packaging, the sacks were stored on a wooden pallet covered by a plastic stretch film. The sacks were exposed to changing climatic conditions. Temperatures varied between –17.9 °C and 32.1 °C and the relative moisture ranged from 28% to 96% (sampling of humidity on a random basis).
Sampling and homogenisation
After 9, 12 and more than 18 months of storage, cement samples from both sack types were collected and analysed. The sampling and homogenisation of the cement was conducted as follows: In the first step, the cement was divided into four equal parts. Secondly, two diagonally opposed quarters were recombined and manually homogenised (mixed). Step 1 and 2 were repeated three times, resulting in sampling approximately 3 kg of cement from each sack which was sent for analysis.
Test methods to evaluate cement properties
The samples were subjected to three different test methods to determine the water content in the cement and the performance of the cement after each storage period.
1) Thermogravimetric analysis
In a thermogravimetric analysis, the total amount of physically and chemically bound water in the cement was measured by registering the weight of the sample as it was heated from 30 °C to 950 °C. A weight loss (loss on ignition) would indicate the formation of a hydrate product due to the release of H2O (decomposition of hydrates) or CO2 (decomposition of carbonates).
2) Calorimetric analysis
Secondly, a calorimetric analysis was conducted on the cement paste mixed externally from the samples to measure the amount and rate of heat which evolved during cement hydration. Measurements were performed up to 24 hours from the point of first contact between dry powder and water against a calibrated reference of similar mass and heat capacity. As the hydration reactions proceed, heat is released. The heat production rate, which is proportional to the hydration rate of the cement, is recorded in the isothermal calorimeter.
3) Mortar casting
The third analysis was mortar testing, which measured the initial flow and 28-day compressive strength according to DIN EN 196. This test method indicates if the performance of cement varies or remains constant over the defined period.
1) Thermogravimetric analysis
According to the thermogravimetric analysis, with increasing storage time the total weight loss for paper sacks increased slightly more than for plastic sacks. It was 3.17% for plastic sacks and 3.61% for paper sacks. However, this latter result is an increase of only 0.55 percentage points compared to the fresh cement and is still within the requirement of ≤ 5.0% loss on ignition for cements according to DIN EN 197-1.
2) Calorimetric analysis
The calorimetric tests showed that the levels of hydration within 24 hours were essentially unchanged. The cumulative heat of hydration after 24 hours, which correlates to the mortar strength at 24h, was within the standard repeatability (5–7 J/g) for all the tested cement samples.
3) Mortar casting
The mortar strength at 24h was also within the standard repeatability for all the tested cement samples. The flow and the 28-day compressive strength for mortar cast with the cements did not change significantly after storage in both tested sacks.
The outcome of the tests revealed that the quality and performance of the periodically sampled cement from both types of sacks was well within the requirements of the cement industry concerning the properties of the product. The study showed that paper sacks provided an equivalent shelf life of at least 18 months for cement as form-fill-seal (FFS) polyethylene sacks.
Strong environmental credentials
At the same time, the carbon footprint of paper sacks is 2.5 times smaller than that of plastic sacks. This is the result of another recent study by the Swedish research institute RISE. Read more details here.