Concrete admixtures for 3D concrete act like a magician’s potion. With just a dash of these chemicals, the properties of concrete mix being 3d printed can be transformed. Admixtures make 3d concrete more workable, accelerate or slow down setting and curing, improve strength, and allow specialized 3d concrete types to be produced. Let’s take a detailed look at some common admixtures and how they affect concrete characteristics.  
Water Reducing Admixtures for 3D concrete
Water reducers decrease the amount of water required in a concrete mix to reach a given slump or consistency. They make the concrete more flowable without adding extra water. The major benefits of water reducers include:   
- Allowing lower water-cement ratios leading to higher strength. Less water means less space taken up in the concrete mix, so it is denser and stronger. Decreasing permeability and improving durability by reducing capillary voids where water can travel. Reducing cracking due to lower concrete shrinkage from less excess water.
- Increased workability without adding water makes concrete placing and finishing easier. Accelerated curing and higher early strength gain due to a reduced water amount that reacts more quickly with the available cement. Normal water reducing admixtures lower the water requirement by 5-10%. High range water reducers, also called superplasticizers, can reduce the water content by 15-30% while maintaining workability.
Retarding Admixtures for 3D concrete
Retarders slow down the initial setting time and hydration reactions of concrete. This delays stiffening and hardening of the concrete after it has been placed. Retarders are useful in hot weather concreting to offset the accelerating effects of high temperatures on curing. Benefits include:  
- Counteracting the accelerated setting caused by high temperatures, allowing more time to place and finish concrete properly before it hardens prematurely. Keeping concrete workable in the delivery truck or concrete pump lines during transportation in hot weather. This reduces rejection of loads.
- Allowing longer vibration and settlement of concrete for better consolidation quality. Providing more time for aligning slip form pavers used to continuously place concrete roads and runways.
- Normal dosage of retarders will delay initial setting by 1-3 hours. Extended retardation for several hours is possible with higher dosages.
Accelerating Admixtures for 3D concrete
Accelerators speed up the rate of early strength development in concrete. They shorten the setting time and allow faster hardening of concrete after placement. Benefits of accelerators include: 
- Earlier development of concrete strength allowing faster formwork removal and reduced turnover time between construction activities. Colder weather concreting by offsetting the slowing effects of low temperatures on concrete curing and hardening. This improves strength. Quicker hardening reduces damage to fresh concrete from rain and aids construction. Rapid strength development is useful in precast concrete applications allowing faster mold turnover and productivity. Reduced protection period against freezing conditions as the critical first few days of curing are accelerated. Accelerators typically allow up to 20% higher strength gain within the first 7 days of curing compared to plain concrete. Their effect diminishes after that.
Air Entraining Admixtures for 3D concrete
Air entraining agents cause hundreds of tiny and stable air bubbles to form throughout the concrete mix. The air voids are typically between 10-500 microns in size. The benefits of entrained air include:
- Air voids provide space into which water from the concrete mix can expand on freezing. This minimizes the hydraulic pressures that damage concrete. Resistance to surface scaling from deicing salts is improved as there are empty voids in the outer concrete to accommodate salt crystallization pressures.
- Added protection in concrete from freeze-thaw deterioration. This ensures long term durability in cold climates. Improved workability and cohesion of concrete through the “lubricating” effect of air bubbles between concrete particles. Reduced segregation and bleeding as the air bubbles act like miniature ball bearings between the mix constituents. The recommended dosage is typically 0.005 to 0.02% by weight of cement to entrain 3-6% air in concrete to get sufficient freeze-thaw protection. Excess air can reduce strength.
Superplasticizers for 3D concrete
Superplasticizers, also called high range water reducers, are advanced admixtures that allow large water reductions up to 30% while maintaining normal slump. Their effects include:
- Greatly reduced water-cement ratios resulting in increased strength, up to 50% higher at later ages. This also improves durability against aggressive environments.
- Accelerated curing and higher early strength development, typically 20% higher in the first 7 days compared to plain concrete. High workability retention over 90 minutes makes them ideal for ready-mix concrete transportation. Slump loss is minimized. Lower permeability from reduced water content improves chemical resistance. Allows special low water-cement ratio flowing concretes that are self-compacting or self-consolidating with minimum vibration. Superplasticized high performance concrete has replaced normal concrete for many demanding applications like high rises and bridges.
Effect of Admixtures on Curing
Most chemical admixtures affect the rate of hydration and setting to some degree. Their influence on concrete curing includes:
- Water reducers lower water content and result in faster curing compared to plain concrete with higher w/c ratios. Retarders slow down the initial curing rate and prolong setting. This delays strength gain in early stages of curing. Accelerating admixtures speed up curing reactions giving higher early strength but benefits diminish after 7 days.
- Air entrainers slightly retard the hydration rate due to surface effects on cement grains and interfere minimally with curing. Superplasticizers accelerate early curing significantly due to reduced w/c ratios. This continues over longer periods and also benefits later strength. Thus admixtures provide a means of tailoring the concrete curing rate and strength gain to suit the application and construction schedule requirements. Careful selection and proportioning is needed to get the right balance of properties.
Compatibility between Admixtures
Chemical admixtures can interact with each other, potentially reducing their effectiveness or even causing unintended effects. Some guidelines for compatibility include:  
- Accelerators combined with water reducers may diminish the effect of the accelerator. The water reducer should be dispensed first.
- Air entrainers work better when added to concrete prior to other admixtures. Extended set retarders may disrupt proper hydration and curing when combined with accelerators. Admixtures from different manufacturers should not be mixed together due to possible chemical interactions. Pretesting of admixture combinations on a small scale is recommended to check for any unwanted interactions before full scale use. Alternate admixtures or adjusting dosages may be needed.
Optimizing Admixture Use for Curing
To optimize admixture use, it is important to understand how each influences concrete properties over time during curing:
- Water reducers provide continuous benefits by reducing capillaries and improving density throughout curing.
- Retarder effects diminish after initial set. Extended retardation may not provide benefits in later curing stages. Accelerators mainly influence the first 1-3 days when concrete gains significant early strength. Later curing is less affected. Air entrainers have a relatively consistent effect over the full curing period by providing freeze-thaw protection. Superplasticizers benefit curing both early and long term due to reduced water-cement ratios. This allows selection of the right admixtures and dosages to achieve the desired curing effects whether early strength gain or long term durability improvement is the priority.
Further readings: For those captivated by the transformative potential of 3D printed concrete, a deeper dive into related topics can broaden your horizon. Explore the exciting prospect of 3D Printed Floating Homes that exemplify the marriage of innovation with sustainability. Uncover the mechanics behind this revolutionary technology in How 3D Concrete Works, a piece that demystifies the complex processes involved. While the allure of 3D printing is undeniable, understanding the True Costs of 3D Printing is essential for anyone considering to venture into this domain. Lastly, delve deeper into the curing journey of 3D printed concrete in How Long to Cure 3D Printed Concrete, which sheds light on the patience and precision required to achieve the desired structural integrity.
 PCA – Chemical Admixtures
 The Spruce – Seven Must-Use Concrete Admixture
 GCP – Water reducers: Where did they come from
 Sika – HIGH RANGE & ULTRA-HIGH RANGE WATER REDUCERS
 Cemex USA – Concrete
 NPCA – Revealing the Mystery of Admixtures
 Wikipedia – Superplasticizer
 Natallia Shanahan – Interaction of Cementitious Systems with Chemical Admixtures
 Cement.org – Types and Causes of Concrete Deterioration