Shell Mix

Fibers Measured

Using a high strength mortar formula for a hand-applied or sprayed-on, plaster-style shell, creates a hard, long-lasting, and waterproof membrane that does not require advanced skill and ownership of professional-level tools to mix and apply.  This particular range of composite design for thinner shells provides several advantages and is fundamentally different from standard concrete or mortar composites. It is a material that is well suited for an improved ferrocement thin shell style, and is easier and less expensive to use because it allows a smaller volume of a higher performance material to replace a larger volume of less durable material that is more costly (in terms of labor expense, safety expense, material expense, equipment expense, design expense, and testing expense). 

The materials and equipment for this mix are readily available to small business professionals, home-owner builders, and non-professional Do-It-Yourself builders.  Special properties that are unique to this composite design make it easier to accurately predict its compressive strength while also making it easier to endure a certain level of mixing errors and curing errors.  On-site monitoring techniques that are unique to this composite design allows the quality of the shell material to be accurately evaluated and improved to compensate when needed.

With a high strength shell, the composite particles in the material matrix are much closer and fit more tightly into one another allowing more chemical bonds so the toughness, or hardness, is greater.  Toughness and durability are greater when the measured compressive strength is higher.  For this particular high strength mix design, described here, the compressive strength is 2 to 6 times higher than conventional cement-based composites depending upon the desired density. 

Predictable Compressive Strength 
The compressive strength is highly predictable and directly dependent upon the percentage of aeration or, in this case, Closed Spherical Cell Aeration. Percent aeration can be measured by determining the density of the wet mortar as it is made.  Simply weighing a measured portion of the mortar achieves this.   This measure should be done when making normal test cubes with fresh mix in the field (construction site) by recording the gram weight of a properly tamped, void-free, two-inch test cube. 

It is surprising and fortuitous that, for special reasons explained below, the compressive strength of this particular mix design can be accurately and reliably predicted just by obtaining the gram weight, or density, of a fresh test cube. This effect of reliably predictable compressive strength is unusual and the discovery is important. This predictable compressive strength effect helps this high-strength mortar formula to be used with greater reliability by new builders and it helps it to be trusted more by building inspectors (due to the ability to document). 

Mortar cube being crushed to test compressive strength.  Cubes retain most of their strength after they initially give way to pressures as high as 50,000 pounds then slowly crush down like a beer can while still holding substantial weight. Broken Test Cube

After a test cube is broken to monitor the compressive strength, the material still retains 60 to 80 percent of its highest strength until the cube is crushed down to a smaller size.  This shows that even if the shell endures significant damage, it does not shatter and crack and instead  retains most of its strength and is easily repaired.

With standard concrete and mortar, exact proportions of ingredients, especially water, exact mixing conditions and proper curing conditions, are critical for consistent results.  Consistent results are still difficult to obtain for certain reasons.  Open-pores, or open capillaries, of traditional mortar, can cause the shells to dry out too quickly making proper curing more dependent upon factors such as temperature, humidity and wind.  Also, other factors can cause unpredictable results, such as difficulty in adding the same amount of water because of variations in the amount of moisture already contained in the sand or, the effect of a composite particles ability to absorb or isolate water during the chemical hydration process. 

The components of this mix design addresses these issues of variations in material and curing condition by achieving a capillary-free matrix where all the water is reacted and gel water removed despite the ability to know for certain how much water is present.

The compressive strength is also more predictable because there is assurance that all the cement particles are fully hydrated where as, in traditional concrete, an uncertain amount of dry Portland cement remains nonreactive, depending upon factors such as a water content, composite characteristics, and mixing speed and method.  These variations in moisture content and cement hydration that cause fluctuating results are eliminated with this mix design.

The following is an older video about making plaster mix

Play Video

Eucon MSA (Silica Fume)
Eucon MSA was originally (formerly called Eucon SPJ) made by Boral Materials Technology, but is now made by Euclid Chemical Company. The MSA is a .3 micron sized particle that has crystal spins, so it chemically binds the other crystal forming around it in a myriad of ways, which greatly enhances the reactive powder advantage that causes late-term compressive strength gain.  The CO2 carbonate reacts with the atmosphere, which causes cracks to close tight and a steep rise in compressive strength after 90 days. From there, a gradual increase in compressive strength continues ad infinitum due to compact density of particles, a denser mix that is typical of ultra low water mixes

Plastol 5000
The compressive strength is, in part, more predictable with this mix because it uses a special polycarboxylate based, high-range water reducer/superplasticizer, called Plastol 5000 (from Euclid Chemical). Adding a few ounces of Plastol 5000 (12 to 20 oz. for a one bag mix) to wet mortar mix that has a minimal amount of water and so has a dry, damp-sand consistency, will cause the damp mortar to gradually become fluid as it mixes, transforming into a molten and often drippy consistency.  This magic-like fluidity effect greatly minimizes the amount of extra mix water needed to make a mortar workable and easy to place, and also causes much more of the dry cement particles to react with water by allowing the composite particles to churn and flow as a liquid. When more of the remaining gel water is eliminated (the water that is otherwise left unreactive within the composite matrix), the percentage of open capillary voids is reduced or eliminated so the compressive strength is increased, the permeability of the composite is improved, and the shell does not shrink when it dries.  It is this gel water that causes the open capillary voids because when that extra water dries away the composite shrinks and leaves connecting capillary voids behind where the extra water was before.  When the composite dries and causes the concrete to shrink significantly, it stores up internal strains, like stretched springs, inside the matrix, making it more prone to cracking and it causes shrinkage cracks.  When a mix has little or no gel water, or extra mix water, it shrinks very little or doesn’t shrink at all.  No-shrink composite mixes are synonymous with high strength mixes.  Mixes that have no or less shrinkage have improved chemical bonding in between the composite components because the particles are closer.  Less water equals tougher concrete.

The Plastol 5000 water reducer also increases the percentage of dry Portland cement hydration, which reduces the percentage of dry Portland weak spots in the composite matrix, an effect that also translates to variable compressive strength numbers due the unpredictable amount of this weakening effect. This extra hydration of Portland cement, in turn, increases the percent of chemical bonding, which also improves compressive strength or composite integrity.


SEFA Star Micron 3 (Fly Ash)
The SEFA Group, Inc. Star Micron 3 (Fly Ash) a refined micro pozzolan, called Star Micron 3, is also added to the shell mix.  The SEFA Group makes Star Micron 3.  Star Micron 3 takes hydration nearly all the way to boundary of what is possible, eliminates mostly all of the remaining free and unreacted gel water in the composite, and increases the density of the composite in other ways as well.   Star Micron 3 is a tiny particle compared to the cement particles and other composite components. It acts like tiny ball bearings helping the larger particles to roll, loosen and move, which helps the suspended gel water and the dry Portland cement (that may otherwise remain dry) meet up and react.  Star Micron 3 also helps the larger particles settle in closer to one another while filling in the small spaces in between the larger composite particles.  This results in a denser matrix with fewer voids and better chemical bonding between the aggregate particles.

Star Micron 3 is a pozzolan, similar to regular fly ash, so it reacts and binds with the cement particles to help create more chemical bonding.  It is far superior to regular fly ash because a much smaller portion of it is needed for the pozzolan enhancement effect that is observed, which is a slower but longer duration and higher strength gain. The lower dry powder demand of the refined pozzolan fly ash decreases the water demand needed for the desired pozzolan strength enhancing effect.   This improves the integrity of the composite significantly compared to regular fly ash.  For these reasons, Star Micron 3 is the only fly ash one should consider if the goal is making high strength thin shells.

Mearlcell 3532
Capillary causing gel water is fully eliminated by the final adding of a special synthetic foamed additive called Mearlcell 3532.  The additive is foam that is produced from the concentrated foaming compound in a foam generator or foaming drill paddle. It makes a foamed material that resembles runny shaving cream and provides the property known as Closed Spherical Cell Aeration that is distinct from open capillary aeration or air entrapment in important ways.  It is typically used for making cellular concrete, which is a low-strength cementitious composite.  Mearlcell reduces the water demand even further, much closer to the threshold of the minimum amount possible, and it also reduces the amount of gel water and unreactive dry cement in the same way the Star Micron 3 and Eucon MSA do, by providing a gradation of tiny round bubbles that increases the fluidity of the wet composite dramatically (thus helping non-reacted water and Portland cement to react). However, the Mearlcell does something else that is unique and even more interesting.  It, being the last additive, actually binds with any excess water remaining at this point.   For this reason, Mearlcell 3532, is the key to achieving the unusual and important compressive strength predictability. 

The water drawing effect was first observed in experiments where dry Portland cement was precipitated from wet, fully hydrated Portland.  Mearlcell foam was blended with a pumice aggregate that seemed to have the tendency to soak up additional water from the cement hydration process.  This, combined with water drawing effect of the Mearlcell, seemed to cause the strange reversal of cement hydration where wet cement was magically made dry again.  Joe David Lacy, when experimenting with pumice blended with cellular concrete, conducted this experiment in 1998 and observed the dry nodules of Portland precipitated from a neat mix of Portland and water that had already been fully hydrated before blending with the aggregate.  The foam additive appears to bind with the tiny amounts of extra mix water, and isolates the mix water from the hydration process and mix water from the gel water formation.  If slightly too much water was used initially and lots of gel water was still in the mix after running the gel water elimination gauntlet of the MSA and Micron 3 additives, this extra water appears to be absorbed by the Mearlcell, and turns into smooth, individually sealed spherical bubbles instead of allowed to form complexly shaped and interconnected capillaries. 

Extra water in the mix may cause the aeration percentage to rise, but if the density goes lower than the specifications for the compressive strength needed, the density can be checked while the mix is fresh and corrected in the mixer by adding more mortar to offset the foam content thus increasing the compressive strength.  This water drawing effect of the foam has not been documented in standard cellular concrete that uses closed spherical cell foaming compounds such as Mearlcell 3532, but samples of standard, ultra low density neat cellular concrete has been observed to be able to block moisture intrusion.  For example, we tested six-inch cylindrical samples of 30 pcf cellular concrete that was kept under water for more than a month, and it was still dry in the center when sawed in half.  This indicates that open capillary aeration is largely replaced by closed spherical cell aeration. This type of aeration is shown to block moisture penetration, because the samples would not still be dry in the center if open capillary aeration were present to a large extent around the spherical bubble composite pieces.

The observation and discovery of this effect of compete gel water removal in high strength; ultra low-water mixes may only work for this type of mix and was discovered during the development of this particular mix design by Doug Lacy. 

The development of this mix and the initial idea of the framing material that accompanies this mix were inspired by ideas and discussions with Joe Lacy. 

Overall, this mix uses significantly less water than other low water-to-cement ratios.  The mix uses 2.5 to 3 gallons of water for a whole 94 # bag of Portland cement plus 12.5 pounds of Micron 3.  Excluding the capillary and gel water contribution from the sand, which is unknown relative to the moisture content of the sand, it puts the water-to-cement-ratio (w/c ratio) at: 0.195 w/c to 0.235 w/c. This is much lower than what many consider to be possible and the absolute minimum amount of water needed to theoretically hydrate cement, a number often placed at 0.25 w/c. Even if you were not considering the Micron 3 to be a cementitious material, the w/c would still be very low.  Dry powder Micron 3 does demand water and perhaps should be considered part of the cement reaction because it is included in the cement reaction that consumes water and adds strength and helps prevent gel water induced capillaries that weaken cementitious composites that use more water.  Even without considering Micron 3 a reacting cementitious material, the water-to-cement ratio adjusted to not consider the weight of the Micron 3 is still 0.22 w/c to 0.266 w/c, which is still very low.  This indicates that it is likely that the three water reducing additives, especially the Mearlcell, are indeed effectively removing all the remaining gel water to produce a creamy and workable plaster that has predictable compressive strength and is a high strength material that is ideal for hand plaster application, including overhead plastering. From a certain point of view this may seem like an insignificant advantage because the open capillary aeration and the closed spherical cell aeration both lower the compressive strength but, the resulting compressive strengths are still very high and well into the range of Ultra High Performance Concrete.  Also, the closed cell aeration imparts important properties to the high strength mix. 

For a complete view of all of the materials, check out this video –>

Impact Resistance
The closed cell aeration also provides impact resistance.  When you have a very hard shell, it can be brittle and susceptible to long impact cracks.  The air bubbles in this shell actually absorb impact and prevent cracking. The tiny air bubbles act like air bag cushions and by absorbing kinetic energy in the same way.  A hammer blow or bullet will make a tiny smooth dent instead of breaking and cracking.

Freeze/Thaw or Ice Protection
The closed cell aeration also provides a place for chemical expansion of freezing ice.  The ice crystals form inside the bubbles like little stalactites in a cave instead of wedging apart aggregate by getting in the voids between.

Rust & Chemical Expansion Protection
Ice isn’t the only thing that expands and breaks concrete.  Iron oxide is five times the volume of iron. This is why our steel reinforced concrete infrastructure is failing and only has a service life of a few decades.  The closed spherical cell aeration prevents progressive cracking and moisture intrusion. This aeration also causes the rust to grow inside the spherical cell voids instead of causing internal stress and breaking within.  The rust growth within the cells also creates a chemically stable zone around the steel in or near the matrix. This has a fossilizing effect on the steel and stops the rust after a certain amount has created a stabilized barrier zone.  This effect may help structures made with these shells to last thousands of years instead of hundreds since the both the known mechanism that damage concrete, freeze/thaw damage and chemical expansion, are removed.


Impervious Water-Proof & Vapor-Proof Barrier 
The tighter matrix also means there are less voids or capillaries surrounding the aggregate particles.   This absence of porosity gives the shell a higher surface-scratch hardness. This is because the aggregate particles are bonded more tightly.  This enables the shell to block moisture or vapor from penetrating. 

Absence of Shrinkage Stress and Cracks
The high-strength mortar uses much less water and it is often called ultra low-water mortar.  As a result of using the smallest amount of water possible, the mix shrinks less as it cures. Less shrinkage helps the composite bond more tightly to the supporting reinforcing by pulling away less from the contact edges and surface.  Less shrinkage also helps prevent shrinkage cracks and internal strains inside the shell. More important, the absence of shrinkage in a low-water smooth liquid style mortar, as opposed to a dry-packed grout, indicates the mortar is without open capillaries. These open capillaries, that define the difference between vapor-proof shells and standard concrete mortar shells, are created as the excess mix water and gel water in the wet matrix dries off.  Without the open capillaries, these high-strength mortar shells exhibit radically different properties than standard cement based mortar. They are fundamentally different in terms of structural performance and durability by demonstrating greater resistance to chemical attack, freeze/thaw damage, impact damage and abrasive wear. 

In addition to the Eucon MSA Silica Fume, Star Micron 3, Mearlcell 3532, and Portland cement, plaster sand, Forta Ferro Fibers, Nycon PVA RSC 15 Fibers, Plastol 5000 high-resin polycarboxylate polymer super plasticizer/water reducer are also used in the shell mix.