Recent Advances in Magnesia Cement Claudio Manissero, Premier CPG

SDC Technology Forum # 38October 9th, 2015

Agenda

• Overview of magnesium cements• Historical perspective• MgO grades – product properties• Magnesium Oxychloride Cement (MOC)• Magnesium Phosphate Cement (MPC)• Magnesium ammonium phosphate cement (MAPC)• Magnesium potassium phosphate cement (MKPC)

• Magnesium Oxysulfate Cement (MOS)• Applications

• Magnesium cements are so-called calcined cements• Cements produced from calcined magnesium carbonate

rock (magnesite) and a cross-linking agent to form a cementitious matrix.

• Water is needed for reaction and becomes part of the matrix.

• Reactions are acid-base reactions.• Reactions are exothermic. Heat generated dependent on

nature of crosslinking agent, speed of reaction and nature of fillers.

Overview of Magnesium Cements

• Variations of magnesia cements predates invention of Portland cement.

• Magnesium cement has been used for millennia in Germany, Italy, France, Switzerland, India, China, Mexico, Latin America, and New Zealand, among other countries.

• Original cements based on naturally occurring magnesia and other metal oxides.

• Rediscovered and patented MOC by Stanislav Sorel in 1867• Terrazzo floors of the 18th and 19th century were mainly MOC based• New interest in technology due to their durability and

environmentally friendly attributes.

Historical Perspective

• Ancient Rome – Prime example is the Pantheon

• Source of material was Magnesia, Italy

• Magnesia deposits “calcined” by volcanic activity in area.

• Widespread use in roman buildings

Historical Perspective

• Other examples• Sections of the Great Wall of

China• Some Indian Stupas

Historical Perspective

• Raw material magnesium oxide (MgO) is derives from magnesite (magnesium carbonate) rock or extracted from brine.

• In US only one operating mine at Gabbs, NV by Premier Magnesia

Magnesium Oxide (MgO) grades

The Gabbs location is a world-class mine with 70 years of proven ore reserves and probable reserves over 120 years.

Magnesium Oxide (MgO) grades

Premier Magnesia Gabbs facility

Magnesium Oxide (MgO) grades

Martin Marietta Brine facility, Manistee, MI

Magnesium Oxide (MgO) grades

Light burned MgOBurned at 700-1000oC Most reactive form of MgO

Hard burned MgOBurned at 1000-1500oC Most reactivity eliminated

Dead burned MgOBurned at 1500-2000oC All reactivity eliminated

Unburned magnesite mineral

Magnesium Oxide (MgO) grades

A variety of kilns are used to produce MgO

Rotary Kiln

Shaft Kiln

Inefficient Rotary Kilns or Shaft Kilns can over burn or under burn material which produces inconsistent product.

Magnesium Oxide (MgO) grades

Best available technology available are Multiple Hearth Furnaces for the highest consistency and highest quality product. Used to produce light burned MgO

Equipment and process used for calcining crucial to production of consistent product for magnesium cements.

• Formed by reaction of MgO with magnesium chloride (MgCl2) in the presence of water

• Grade of MgO usually light burned, high reactivity.• The main bonding phases found in hardened MOC pastes

are Mg(OH)2 (magnesium hydroxide), 3Mg(OH)2•MgCl2•8H2O (3-form) and 5Mg(OH)2•MgCl2•8H2O (5-form). The 5-form exhibits superior mechanical properties

• Increased amounts of water will decrease the ultimate strength because the lower strength 3-form starts to develop in competition with the 5-form.

Magnesium Oxychloride Cement (MOC)

Plastic Properties• Neat MOC paste is very fluid and not suitable for casting. The addition of

fillers/aggregates and other components provides ability to control viscosity and workability.

• Set time for neat MOC paste can vary from a few hours to 24-48 hours for a fully formulated product.

• Curing is affected by temperature, air flow and humidity conditions.• MOC tends to be thixotropic . Rheology can be controlled by selection of raw

materials, molar ratio and concentration of the magnesium chloride solution. • To improve flowability, high range water reducers used in OPC concrete are

applicable. • Thorough mixing is necessary to “wet out” the mix.• Since it is an acid/base reaction, MOC develops a significant heat of hydration

(e.g., 60-80 C and temperatures above 100 C have been reported). ̊ ̊

Magnesium Oxychloride Cement (MOC)

Hardened Properties• MOC develops high compressive strength within 48 hours (e.g., 8,000-10,000 psi).

Compressive strength gain occurs early during curing - 48-hour strength will be at least 80% of ultimate strength.

• Flexural strength of MOC is low but can be significantly improved by the addition of fibers. MOC is compatible with a wide variety of plastic, mineral (such as basalt fibers) and organic fibers such as bagasse, wood fibers and hemp.

• MOC is non-shrinking, abrasion and wear resistant, impact, indentation and scratch resistant

• MOC is stable to heat and freeze-thaw cycles and does not require air entrainment to improve durability.

• MOC has excellent thermal conductivity, low electrical conductivity, and excellent bonding to a variety of substrates and additives.

• Excellent fire resistance properties

Magnesium Oxychloride Cement (MOC)

Issues/New Developments• MOC is not stable in prolonged contact with water and can result in the leaching

of magnesium chloride, with reversal of the reaction and loss in strength.• Over a period of time, atmospheric carbon dioxide reacts with magnesium

oxychloride to form a surface layer of Mg2(OH)ClCO3•3H2O. This layer slows the leaching process.

• Additional leaching leads to the formation of hydromagnesite, which is insoluble and results in maintenance of structural integrity.

• A variety of additives have been developed that will significantly slow down or block water penetration during early ages, or expedite formation of hydromagnesite.

• Materials used include: phosphates, fly ash, silica fume, alkali metal sulfates and fatty acids.

Magnesium Oxychloride Cement (MOC)

Magnesium ammonium phosphate cement (MAPC)• MAPC is formed by reaction of MgO with monoammonium dihydrogen phosphate

(NH4H2PO4), also referred to as ADP.• A wide variety of insoluble ammonium and magnesium phosphate phases are formed, but

struvite (NH4MgPO4•6H2O) and dittmarite (NH4MgPO4•H2O) are believed to be the main phases.

• The ratio of these phases is dictated by the speed of reaction, with dittmarite being predominant at a fast rate and struvite being predominate at a slower rate.

• At temperatures above 55 C, struvite decomposes releasing water and ammonia from its ̊structure. The resulting material has an amorphous structure that corresponds chemically to MgHPO4.

• ADP is added in excess to ensure full reaction and provide higher compressive strength in the hardened concrete resulting in the excess ADP being volatilized during curing.

• Due to a very fast rate of reaction, the MgO normally used is dead-burned. • Retarders, normally borates, are used to achieve a manageable reaction time.

Magnesium Phosphate Cement (MPC)

Magnesium potassium phosphate cement (MKPC)• MKPC is formed by reaction of MgO with monopotassium phosphate

(KH2PO4), referred to as MKP.• Final reaction product is identified as magnesium potassium phosphate

hexahydrate (MgKPO4•6H2O)• Various intermediate phases are formed during the reaction as the pH and

temperature vary during the reaction.• Both increasing the molar ratio of magnesium to phosphate (M/P) and

decreasing the weight ratio of liquid to solid can accelerate the reaction rate.• Due to a very fast rate of reaction, the MgO normally used is dead-burned. • Retarders, normally borates, are used to achieve a manageable reaction time.

Magnesium Phosphate Cement (MPC)

Plastic Properties• MPC cements are very fast setting materials with low flow characteristics. • Both plastic and hardened properties are affected by the other components of the

mixture, the grade of MgO used, the ratios of MgO and phosphate, and w/b ratio.• Typical set times range between 2-15 minutes for initial set and up to 20-30 minutes

for final set.• The temperature at placement has a significant effect on set times, with mixtures

that normally set in 10-15 minutes at 20 C (60 F) setting in 5-8 minutes at 30 C (86 ̊ ̊ ̊F). Cooling the mixture water helps control set times.̊

• MPC mixtures are thixotropic in nature and therefore do not flow well. When shear force or vibration is applied, MPC flows easily.

• Since it is an acid/base reaction, MPC develops a significant heat of hydration (up to 80 C). Temperature evolution is significantly decreased when fillers are added to ̊the paste, creating mixtures that are safe to handle and place.

Magnesium Phosphate Cement (MPC)

Hardened Properties• MPC mixtures develop high compressive strength within the range of 5,000-10,000

psi (35-70 MPa). MPC does not lose strength over time under normal exposure conditions.

• A number of factors affect strength development, with the largest effect observed being the ratios of reactants (M/P), w/b ratio, amount of retarders used, and materials added as fillers/aggregates to the binder.

• Flexural strengths of 600-2,000 psi (4-14 MPa) have been reported with little effect on strength due to reactant ratios. Addition of fibers improves flexural strength.

• MKP exhibit minimal shrinkage, excellent freeze-thaw resistance, and very low permeability. It also has low coefficient of thermal expansion, excellent corrosion protection, and high abrasive resistance. Immersion in magnesium sulfate solution increases strength.

• MKP exhibits good adhesion to weathered concrete substrates

Magnesium Phosphate Cement (MPC)

Issues/New Developments• Due to sensitivity of reaction, significant variability has been observed

with MPC mixes. The variability is due to variability in dead burn MgO. • Recent advances have been made to utilize more reactive, but more

consistent light burn MgO. • Changes in reagent ratios and development of alternative retarders as

well as changes in fillers have resulted in mixes that have set times similar to “traditional” MPC formulations with only slight loss in strength.

• Recent developments have been reported on corrosion and fire protection coatings based on MKPC.

• Use of additives and fibers have resulted in formulations useful for flexural strengthening of concrete structures utilizing glass mesh (low pH of mixture prevents ASR reaction).

Magnesium Phosphate Cement (MPC)

• MOS is formed by reaction of MgO with magnesium sulfate in the presence of water. The cement is a homologue of MOC.

• Grade of MgO usually light burned, high reactivity. The magnesium sulfate salt used is the heptahydrate salt, also known as Epsom salt.

• There are four main bonding phases formed during reaction: 5Mg(OH)2•MgSO4•3H2O (5-form), 3Mg(OH)2•MgSO4•8H2O (3-form), Mg(OH)2•MgSO4•5H2O, and Mg(OH)2•2MgSO4•3H2O.

• Only the 3-form and the 5-form are stable at room temperature. The 3-phase is the phase that provides compressive strength.

• Like MOC, MOS has low water resistance and loses strength over time when immersed in water due to a reversal of the reaction.

Magnesium Oxysulfate Cement (MOS)

Plastic Properties• MOS, like MOC, is very fluid and not suitable for casting. The addition of

fillers/aggregates and other components provides ability to control viscosity and workability.

• Vicat initial set times of 30 minutes and final set time of 130 minutes were reported using a stoichiometric mixture of components to maximize formation of the 5-form

• Curing of MOS is less effected by weather conditions of humidity, temperature and wind than MOC.

• Rheology of MOC can be controlled by selection of raw materials, molar ratio and the nature of fillers. In general MOS is less thixotropic and more fluid than MOC. Fly ash has been reported to improve rheology (at the expense of compressive strength)

• Since it is an acid/base reaction, MOS develops a significant heat of hydration which has been shows to result in thermal cracking.

Magnesium Oxysulfate Cement (MOS)

Hardened Properties• MOS has been reported to reach 3000-5000 psi in 1 day and up to

10000 psi at 28 days.• A number of factors affect strength development, with the largest effect

observed being the ratios of reactants (M/P). Addition of fly ash results in significant strength loss.

• Flexural strength has been reported to be 500 psi in 7 days. • MOS has better volumetric stability, less shrinkage, better bond to

substrate and lower corrosivity under a significantly wider range of weather conditions than MOC.

• With traditional formulations MOS has worse water resistivity than MOC limiting its applications.

Magnesium Oxysulfate Cement (MOS)

Issues/New Developments• Due to water sensitivity of reaction and relatively slow reaction and

limited strength gain, MOS has received limited attention over the other magnesium cements.

• Recent work and advances have addressed the issues with MOS • Addition of citric acid significantly improves compressive strength to 12000-

15000 psi • Addition of dolomite, magnesite or other fillers at a level of 40-60% of binder

can absorb some of the heat and reduce the chances for thermal cracking• Flexural strength can be doubled by addition of combination of citrate and

sodium silicate in 1:1 proportions.• These combinations significantly improved water resistance.• Addition of sodium bicarbonate significantly improved water resistance.

Magnesium Oxysulfate Cement (MOS)

MOS Applications• Most widespread

application of MOS is for wallboard, backer slats, ceiling tiles, decorative panels, firewalls, SIP board systems

• Still considerable use in flooring applications by development of suitable coatings and improved water resistance additives.

• Fire proofing coatings.

Applications

MPC Applications• Fast patching mixes for

bridges/roads• Hazardous waste/nuclear

waste encapsulation• Shotcrete applications• Mortar for carbon fiber

reinforced applications• Permafrost, low temperature

applications• Corrosion protective

overlays, coatings• Fire proofing coatings.

Applications