Effect of Cellulose Ether and Starch Ether on Hydration ...

The production of factory-made mortars is a multicomponent system. Viscosity-modifying admixtures (VMAs) are an inherent ingredient of these materials. The correct choice of the amount and type of these admixtures is important from the practical and scientific points of view. In this article, the use of cellulose ether (CE) and starch ether (SE) in cement pastes and mortars is studied. This research focuses on the hydration process and fresh-state properties of mortars because this subject determines the correct choice of the amount and type of admixture used, and the results determine the application and properties of hardened mortars. Polymers were added in the range from 0.056% to 0.22% in relation to the dry ingredients of the mortar. The research showed that cellulose ether had the greatest impact on the consistency, air content, bulk density, and water retention of ordinary dry-mix mortars. On the other hand, starch ether affected the hydration process, delaying the setting and hardening processes much more than cellulose ether. The action of these admixtures rose with the increase in the amount of polymer used in different ways (depending on the type of ether).

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1. Introduction

Chemical admixtures are widely used to modify mortars and concretes in order to obtain standard or special properties. Mortar formulations quite often comprise from a few to over a dozen different ingredients, so the development of such recipes for mortar requires great knowledge and practice. Even small amounts of some admixtures can have a significant effect on the properties of fresh and hardened mortars. Among the wide range of these substances used in the production of factory-made masonry, plastering, and adhesive mortars, there are polymer admixtures, which include, among others, cellulose ethers and starch ethers [1,2,3,4,5,6]. The way in which both of these admixtures affect the properties of mortars is varied and depends on many factors [7,8,9,10,11,12]. The correct selection of the type of admixture and its amount consists of a compromise between obtaining the appropriate consistency and rheological properties (ensuring adequate fluidity, plasticity, and nonsegregation), good workability, and high water retention, which ensures the appropriate conditions for hydration, as well as the mortar setting and hardening processes, and reduces shrinkage deformation.

Cellulose is a natural polysaccharide. It commonly occurs as the basic construction material in plant tissues. Pure cellulose is a white, water-insoluble substance, which must be further processed to be used as a chemical admixture. Cellulose ethers arise from a chemical industrial process. The plants etherified to obtain polymer admixtures include pure cotton (about 90% cellulose), wood (about 40–55% cellulose), and straw or reed (30–40% cellulose) [13,14,15,16,17]. The production of these admixtures consists, inter alia, of grinding, activation with NaOH aqueous solution, a heterogeneous reaction with an etherifying agent, neutralization, the isolation of crude cellulose ether, purification by extraction of the salts and by-products, drying, and confectioning [16,17]. These cellulose derivatives control the water retention in fresh-state mortars and can increase adhesion power [7,12,18,19,20,21,22,23,24,25]. They influence the rheological properties (viscosity and yield stress). Their purpose is the improvement of the workability of mortars [3,7,18,26]. Additionally, cellulose ethers affect the delay of hydration processes, as well as the setting and hardening processes [3,10,27,28,29], and decrease compressive strength [3,5,30,31,32,33].

Starch is a natural polymer derived from the polysaccharide group. It is most often obtained from potatoes or corn. It is found in tubers, seeds, and plant roots. The main component of starch is amylopectin [9,11,34,35]. Due to its physicochemical properties, the possibilities of using starch are quite limited. Therefore, it undergoes physical and chemical modifications that give it desired parameters, such as texture, solubility, and consistency. Starch ethers are added to dry-mix mortars or concretes. They can modify the rheology of fresh-state mortars by bridging flocculation. In general, starch ethers can enhance workability and reduce segregation of the components of mortar [11,34]. As introduced in previous publications [5,8], starch ether has shown a considerable delaying effect without adversely impacting compressive strength (unlike cellulose ether admixtures).

In the literature, one can usually find research on the properties of mortars modified with only one admixture—either starch ether or cellulose ether. Analyses often concern the influence of one type of admixture on the properties of pastes or mortars (for example, taking into account various chemical modifications, quantity, and viscosity) [4,6,10]. Some studies focus on the detailed analysis of a selected problem. In articles [10,20,27,28], the impacts of cellulose ethers on C3A or cement hydration have been examined. These works have shown that cellulose ether has an influence on these processes. This polymer leads to a gradual slowing down of hydration dependent on cellulose ether chemistry. Water retention and rheological measurements have been tested in various reports [12,22,26]. The influence of cellulose ether on water retention is more noticeable than when using starch ether. Research has also tested mortars consisting of combinations of several different ingredients, one of which being either cellulose ether or starch ether [2,4,18,30].

Some research on pastes and mortars can be found in which the authors compare the actions of different VMAs. In [5], mortars with four types of chemical admixtures, including, among others, cellulose ether and starch ether, were tested. The effects of these polymers on the working and mechanical properties of dry-mixed mortars and the relationships between the performances were studied. The amount of admixtures used was in the range of 0.02–0.08% (by mass in relation to all the dry ingredients of the mortar). Consistency, water retention, and 28-day compressive strength tests were carried out for mortars with different amounts of each of the admixtures. On the other hand, the setting time was determined for samples with selected amounts of ethers (three amounts of cellulose ether and one dose of starch ether), which did not allow the full assessment of the setting and hardening processes. As can be seen, the effect of cellulose ether was more varied in the case of the consistency test. The consistency for mortars modified with cellulose ether ranged from 88 mm to 98 mm, whereas for mortars modified with starch ether, it ranged from 60 mm to 110 mm. The water retention results were similar for both ethers (from about 85% to 100%). As the admixtures were added, the setting time increased. In [36], the influences of two polysaccharides were compared. The research reported different properties between welan gum and cellulose ether; fluidity after 0, 60, and 120 min; 3-day and 28-day compressive strength; 7-day and 28-day adhesive strength; rheological properties; and zeta potentials. Mortars modified with welan gum thickened instantly and increased after 60 min, but the fluidity was still smaller than that in the reference sample. After a longer time, the fluidity dropped. The fluidity of mortar modified with cellulose ether after 0 min was the same as that of the reference sample (without admixture), but the flow decreased with time. Comparing the fluidity loss measurements between 0 min and 180 min, the mortar without admixture had 25.3% fluidity loss, but the results of modified mortars were only about 10.2–11.5%. The results for compressive strength and adhesive strength were different for cellulose ether and welan gum. For example, the strength loss of mortar with cellulose ether amounted to about 47%, whereas mortar with welan gum reached about 7%. On the other hand, the adhesive strength of mortar with welan gum was similar to that of the reference mortar, but the tested parameter was doubled at 7 days and 14 days for mortar with cellulose ether. In [26], the authors reported the influences of cellulose ether, starch ether, and guar gum on the rheological behavior of render mortar and their relationships with water retention. The amounts of admixtures used were from 0.03% to 0.24% (cellulose ether) and from 0.03% to 0.12% (starch ether and guar gum). Increase in the amount of cellulose ether or guar gum resulted in a significant increase in the water retention value (in the range from about 80% to about 100%), which was not as noticeable when adding starch ether (in the range from about 74% to about 86%). Starch ether increased the gel feature of mortars and had more impact on cohesion, while the other two admixtures decreased the gel-like responses of the tested samples. The research concentrated only on the parameters of mortars in the plastic state (application properties), and the setting and hardening processes were not taken into consideration.

The goal of the present work is the study and comparison of the uses of two types of viscosity-modifying polymer admixtures, which have important roles in dry-mixed mortars. The variables in the study are the type of admixture used and its amount. The paper reports different properties between mortars with cellulose ether and starch ether, as well as consistency, air content, and bulk density. The WRV value is defined as water retention measurements tested after, 10, 20, and 30 min. On this basis, the application properties of these materials are analyzed. Additionally, the influences of admixtures on the setting and hardening processes of the pastes are examined on the basis of calorimetric measurements and setting-time research. Setting-time research gives information about the initial, final, and total setting times, but the calorimetric method allows the full introduction of the hydration process in the first several hours of research. These parameters can allow a full understanding of the actions of cellulose ether and starch ether, which is necessary for the correct choice of the type and amount of admixture used. Additionally, if starch ether is added to dry-mix mortars with cellulose ether, understanding of its action and choosing the amount of starch ether should be preceded by research on the separate effect of each ether on the properties of pastes and mortars. In the present work, we assess how the same amount of cellulose ether and starch ether admixture affects the tested properties, as well as whether the scope of their activity is the same. In addition, the statistical influence of the amount of admixture on the tested parameters of pastes and mortars is verified, and on this basis, trends in changes of the tested parameters are determined. In summary, the article supplements the knowledge on the scope of operation of cellulose ether and starch ether based on the standard and nonstandard properties of pastes and mortars in the plastic state, as well as during the setting and hardening processes. On the basis of the research performed, it can be stated how the properties of pastes and mortars change, taking into account variable doses of admixtures, and it can be assessed how the same amount of both cellulose ether and starch ether affects the tested parameters, which complements the knowledge on the scope of operation of these polymers.

Introduction to starch ether:

 

Starch ether is a modified starch that introduces ether groups through a chemical process. The modification process gives the starch specific properties, making it more suitable for a variety of applications, including the construction industry. In mortars, starch ethers are used as additives to modify the rheological, mechanical and thermal properties of the mixture.

 

Types of Starch Ethers Used in Mortar:

There are various types of starch ethers used in mortars, and the choice depends on the properties required of the final product.

 

Common types include:

 

Hydroxyethyl starch ethers (HEC): These ethers enhance water retention and improve processability. They are effective in preventing rapid moisture loss, especially in hot and windy conditions.

 

Hydroxypropyl Starch Ether (HPC): HPC is known for its thickening properties and ability to improve mortar cohesion. It helps prevent ingredients from separating.

 

Carboxymethyl Starch Ether (CMS): CMS is widely used for its water retention and thickening properties. It can effectively improve the consistency and workability of mortar.

 

The role of starch ether in mortar:

 

Water retention:

 

Starch ethers act as water-retaining agents in mortars. They form a thin film around the water particles, preventing rapid evaporation and ensuring that the water is available for hydration of the cement over a longer period of time. This is essential for the mortar to cure and set properly.

 

Workability improvements:

 

Adding starch ether can improve the consistency and ease of use of the mortar, thereby enhancing the workability of the mortar. This is especially important for masons and construction workers as it allows for better handling and placement of mortar.

 

Thickening and segregation prevention:

 

Starch ethers help thicken the mortar and prevent its components from separating. This ensures a more even distribution of particles, resulting in a uniform and structurally sound final product.

 

Adhesion and cohesion:

 

Starch ethers improve the adhesion between mortar and various substrates. This is critical to the overall bond strength of the mortar to bricks, stones or other building materials. Additionally, they enhance the cohesion of the mortar itself, improving its overall structural integrity.

 

Reduce shrinkage:

 

Starch ethers can help reduce the shrinkage of mortar during drying. This is important to minimize cracks and ensure the long-term durability of the masonry.

 

Effect on the freshness of mortar:

 

consistency:

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Starch ethers help maintain a consistent and desirable consistency of mortar, making it easy to apply and form.

 

Opening hours:

 

Opening time is the duration that the mortar remains usable. Starch ethers extend the opening time by slowing the evaporation of water, providing longer time for construction activities.

 

Effect on the hardening properties of mortar:

 

Strength development:

 

Starch ethers can influence the early and long-term strength development of mortars. The controlled water retention properties of these additives promote proper cure and hydration.

 

Durability:

 

Improved water retention and reduced shrinkage contribute to the long-term durability of the mortar. This is critical to the structural integrity and longevity of masonry structures.

 

Crack resistance:

 

The cohesive and adhesive properties imparted by starch ethers help minimize the development of cracks, especially during the drying and curing stages.

 

Resistant to harsh conditions:

 

Mortars modified with starch ethers exhibit enhanced resistance to harsh environmental conditions, such as temperature changes, wind and sun exposure. This is especially important for structures in harsh climates.

 

Application notes:

 

dose:

 

The appropriate amount of starch ether depends on a variety of factors, including the type of ether used, local climate conditions, and the specific requirements of the mortar mix. For optimal performance, manufacturer's recommendations must be followed.

 

Mixing procedure:

 

The starch ether should be evenly dispersed throughout the mortar mixture. Proper mixing procedures are critical to ensuring the effectiveness of these additives.

 

compatibility:

 

Starch ethers should be compatible with other additives and components of the mortar mixture. Compatibility testing should be performed to avoid any unwanted interactions.

 

QC:

 

Quality control measures should be implemented regularly to monitor the performance of starch ether modified mortars. This includes consistency, strength and durability testing.

 

in conclusion:

 

In summary, starch ethers play a multifaceted role in improving the performance of mortars in construction applications. Their effect on water retention, workability, strength development and durability make them valuable additives for those seeking high-quality, long-lasting masonry structures. As construction practices continue to evolve, the use of starch ethers in mortar formulations may remain a key strategy to achieve optimal performance and address the various challenges of the construction industry. Understanding the nuances of starch ethers in mortars enables architects, engineers and construction professionals to make informed decisions and take advantage of the benefits of these additives in their projects.

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