Effect of MgO on Clinker

1. Effect of High MgO on Cement expansion
(Autoclave Expansion)
Introduction
 The Autoclave expansion test is used to provide an index of potential
delayed expansion caused by the hydration of CaO and MgO in hydraulic
cements.
 Cement soundness may be monitored by cement paste expansion in boiling
water with the Le-chatlier test apparatus.
 Lechatlier procedure determines unsoundness from the hydration of the free
CaO only
 Autoclave procedure measures the expansion from either free Cao, free
MgO, or both.
 However it should be noted that a High C3A and low C4AF content in
cement, in addition to the known additive effects of CaO and MgO may also
contribute to the total autoclave expansion.
 Dreizer(1981) has investigated the causes of the cement expansion due to
periclase ( Crystalline MgO), and identified the following factors as
influencing cement soundness:
 MgO content of the raw materials
 Mineralogical composition of the raw materials.
 Chemical composition of the raw materials.
 Raw meal fineness
 Clinker burning
 Clinker cooling
 Periclase size and size distribution in the clinker
 Cement fineness
 Cement storage
 Cement additives

2.  Technical Literature
 The MgO in cement is a dangerous ingredient, if present in large
quantities due to its slower hydration.
 MgO in excess of 6 % deteriorated with the age and exhibited both
reduced tensile strength and expansion after five years of storage.
 Autoclave Expansion Test
 The failure of Autoclave test is due to either free CaO( Lime )or free
MgO ( magnesia) but rather to the amount of Coarse( greater than 200-
mesh sized) particles in the cement.
 On Le-Chatlier test for cement unsoundness demonstrated that free lime
(CaO) alone was responsible any abnormal expansion, cracking, or
distortion of the cement paste.
 If the materials was fine enough, the cement in every case became
sound.
 In 1963, the autoclave expansion limit was increased to 0.8 %.
 It is mainly hard-burned lime (CaO) rather than hard-burned magnesia
(MgO) that cause expansion under autoclave conditions.

 Correlation with concrete
 It is found that MgO recrystallized as dense, compact periclase crystals
at rotary kiln.
 Its harmful expansion seemed to be inhibited by chilling the clinker so
that MgO was retained in the solution in the glass form.
 If Clinker was cooled slowly, more periclase crystallized out of the melt
and larger crystals were formed.
 Blended Portland cements produced with a clinker containing 7.5 % MgO
showed satisfactory properties in a 4- year old concrete, with no
evidence of deterioration.
 Conclusion: Rapid Chilling clinker causes the MgO in the
solution in the glass form and reduce the autoclave
expansion.
 Cement fineness effects
 Coarser ground cements have always exhibited a greater amount
autoclave expansion, all other parameters remains same.
 The effects of cement fineness on autoclave expansion as follows.
 High MgO- content cement ground to a fineness of
225m2/Kg showed an autoclave expansion of 7.06 %.
 When the cement was ground to a higher fineness of 350
M2/Kg, the autoclave expansion dropped to 1.39 %

3.  When the fineness increased to a value of 400 M2/Kg, the
resulting autoclave expansion was only 0.24 %.
Conclusion: when the cement ground more finer
then the autoclave expansion will reduce.
 Periclase size and Distribution
 No autoclave expansion were found for smaller periclase crystals
which were predominately located in the interstitial phases.
 The clinker cooled by more rapid cooling process results in smaller
periclase crystals.
 At constant 5 % MgO content, as the lime saturation factor of
clinker increased, the free MgO (periclase) content increase and
resulting cement autoclave expansion increased.
 Hard burning mixes with only 1.5% to 3 % MgO often failed the
autoclave test due to excessive amounts of free lime.
 Addition of fluorspar to high raw mix has aided in the formation of
smaller periclase crystals (1 to 7 µm) and more uniform distribution
in the clinker.
 Portland cement ground from Clinker containing 6% MgO can pass
the autoclave test by the use of 0.8 % CaF2 in the raw mix.
 A high MgO Clinker ground with 2.0 % SO3 containing and the
cement was having 6.5 % MgO (5.4 % free periclase) and free CaO
content 0.47 %. The particle size distribution showed 80 % of
periclase is of size less than 5 µm and remaining 20 % periclase is in
the size 5 to 15 µm range. The autoclave expansion of the same
cement found to be 0.47 %
 Conclusion :
1. Rapid cooling clinker decreases the size of periclase
crystals and ultimately decreases the autoclave
expansion.
2. At constant MgO %, lowering the LSF reduce the free
MgO content resulting the less autoclave expansion.
3. Hard burning mixes failed autoclave expansion.
4. CaF2 (fluorspar) reduce the Periclase size and hence
reduce the effect of MgO expansion. Addition of
0.8 % CaF2 in raw mix can pass the autoclave
expansion.

4.  Long-term Storage and hydration
 Cement (containing 5.0 % MgO and 0.22 % free CaO) paste bar were
autoclave at three different periods and other bar cured in water
for a longer period (1 years). A paste bar that was autoclave after
24 hours disintegrated from excessive expansion. A Paste bar
autoclave after 3 days of water curing was 0.48 % and that of 7 days
water curing was 0.22 %. A paste bar after 1 year curing shows an
expansion of 0.03
 Conclusion – Long curing of cement decreases the
autoclave expansion.
 Hard –burned Periclase additions
 Hard burn MgO more than 2.5 % added to cement failed the
autoclave expansion.
 However no expansion was found for cement mixture containing up
to 5 % of light-burned magnesia, as this materials hydrated before
set had taken place
 Conclusion: - light –burned MgO hydrate rapidly before the
cement set.
 Raw Mix Composition factors
 As a thumb rule “approximately 2 % of the total MgO content in a
cement clinker will remain in solid in the other clinker phases, and
the remainder of the MgO is expected to be present in the form of
periclase crystals.
 The amount of periclase in clinker , particle size and particle size
distribution will varying based upon such factors as
(a) Chemical composition of the raw feed ,
(b) raw feed fineness,
(c) raw feed mineralogy,
(d) Clinkering temperature.
(e) Clinkering cooling history. This may have direct influence on
the amount of autoclave expansion.
 An elevated –SO3 clinker (from CaSO4 additions to the raw mix or
by the use of high sulfur fuel) also prevented the excessive
autoclave expansion of high MgO cement.
With 5 % MgO-containing clinker with
(a) 2.17 % SO3 the autoclave expansion was 0.44 %
(b) 1.01% SO3 the expansion was 4.0 %
(c) 0.37 % SO3 the specimen was disintegrated.

5.  With lower LSF (0.94 to .97) increasing MgO has no influence on
the free CaO. With a higher LSF (0.97 to 1.00), any increase MgO
content result in free CaO and the risk of unsoundness.
 Conclusion :
 1. High sulfur fuel (pet coke) or CaSO4 in raw mix prevent the
excessive expansion of high MgO cement.
 2. With lower LSF (0.94 to 0.97) increasing MgO has no
influence on the free CaO. With a higher LSF (0.97 to 1.00), any
increase MgO content result in more free CaO and the risk of
unsoundness.
 Iron addition effects
 The free MgO will decrease as Fe2O3 increases in the raw mixture.
 A cement containing 5.0 % MgO and2.3 % Fe2O3 had autoclave
expansion of 1.28 % whereas another cement containing 4.9 5 MgO
and 3.6 % Fe2O3 expanded 0.15 %.
 C4AF is an excellent stabilizer for MgO and capable of
transformation of considerable amount of MgO into a non-
expanding compound.
 It is suggested that lowering the alumina modulus and thereby
lowering the C3A and increasing the C4AF contents, could readily
counteract the autoclave expansion influence of MgO , but it was
conceded that rapid cooling of clinker also has very beneficial
effect on the stabilization of MgO.
 Conclusion :
 1. Free MgO will decreases as Fe2O3 increases in the raw
mixture.
 2. Lowering the alumina modulus and thereby lowering the C3A
and increasing the C4AF contents, could readily counteract the
autoclave expansion influence of MgO.
 Mixed Design Guidelines
 Different plant have different design mix control criteria and
limit. The ratio between (alumina+ magnesia) and Iron oxide
should be maximum 2.7
 The ratio of MgO to Fe2O3 is to be considered for raw mix
design
 MgO: Fe2O3 > 1.53 – plant in trouble , autoclave expansion
prevalent danger zone
 MgO: Fe2O3 > 1.40 – High probability of expansion, any
appreciable free CaO gives excessive expansion.

6.  MgO: Fe2O3 < 1.40- Probability of expansion failure decreases
rapidly.
 MgO: Fe2O3 < 1.20- Standard control target (maximum value)
failure due to MgO very unlikely.
 Conclusion : Raw Mix guidelines
 1. Ratio: (Al2O3+MgO)/Fe2O3 ≤ 2.7 % (maximum).
 2. Ratio : MgO: Fe2O3 < 1.20
The effect of MgO on the composition and properties of alite-
sulphoaluminate cement clinker with commercial-grade raw materials such
as limestone, clay, gypsum and fly ash as starting materials has been
investigated in order to confirm results obtained with pure raw materials.
What will happen if MgO >2% in raw mix? How it will affect
the clinker formation process in kiln?
(i) The addition of MgO retards the initial hydration of cement and
increases the setting time of cement.
Two causes for the retardation of hydration are proposed:
(a) As the solubility product constant of Mg (OH)2 is far smaller
than Ca(OH)2, Mg(OH)2 precipitates earlier than Ca(OH)2. The
formation of Mg (OH) 2 reduces the Ca (OH)2 saturation ratio, thus
delaying the initiation of the maximum of Ca(OH)2 saturation ratio.
(b) When MgO hydrates in high-alkali medium such as the liquid
phase of hydrating cement, Mg (OH)2 with tiny crystals precipitates
around the cement grains to form a protective layer, hence retarding
further hydration of the cement grains.)
(ii) MgO is good for nothing in raw mix. it gives some darker colour to
the clinker. More MgO consumes extra thermal energy, if the
particle size also more the clinker will get disintegrate quickly
because of expansion.
(iii) Below about 2% in the clinker, MgO is incorporated into the crystal
structure of the clinker minerals. MgO in excess of about 2% appears in
the clinker as free MgO (the mineral periclase).
(iv) MgO present 1 - 1.5 % in clinker is a very good mineralizer and improve
nodulisation. If it is quenched it does not give expansion
problems .Good nodules are always preferred in burning and it helps in
improving cooling efficiency and there by recovering thermal energy
from clinker.

7. (v) If the clusters of MgO crystals are numerous and large enough this
can cause excessive expansion in the cement resulting in
unsoundness. Finer grinding of the cement can offset this problem
up to a point.
(vi) Withinthe kiln MgO increases the liquid phase and may therefore
promote the formation of clinker minerals. However, too much MgO
can increase the liquid phase to problematic amounts and cause
excessive coating and even balling of the clinker.
(vii) Most plants therefore try to keep MgO levels in the raw mix to
below 2 or 3%.
(viii) MgO at about 2–5% can
(a).Improve the burnability of raw meal, promote the absorption of free
lime and the formation of C3S and C4A3S¯.
(b)Increase the strength development of the cement
(c) Shorten the setting time.
(ix) When the content of MgO reaches about 8%, the strength of the
cement decreases slightly and the setting time is extended.
(x) Clinkers with 2% MgO it helps in better distribution of C3S and C2S in
clinker as per clinker microscopy studied. The C3S and C2S contents
were linearly related to the SO3/MgO ratio. Adding SO3 hindered the
formation of C3S, the formation temperature of which increased by
approximately 100 °C.
MgO enhanced the formation of C3S by both accelerating the
clinkerization reactions and lowering the formation temperature of C3S.
The C4AF content increased when MgO was added. Microscopically, MgO
led to the formation of small C3S crystals, whereas SO3 enlarged the size
of C3S crystals and promoted the formation of C2S rims. M3 alite was the
primary component of the MgO-doped clinkers, whereas the M1 type was
stabilized by SO3;
(xi) Cement clinker when it is stored outside as we see clinker cracks and
rate of cracking depends on the % of free lime. Periclase hydrates
slowly forming Brucite, which has more expansion (delayed) than free
CaO. So after many days the concrete structures expand. The size of
periclase and % is the most influential parameters for delayed
expansion and can be reduce by
(a) Grinding the raw meal finer,
(b) Increase Rate of burning,
(c) Rapid Annealing or quenching of clinker

8. (xii) By screening in crusher, small amount of MgO will be separated.
(xiii) If corrective (Iron ore) use also reduce the MgO % to some extent.
(xiv) MgO periclase crystalline would not reacted while on hydration.
(xv) Clinker cooling have to be done properly (clinker outlet temperature
-100 0C to convert MgO should convert glassy content form.
(xvi) MgO should be freezed as glassy content on cooling
(xvii) MgO should be bring to form smaller size periclase.
(xviii) Ash Content should be maintain above 32 % (if limestone TCO3 is
above 83 %), If maintain low ash, MgO % will increase due to low ash
absorption.
(xix) Low MgO Limestone grade to be consumed (either Kukurdih or
Sweetener).
(xx) Cement fineness to be increased.
(xxi) Ash abortion should be maintained 4-5 %
(xxii) C3A is to be maintained :6-8 %
(xxiii) MgO reacts with water to form Mg(OH)2 that cause volume expansion
of about 118 %(2-4)
(xxiv) The hard burnt MgO in cement reacts with water very slowly and the
expansion caused by MgO hydration takes place after the concrete
has hardened, so that the shrinkage of cement could be compensated
effectively.
(xxv) MgO content in the ordinary cement is restricted to less than 5 % and
can be 6 %.
(xxvi) Free lime should be maintained 1-1.5 %, if MgO is more in cement
then PPC, Slag cement is to be ground.
(xxvii)Up to around 2% the MgO will dissolve in the clinker minerals,
predominantly in the C4AF and therefore contribute to the liquid
phase in the kiln. That is why changes in MgO content of kiln feed
can sometimes cause clinker ball or coating ring formation. Above 2%
the MgO will be present as periclase, MgO, as a solid phase. It is this
solid MgO that can lead to long-term unsoundness in hydrated
cement due to slow hydration of the periclase. When MgO is higher
than 2% then rapid heating and cooling is beneficial to ensure the
MgO is present in small, reactive crystals of periclase, which will
hydrate while the cement is hardening and therefore not cause
unsoundness by later expansion of periclase.