# What Is The Optimal Value Of Lime Saturation Factor For Cement?

Lime Saturation Factor (LSF) – The Lime Saturation Factor is a ratio of CaO to the other three main oxides. Applied to clinker, it is calculated as: LSF=CaO/(2.8SiO 2 + 1.2Al 2 O 3 + 0.65Fe 2 O 3 ) Often, this is referred to as a percentage and therefore multiplied by 100.

The LSF controls the ratio of alite to belite in the clinker. A clinker with a higher LSF will have a higher proportion of alite to belite than will a clinker with a low LSF. Typical LSF values in modern clinkers are 0.92-0.98, or 92%-98%. Values above 1.0 indicate that free lime is likely to be present in the clinker.

This is because, in principle, at LSF=1.0 all the free lime should have combined with belite to form alite. If the LSF is higher than 1.0, the surplus free lime has nothing with which to combine and will remain as free lime. In practice, the mixing of raw materials is never perfect and there are always regions within the clinker where the LSF is locally a little above, or a little below, the target for the clinker as a whole.

This means that there is almost always some residual free lime, even where the LSF is considerably below 1.0. It also means that to convert virtually all the belite to alite, an LSF slightly above 1.0 is needed. The LSF calculation can also be applied to Portland cement containing clinker and gypsum if (0.7 x SO 3 ) is subtracted from the CaO content.

NB: This calculation (0.7 x SO 3 ) is based on the ratio of the molar masses of calcum oxide and sulfur trioxide, ie: 56/80 = 7/10. It therefore assumes that all the sulfate in the clinker is present as anhydrite; it does not account for sulfate present as clinker sulfate in the form of potassium and sodium sulfates, or for water in gypsum, and the calculation will therefore not be exact.

### What is the lime saturation factor in cement kiln?

1. Lime Saturation Factor – The lime saturation factor (LSF) is a ratio of CaO to other oxides, it is used to control the proportion of cement raw meal. In the operation of the cement kiln, the thermal system of the kiln can be affected by the fluctuation of LSF. The high saturation ratio will make the raw meal difficult to burn to clinker.

 Oxide CaO SiO 2 Al 2 O 3 Fe 2 O 3 others Ratio 62%~67% 20%~24% 4%~7% 5%~6% <5%

The proportion of oxides in cement clinker

### What is the strength of alumina with high lime saturation factor?

Mineralogical Characterisation of Cements Containing Copper Slag – In the manufacturing of PC clinker, the raw materials are mixed and heated to temperatures up to 1450°C. To identify the potential phases after heating the raw mix blend, the lime saturation factor (LSF) is often used to verify the ratio of C 3 S to C 2 S.

It also shows whether the clinker is likely to contain an unacceptable proportion of free lime. Values between 0.92 and 0.98 are typical of modern clinkers, and a mix with an LSF greater than 1.0 will yield free CaO, which is liable to persist in the final product, regardless of the degree of mixing and time during which the clinkering temperature is maintained ( Taylor, 1997 ).

The silica ratio and alumina ratio (also respectively called silica modulus and alumina modulus) are empirically used to characterise the potential mineralogical composition of the cement clinker. The silica modulus mainly governs the proportion of silicate phases in the clinker, whilst the alumina modulus governs the ratio of aluminate to ferrite phases in the clinker; for normal PC clinker, the silica and alumina moduli usually vary from 2.0 to 3.0 and from 1.0 to 4.0, respectively ( Taylor, 1997 ).

1. Ali et al,
2. 2013) studied the effect of incorporating up to 2.5% CS in the raw mix by replacing limestone, bauxite and iron ore.
3. An analysis of the LSF and silica and alumina moduli showed that, although the LSF was not affected by the incorporation of CS, the silica and alumina moduli decreased mainly because of the greater Fe 2 O 3 content of CS, when compared to that of the raw ingredients that were replaced.

This would suggest that the amount of silicate and aluminate phases would decrease with an increase of the C 4 AF phase. This was also shown by Bogue’s method, in which the estimations for the amount of tricalcium silicate (C 3 S), dicalcium silicate (C 2 S), tricalcium aluminate (C 3 A) and tetracalcium aluminoferrite (C 4 AF) phases were in the ranges of 54.7–58.9%, 16.3–19.3%, 4.6–6.3% and 12.9–15.3%, respectively, with liquid content varying between 27.4% and 28.9%.

Although the aforementioned methods suggested a decrease of the silicate phases with increasing CS content, the results of the X-ray diffraction analysis indicated that the incorporation of CS resulted in relatively rapid clinker mineral phase formations. Indeed, C 3 S and C 2 S contents in samples containing CS, heated at 1400°C, were found to be within the range of 52–58% and 23–28%, respectively, and were comparable to those of the control PC clinker, with C 3 S and C 2 S contents of 56% and 26%, respectively, calcined at 1450°C.

In the study of Medina et al. (2006), the amount of each phase produced during clinkerisation at 1350 and 1450°C was quantified by means of the Rietveld method ( Table 5.4 ). Although no significant changes were found in the C 3 S phase, a slight increase in the C 2 S phase was observed when 1.85% CS was used, which would explain the slight decrease in the free CaO content ( Table 5.5 ) in comparison to that of the control PC clinker.

Temperature, °C Sample Phase, % by Weight
C 3 S C 2 S C 3 A C 4 AF
1350 Control 62.32 18.12 6.36 10.10
1.25% CS 61.22 18.70 8.21 9.07
1.85% CS 62.21 18.75 6.92 9.27
1400 Control 69.18 12.66 5.76 10.55
1.25% CS 70.90 10.37 8.34 8.86
1.85% CS 69.05 13.26 6.60 9.43
1450 Control 73.57 9.20 6.30 10.11
1.25% CS 74.21 7.64 8.44 8.95
1.85% CS 73.61 9.47 5.88 10.18

CS, copper slag. Adapted from Medina et al. (2006), Table 5.5, Free lime content of cement with increasing copper slag (CS) content

Publication CS Content, % Free CaO Content (%) for Temperatures of
1250°C 1300°C 1350°C 1400°C 1450°C
Ali et al. (2013) 3.18 1.42 0.99 0.40
1.50 3.10 1.34 0.96 0.38
2.00 2.98 1.23 0.78 0.37
2.25 2.76 1.14 0.69 0.35
2.50 2.75 1.12 0.68 0.33
Medina et al. (2006) 3.11 1.85 0.82
1.25 2.80 1.53 0.79
1.85 2.86 1.66 0.88
Sahu et al.(2011) 4.64 2.56 1.20
1 4.25 2.45 1.24
2 3.85 2.22 1.01
Supekar (2007) 0.71 0.51 0.35 0.21
1.5 0.35 0.31 0.19 0.10
2.0 0.45 0.34 0.22 0.10
2.5 0.35 0.30 0.23 0.11
3.0 0.41 0.27 0.13 0.08
Taeb and Faghihi (2002) 16.72 10.1 3.83 2.09 0.87
2 10.60 8.50 1.67 1.01 0.32
4 9.30 5.07 1.01 0.34 0.27
6 7.30 2.25 0.55 0.28 0.30
8 5.30 1.03 0.30 0.20 0.18
10 2.80 0.41 0.21 0.17 0.09

The cement clinker must be correctly burned, to minimise its free lime (CaO) content with the least expenditure of energy ( Taylor, 1997 ). The free lime content of clinker is regarded as a practical measure of the degree of raw mix clinkerisation and is used as a means of controlling the quality of clinker produced.

• The typical range of free lime content in PC is 0.5–3%.
• Table 5.5 presents the free lime content in clinker manufactured with and without CS at different temperatures.
• As expected, the free lime content decreased with increasing clinkerisation temperatures and it decreased even further when increasing CS was incorporated in the raw mix ( Figure 5.4 ).

This trend was explained by the enhanced lime combinability at lower temperatures with the incorporation of CS containing copper oxide (CuO) as well as a decrease in the liquid phase’s viscosity ( Kakali et al., 1996; Kolovos et al., 2005; Ma et al., 2010 ). Figure 5.4, Effect of copper slag (CS) incorporation on free lime content of cement clinker subjected to increasing temperatures based on the results of (a) Ali et al. (2013) (b) Supekar (2007) (c) Sahu et al. (2011) (d) Medina et al. (2006) (e) Taeb and Faghihi (2002),

Figure 5.5, which reflects the results presented in Table 5.5, presents the relative free lime of cement clinker samples taken from several studies, in which the CS was ground with the other raw mix components and subjected to normal clinkerisation temperatures ( Ali et al., 2013; Medina et al., 2006; Sahu et al., 2011; Supekar, 2007; Taeb and Faghihi, 2002 ).

The results indicate a clear decrease in free CaO content with increasing CS content, revealing greater lime combinability at lower temperatures as observed in other studies ( Kakali et al., 1996; Kolovos et al., 2005; Ma et al., 2010 ). The presence of CuO, which acts both as mineraliser and as flux, decreases the melting temperature by at least 50°C and favours the combination of free lime, resulting in accelerated C 3 S formation ( Kolovos et al., 2005 ). Figure 5.5, Effect of copper slag (CS) content on the relative free CaO of cement clinker. Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780081009864000055

#### What is the percentage of lime in Belite cement?

8.4.6 EFFECT OF PROPORTION OF MAIN CLINKER MINERALS – The relation between the chemical characteristics of laboratory clinkers, burning at 1450°C for 2 h, and their strength-generating properties has been investigated 67–70 by modifying the three parameters: lime saturation factor, alumina ratio and silica ratio.

One of these factors was changed while keeping the other two constant. Cements were prepared by grinding the clinkers to 320 m 2 /kg with 6 per cent gypsum. Compressive strength testing was carried out on small mortar prisms, 15 × 15 × 60 mm in dimension, using a mix with sand/cement = 3 and water/cement = 0.5.71 Investigations have been carried out on laboratory clinker made from cement raw meals with a lime saturation factor of 93 per cent, and an alumina ratio of 2.0.

The silica ratio (1.6–3.2), the alkali contents and the degree of sulfatisation were varied. Strength tests showed the following results. • With increasing silica ratio strength was generally increased, and reduced with increasing SO 3 content. • Strength at 2 days was increased by K 2 O additions of up to 1.5 per cent, and Na 2 O additions of up to 1.5 per cent when the degree of sulfatisation was 100 per cent.

The strengths at 28 and 90 days were reduced by additions of Na 2 O and K 2 O; for equal additions, Na 2 O had a greater influence than K 2 O. • For K 2 O, the optimum degree of sulfatisation was 60–70 per cent and for Na 2 O it was 90–100 per cent. • Supersulfatisation of the alkalis results in strength reduction.

• The reduction in strength at 28 and 90 days in cements containing K 2 O can be offset by increasing the silica ratio, regardless of whether there was an optimum degree of sulfatisation. This action is only successful in cements containing Na 2 O. Investigations have been carried out on laboratory clinker made from cement raw meals with a uniform lime saturation factor of 93 per cent and a silica ratio of 2.4.

• The alumina ratio (1.3–2.7), alkali contents (Na 2 O and K 2 O each ∼0–2 per cent) and the degree of sulfatisation were varied.
• Strength tests showed the following results.
• Strengths always increased with falling alumina ratio.
• Strengths after 2 and 7 days were increased by K 2 O and Na 2 O contents up to about 1.5 per cent.

Higher alkali contents and supersulfatisation reduced strengths. • The strengths after 28 and 90 days were increasingly reduced by increasing K 2 O and Na 2 O contents. • For equally high contents of alkali oxides, the strengths after 28 and 90 days were reduced to a greater extent by Na 2 O than by K 2 O.

1. Equal molar proportions of the two alkali oxides lowered the strengths by equal amounts where the degree of sulfatisation was below 100 per cent.
2. For alumina ratios between 1.3 and 2.7, the optimum degree of sulfatisation was about 60 per cent for K 2 O and about 100 per cent for Na 2 O.
3. Supersulfatisation of the alkalis reduced the strengths considerably; this was especially true of cements containing Na 2 O.

• The reduction in strength after 28 and 90 days by K 2 O contents up to ∼1.5 per cent can be offset by lowering the alumina ratio where there is an optimum degree of sulfatisation, but the reduction in strength caused by Na 2 O contents up to 1.5 per cent is only partly compensated.

Investigations have been carried out on laboratory clinkers made from cement raw meals with a silica ratio of 2.4 and an alumina ratio of 2.0. The lime saturation factor, the alkali contents and the degree of sulfatisation were varied. Strength tests showed the following results. • Strengths were increased with increasing lime saturation factor.

• Strengths after 2 and 7 days were increased by K 2 O and Na 2 O contents up to ∼1.5 per cent. Higher alkali contents and supersulfatisation reduced strengths. • The strengths after 28 and 90 days were increasingly reduced by increasing K 2 O and Na 2 O contents.

• For equally high contents of alkali oxides, the strengths after 28 and 90 days were reduced to a greater extent by Na 2 O than by K 2 O. Equal molar proportions of the two alkali oxides lowered the strengths by equal amounts. • The optimum degree of sulfatisation of the alkalis was dependent on the lime saturation factor: • Lime saturation factor 86%, 90–100% for Na 2 O and K 2 O; • Lime saturation factor 93%, 50–70% for K 2 O and 90–100% for Na 2 O; • Lime saturation factor 98%, 50–70% for K 2 O and Na 2 O.

These data are valid for alumina ratios between 1.3 and 2.7, and for silica ratios between 1.6 and 3.2, where the lime saturation factor is 93 per cent. Supersulfatisation reduced the strength, especially at a high lime saturation factor and in cements containing Na 2 O.

• The reduction in the strengths at 28 and 90 days by K 2 O levels up to 1.5 per cent can be offset by a significant increase in the lime saturation factor where there is an optimum degree of sulfatisation.
• Lowering the alumina ratio, or raising the silica ratio, improved the strength of alkali-rich cements more effectively than raising the lime saturation factor.

• Cements with lime saturation factors of 93 and 86 per cent, with alkali oxide contents of 1.0–1.5 per cent lead to virtually identical strengths after 28 days and 90 days where there was an optimum degree of sulfatisation. However, strengths after 2 and 7 days were significantly lower with cements with lime saturation factors of 86 per cent.

• As expected, the compressive strength of the alkali- and sulfate-free cements increased at all test dates with increasing silica ratio and lime saturation factor.
• With increasing alumina ratio, the strengths were virtually constant.
• A comparison 72, 73 of the strengths developed in a series of laboratory-made cements, composed of CaO, SiO 2, Al 2 O 3, Fe 2 O 3 and MgO, all ground to the same fineness and with the same amount of gypsum added, indicated that strength is primarily a function of the contents of 3CaO·SiO 2 and 2CaO·SiO 2,

The strengths at ages up to 28 days were found to be a function of the content of 3CaO·SiO 2, while the increases in strength between 28 days and 6 months were roughly linearly related to the content of 2CaO·SiO 2, With more finely ground cements, the dicalcium silicate probably began to produce its effect somewhat earlier.

Cements rich in tricalcium silicate showed high strength at early ages, while those which were low in this compound showed much lower strengths at early ages, but a progressive increase with age, so that at 6 months the differences in the strengths were relatively small. At 12 months the strengths of the two groups of cements were about equal.

Table 8.13, Clinker phase proportions for lime saturation factor = 95%, alumina ratio = 2.0 and a range of silica ratios 71

Silica ratio Alite Belite C 3 A C 4 AF
1.5 50 19 21 10
1.7 54 17 20 9
2.0 58 16 17 9
2.3 61 15 16 8
2.5 63 14 16 7
2.8 65 14 15 6

Table 8.14, Clinker phase proportions for laboratory clinkers with lime saturation factor = 95%, silica ratio = 2.0 and a range of alumina ratios 71

Alumina ratio Alite Belite C 3 A C 4 AF
0.5 68 14 18
1.0 65 14 6 15
1.5 63 15 10 12
2.0 58 16 17 9
2.5 56 16 20 8

Table 8.15, Clinker phase proportions in laboratory clinkers with silica ratio = 2.0 and alumina ratio = 2.0, and a range of lime saturation factors 71

Lime standard Alite Belite C 3 A C 4 AF
85 39 37 15 9
90 50 26 15 9
95 58 16 17 9
100 69 4 17 10
105 71 6 17 CaO*

The interrelationships between the proportions of clinker phases and compressive strength have been studied 71 for laboratory cements made from chemically pure substances (CaCO 3, SiO 2, Al 2 O 3 and Fe 2 O 3 ) fired at 1450°C for 2 h. The free lime in the clinker was very low (<0.1 per cent); the cement was ground to a specific surface area of 320 ± 4 m 2 /kg with 6 per cent gypsum. The compressive strength was determined on small prisms (15 × 15 × 60 mm) made from mortar (sand/cement = 3, water/binder = 0.5). Fig.8.12, Influence of silica ratio on compressive strength.71 Attempts have been made, by analysing data from a series of cements, to derive factors representing the contribution of each of the four major cement compounds to a particular property at a given age, and under given conditions.

1. In this analysis the assumption is made that the contributions made by the different compounds are additive, i.e.
2. That a property of a cement may be expressed quantitatively by an equation: (8.9) P = a A + b B + c C + d D where P represents the numerical value of the property, e.g.
3. Strength, at a given age and under given conditions; A, B, C and D are the percentage contents of 3CaO·SiO 2, 2CaO·SiO 2, 3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 present in the cement, and a, b, c and d are coefficients representing the contribution of 1 per cent of the corresponding compound to the property considered.

The use of such an equation also assumes that the contribution of a unit mass of any given compound to a particular property under the conditions postulated remains unchanged, i.e. that the coefficients a, b, c and d are constants. Errors in the calculated amount of the compounds present do not invalidate this method of analysis, but they will be reflected in the degree of uncertainty attached to the coefficients deduced.

The above approach has been more successful when applied to the heat of hydration than when applied to strength or shrinkage data. The relative contributions to strength of the four cement compounds derived from different sets of data are very variable, and it cannot be claimed that they do more than confirm the conclusion that 3CaO·SiO 2 makes the major part of its contribution to strength in the first 28 days, and that 2CaO·SiO 2 contributes from 28 days onwards.

The results for 3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 are very erratic; this may not be surprising since the content of these compounds calculated by the Bogue formula can be wide of the truth. All that can be said is that the available data indicated that these two compounds contributed positively to early strength, but under moist storage caused regression in strength after longer periods. Fig.8.13, Variation of compressive strength with lime saturation factor.71 In an experimental investigation to study the early rate of strength gain, a series of 53 laboratory clinkers was prepared from the system CaO-SiO 2 -Al 2 O3-Fe 2 O3 and were formed into cements by intergrinding with 3 per cent CaSO 4,74 Compressive strengths on compacts (pressed minicylinders allowed to hydrate by flooding with distilled water), 11 mm diameter and 11 mm in height were obtained after hydration for 24 h at 20° C.

The best relationship between this crushing strength and Bogue composition was given by (8.10) Strength = 1.66 × %   C 3 S – 1.52 × %   C 4 AF – 0.72 × %   C 3 A + 0.12 × fineness – 3.2 where fineness (m 2 /kg) is the specific surface area of the cement. The coefficient of determination for this regression, adjusted for degrees of freedom, was 89.6 per cent.

The effects of firing time and firing temperature were found to be negligible under the conditions of this laboratory test. The overriding importance of C 3 S content to cement reactivity and early strength development was confirmed by these results. Some clinkers showed anomalous expansions after 24 h and low strengths; most of these clinkers were fired at relatively low temperatures and free CaO was present in at least one.

1. All contained substantial quantities of C 3 A.
2. The relationship between clinker phase composition and strength has been much investigated.75, 76 Data from 30 cements containing 46–62 per cent C 3 S and 1–11 per cent C 3 A have been analysed and the most significant association between strength and compound composition lay in an expression involving the percentage of C 3 A alone.77, 78 The percentage of C 4 AF was found to have little effect on strength.

The contribution of C 2 S matched that of C 3 S only after 90 days’ hydration. The correlation between strength and the percentage of C 3 S passed through a maximum at 7 days’ hydration, while the correlation with the percentage of C 3 A was most significant after 28 days’ hydration.

It may be assumed that each of the phases of Portland cement clinker contributes either directly or indirectly to the development of strength and the ultimate strength of the resultant cement. It is conceivable that this effect may be an additive one, in which each of the phases present acts individually, or a cumulative one, in which an interaction between the individual phase takes place, enhancing or weakening their influence.

Several investigators studying the relationship between clinker phase composition and strength have used multiple linear regression analysis, and have assumed that the action of the phases involved is additive, i.e. that the relationship between strength and cement composition may be expressed by an equation of the type (8.11) σ t = a 0 + a 1 c 1 + a 2 c 2 + a 3 c 3 + where σ t is the cement strength after hydration time t, a 0, a 1, a 2, a 3 are regression coefficients and c 1, c 2, c 3 are the contents of phases 1, 2 and 3 in a cement.

A series of equations has been obtained by evaluating cement strength against a number of clinker parameters (phase composition, ignition loss, insoluble fraction, air content, alkali content).79 A multiple correlation analysis 80 has been published between mortar compressive strength after up to 10 years’ water curing and chemical composition parameters for setting of up to 199 commercial US cements.

The relationship between strength and cement composition has been studied for 30 Australian cements (containing 46–62 per cent C 3 S and 1–11 per cent C 3 A).77 It was concluded that the most significant association between strength and compound composition lay in the regression involving C 3 A alone.

In contrast, C 4 AF was found to have little or no effect on strength. The contribution of C 2 S matched that of C 3 S only after hydrating for 90 days. In a subsequent work 78 it was found that the significance of the correlation of strength with C 3 S passed through a maximum at 7 days, and the correlation with C 3 A was most significant at 28 days.

Schrämli, 81 in analysing his own data and data from the literature, concluded that C 3 A affected initial strengths positively but the ultimate strengths were affected negatively. C 3 A had a negative effect on strengths at 7 and 28 days (31 cements), while the effect of C 3 S is positive.76 The correlation coefficient between strength and C 3 S and C 3 A content was not significant at 1 and 3 days.

The relationship between strength and phase composition has been studied on 114 cements from New Zealand.75 It was concluded that an equation of the type (8.12) σ t = a 0 + a 1 ⋅ C 3 S + a 2 ⋅ ( C 3 S + C 2 S ) + a 3 ⋅ C 3 A + a 4 ⋅ SA yielded better fits than the simple equation (8.13) σ t = a 0 + a 1 ⋅ C 3 S + a 2 ⋅ C 3 A + a 3 ⋅ SA C 4 AF was found to be influential on strength development.

Alexander and Ivanusec 82 established that the association between strength and C 3 S content did vary greatly with the SO 3 content of the cement. However, the effect of C 3 A on strength was insensitive to the SO 3 content. As expected, the regression coefficient associated with C 3 S was consistently positive, confirming the positive effect of C 3 S on strength.

• The coefficient associated with C 3 A was also positive, its magnitude always being greater in the equation expressing 28-day strengths than that for 3-day strengths.
• The coefficient for the effect of C 2 S after 28 days was either positive, negative or equal to zero.
• Finally, the coefficient associated with C 4 AF was positive, however, its magnitude was significantly lower than that of the coefficient associated with C 3 A.

Even though multiple linear analysis has yielded reasonably good fits to data for the relationship between strength and clinker composition, doubts have arisen whether the effect of single clinker phases is linear, independent and additive. C 3 A may enhance the contribution of C 3 S to strength either by modifying the hydration products or by accelerating the progress of C 3 S hydration.77, 78 The best fit for the data at 3, 7 and 28 days was obtained by an equation of the type (8.14) σ t = a 0 + a 1 ( 1 + b C 3 A ) C 3 S + a 2 where a 0, a 1, a 2 are empirical constants.

It was also found 78 that the cement strength increased with increasing C 3 A content (linearly) only up to an amount of 14 per cent, and decreases as the C 3 A content becomes even higher. A large number of experimentally made clinkers were investigated, 83 varying widely in phase composition. These clinkers were ground with constant amounts of gypsum (SO 3 = 2.5 per cent) to identical finenesses (300 m 2 /kg).

The following conclusions were reached. • The overall correlation of strength properties with phase quantities was never linear. • The positive effect of silicates and particularly of C 3 S was the most important one at all ages. Nevertheless, the strength started to decline when the C 3 S content exceeded 70 per cent.

1. The contribution of C 3 A and C 4 AF to strength development was relatively small.
2. The strength increased with increasing C 3 A content only to some limiting value, and decreased with higher C 3 A contents.
3. The optimum proportion of C 3 A was not constant and changed with the amount of C 3 S and C 4 AF present.

A catalytic action of C 3 A on C 3 S hydration has been proposed and the following equation for calculating the compressive strength was presented: 84 (8.15) σ t = 1 – p e – bt – ( 1 – p ) e – b 2 1 – p e 90 b t – ( 1 – p ) e – 90 b 2 where σ = strength after t days, σ 90 = strength after 90 days, p = C 3 S content of the cement and b, b 2 = rate parameters which are functions of temperature, C 3 A content and fineness.

A series of laboratory-made cements showed a non-linear effect of the individual clinker phases on cement strength and evidence of an interaction between the phases present.85 The resultant strength increased at all ages if C 3 S was replaced with C 3 A, up to an amount of 10 per cent, and declined at higher degrees of substitution.

Substitution of C 3 S by C 4 AF up to 30 per cent had little effect on 1-day strength, but at longer hydration times the resultant strength increased significantly with increasing amount of C 4 AF in the clinker. At constant C 3 S + C 2 S content, the strength in the presence of both C 3 A and C 4 AF was higher than in the presence of each phase alone.

• It may be concluded that the available data on the effect of clinker phase composition on strength are rather contradictory.
• Even though equations which express the relationship between strength and phase composition of the clinker can be derived from the experimental data using multiple regression analysis, a model that assumes independent and additive contribution of the individual clinker constituents to strength without the existence of any interactions, appears highly unlikely.

These relationships are of little practical importance. Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780750662567500205

#### How long does Lime take to dissolve in cement?

8.4.6 EFFECT OF PROPORTION OF MAIN CLINKER MINERALS – The relation between the chemical characteristics of laboratory clinkers, burning at 1450°C for 2 h, and their strength-generating properties has been investigated 67–70 by modifying the three parameters: lime saturation factor, alumina ratio and silica ratio.

One of these factors was changed while keeping the other two constant. Cements were prepared by grinding the clinkers to 320 m 2 /kg with 6 per cent gypsum. Compressive strength testing was carried out on small mortar prisms, 15 × 15 × 60 mm in dimension, using a mix with sand/cement = 3 and water/cement = 0.5.71 Investigations have been carried out on laboratory clinker made from cement raw meals with a lime saturation factor of 93 per cent, and an alumina ratio of 2.0.

The silica ratio (1.6–3.2), the alkali contents and the degree of sulfatisation were varied. Strength tests showed the following results. • With increasing silica ratio strength was generally increased, and reduced with increasing SO 3 content. • Strength at 2 days was increased by K 2 O additions of up to 1.5 per cent, and Na 2 O additions of up to 1.5 per cent when the degree of sulfatisation was 100 per cent.

• The strengths at 28 and 90 days were reduced by additions of Na 2 O and K 2 O; for equal additions, Na 2 O had a greater influence than K 2 O. • For K 2 O, the optimum degree of sulfatisation was 60–70 per cent and for Na 2 O it was 90–100 per cent. • Supersulfatisation of the alkalis results in strength reduction.

• The reduction in strength at 28 and 90 days in cements containing K 2 O can be offset by increasing the silica ratio, regardless of whether there was an optimum degree of sulfatisation. This action is only successful in cements containing Na 2 O. Investigations have been carried out on laboratory clinker made from cement raw meals with a uniform lime saturation factor of 93 per cent and a silica ratio of 2.4.

The alumina ratio (1.3–2.7), alkali contents (Na 2 O and K 2 O each ∼0–2 per cent) and the degree of sulfatisation were varied. Strength tests showed the following results. • Strengths always increased with falling alumina ratio. • Strengths after 2 and 7 days were increased by K 2 O and Na 2 O contents up to about 1.5 per cent.

Higher alkali contents and supersulfatisation reduced strengths. • The strengths after 28 and 90 days were increasingly reduced by increasing K 2 O and Na 2 O contents. • For equally high contents of alkali oxides, the strengths after 28 and 90 days were reduced to a greater extent by Na 2 O than by K 2 O.

• Equal molar proportions of the two alkali oxides lowered the strengths by equal amounts where the degree of sulfatisation was below 100 per cent.
• For alumina ratios between 1.3 and 2.7, the optimum degree of sulfatisation was about 60 per cent for K 2 O and about 100 per cent for Na 2 O.
• Supersulfatisation of the alkalis reduced the strengths considerably; this was especially true of cements containing Na 2 O.

• The reduction in strength after 28 and 90 days by K 2 O contents up to ∼1.5 per cent can be offset by lowering the alumina ratio where there is an optimum degree of sulfatisation, but the reduction in strength caused by Na 2 O contents up to 1.5 per cent is only partly compensated.

Investigations have been carried out on laboratory clinkers made from cement raw meals with a silica ratio of 2.4 and an alumina ratio of 2.0. The lime saturation factor, the alkali contents and the degree of sulfatisation were varied. Strength tests showed the following results. • Strengths were increased with increasing lime saturation factor.

• Strengths after 2 and 7 days were increased by K 2 O and Na 2 O contents up to ∼1.5 per cent. Higher alkali contents and supersulfatisation reduced strengths. • The strengths after 28 and 90 days were increasingly reduced by increasing K 2 O and Na 2 O contents.

• For equally high contents of alkali oxides, the strengths after 28 and 90 days were reduced to a greater extent by Na 2 O than by K 2 O.
• Equal molar proportions of the two alkali oxides lowered the strengths by equal amounts.
• The optimum degree of sulfatisation of the alkalis was dependent on the lime saturation factor: • Lime saturation factor 86%, 90–100% for Na 2 O and K 2 O; • Lime saturation factor 93%, 50–70% for K 2 O and 90–100% for Na 2 O; • Lime saturation factor 98%, 50–70% for K 2 O and Na 2 O.

These data are valid for alumina ratios between 1.3 and 2.7, and for silica ratios between 1.6 and 3.2, where the lime saturation factor is 93 per cent. Supersulfatisation reduced the strength, especially at a high lime saturation factor and in cements containing Na 2 O.

The reduction in the strengths at 28 and 90 days by K 2 O levels up to 1.5 per cent can be offset by a significant increase in the lime saturation factor where there is an optimum degree of sulfatisation. Lowering the alumina ratio, or raising the silica ratio, improved the strength of alkali-rich cements more effectively than raising the lime saturation factor.

• Cements with lime saturation factors of 93 and 86 per cent, with alkali oxide contents of 1.0–1.5 per cent lead to virtually identical strengths after 28 days and 90 days where there was an optimum degree of sulfatisation. However, strengths after 2 and 7 days were significantly lower with cements with lime saturation factors of 86 per cent.

1. As expected, the compressive strength of the alkali- and sulfate-free cements increased at all test dates with increasing silica ratio and lime saturation factor.
2. With increasing alumina ratio, the strengths were virtually constant.
3. A comparison 72, 73 of the strengths developed in a series of laboratory-made cements, composed of CaO, SiO 2, Al 2 O 3, Fe 2 O 3 and MgO, all ground to the same fineness and with the same amount of gypsum added, indicated that strength is primarily a function of the contents of 3CaO·SiO 2 and 2CaO·SiO 2,

The strengths at ages up to 28 days were found to be a function of the content of 3CaO·SiO 2, while the increases in strength between 28 days and 6 months were roughly linearly related to the content of 2CaO·SiO 2, With more finely ground cements, the dicalcium silicate probably began to produce its effect somewhat earlier.

1. Cements rich in tricalcium silicate showed high strength at early ages, while those which were low in this compound showed much lower strengths at early ages, but a progressive increase with age, so that at 6 months the differences in the strengths were relatively small.
2. At 12 months the strengths of the two groups of cements were about equal.

Table 8.13, Clinker phase proportions for lime saturation factor = 95%, alumina ratio = 2.0 and a range of silica ratios 71

Silica ratio Alite Belite C 3 A C 4 AF
1.5 50 19 21 10
1.7 54 17 20 9
2.0 58 16 17 9
2.3 61 15 16 8
2.5 63 14 16 7
2.8 65 14 15 6

Table 8.14, Clinker phase proportions for laboratory clinkers with lime saturation factor = 95%, silica ratio = 2.0 and a range of alumina ratios 71

Alumina ratio Alite Belite C 3 A C 4 AF
0.5 68 14 18
1.0 65 14 6 15
1.5 63 15 10 12
2.0 58 16 17 9
2.5 56 16 20 8

Table 8.15, Clinker phase proportions in laboratory clinkers with silica ratio = 2.0 and alumina ratio = 2.0, and a range of lime saturation factors 71

Lime standard Alite Belite C 3 A C 4 AF
85 39 37 15 9
90 50 26 15 9
95 58 16 17 9
100 69 4 17 10
105 71 6 17 CaO*

The interrelationships between the proportions of clinker phases and compressive strength have been studied 71 for laboratory cements made from chemically pure substances (CaCO 3, SiO 2, Al 2 O 3 and Fe 2 O 3 ) fired at 1450°C for 2 h. The free lime in the clinker was very low (<0.1 per cent); the cement was ground to a specific surface area of 320 ± 4 m 2 /kg with 6 per cent gypsum. The compressive strength was determined on small prisms (15 × 15 × 60 mm) made from mortar (sand/cement = 3, water/binder = 0.5). Fig.8.12, Influence of silica ratio on compressive strength.71 Attempts have been made, by analysing data from a series of cements, to derive factors representing the contribution of each of the four major cement compounds to a particular property at a given age, and under given conditions.

In this analysis the assumption is made that the contributions made by the different compounds are additive, i.e. that a property of a cement may be expressed quantitatively by an equation: (8.9) P = a A + b B + c C + d D where P represents the numerical value of the property, e.g. strength, at a given age and under given conditions; A, B, C and D are the percentage contents of 3CaO·SiO 2, 2CaO·SiO 2, 3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 present in the cement, and a, b, c and d are coefficients representing the contribution of 1 per cent of the corresponding compound to the property considered.

The use of such an equation also assumes that the contribution of a unit mass of any given compound to a particular property under the conditions postulated remains unchanged, i.e. that the coefficients a, b, c and d are constants. Errors in the calculated amount of the compounds present do not invalidate this method of analysis, but they will be reflected in the degree of uncertainty attached to the coefficients deduced.

• The above approach has been more successful when applied to the heat of hydration than when applied to strength or shrinkage data.
• The relative contributions to strength of the four cement compounds derived from different sets of data are very variable, and it cannot be claimed that they do more than confirm the conclusion that 3CaO·SiO 2 makes the major part of its contribution to strength in the first 28 days, and that 2CaO·SiO 2 contributes from 28 days onwards.

The results for 3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 are very erratic; this may not be surprising since the content of these compounds calculated by the Bogue formula can be wide of the truth. All that can be said is that the available data indicated that these two compounds contributed positively to early strength, but under moist storage caused regression in strength after longer periods. Fig.8.13, Variation of compressive strength with lime saturation factor.71 In an experimental investigation to study the early rate of strength gain, a series of 53 laboratory clinkers was prepared from the system CaO-SiO 2 -Al 2 O3-Fe 2 O3 and were formed into cements by intergrinding with 3 per cent CaSO 4,74 Compressive strengths on compacts (pressed minicylinders allowed to hydrate by flooding with distilled water), 11 mm diameter and 11 mm in height were obtained after hydration for 24 h at 20° C.

• The best relationship between this crushing strength and Bogue composition was given by (8.10) Strength = 1.66 × %   C 3 S – 1.52 × %   C 4 AF – 0.72 × %   C 3 A + 0.12 × fineness – 3.2 where fineness (m 2 /kg) is the specific surface area of the cement.
• The coefficient of determination for this regression, adjusted for degrees of freedom, was 89.6 per cent.

The effects of firing time and firing temperature were found to be negligible under the conditions of this laboratory test. The overriding importance of C 3 S content to cement reactivity and early strength development was confirmed by these results. Some clinkers showed anomalous expansions after 24 h and low strengths; most of these clinkers were fired at relatively low temperatures and free CaO was present in at least one.

1. All contained substantial quantities of C 3 A.
2. The relationship between clinker phase composition and strength has been much investigated.75, 76 Data from 30 cements containing 46–62 per cent C 3 S and 1–11 per cent C 3 A have been analysed and the most significant association between strength and compound composition lay in an expression involving the percentage of C 3 A alone.77, 78 The percentage of C 4 AF was found to have little effect on strength.

The contribution of C 2 S matched that of C 3 S only after 90 days’ hydration. The correlation between strength and the percentage of C 3 S passed through a maximum at 7 days’ hydration, while the correlation with the percentage of C 3 A was most significant after 28 days’ hydration.

• It may be assumed that each of the phases of Portland cement clinker contributes either directly or indirectly to the development of strength and the ultimate strength of the resultant cement.
• It is conceivable that this effect may be an additive one, in which each of the phases present acts individually, or a cumulative one, in which an interaction between the individual phase takes place, enhancing or weakening their influence.

Several investigators studying the relationship between clinker phase composition and strength have used multiple linear regression analysis, and have assumed that the action of the phases involved is additive, i.e. that the relationship between strength and cement composition may be expressed by an equation of the type (8.11) σ t = a 0 + a 1 c 1 + a 2 c 2 + a 3 c 3 + where σ t is the cement strength after hydration time t, a 0, a 1, a 2, a 3 are regression coefficients and c 1, c 2, c 3 are the contents of phases 1, 2 and 3 in a cement.

A series of equations has been obtained by evaluating cement strength against a number of clinker parameters (phase composition, ignition loss, insoluble fraction, air content, alkali content).79 A multiple correlation analysis 80 has been published between mortar compressive strength after up to 10 years’ water curing and chemical composition parameters for setting of up to 199 commercial US cements.

The relationship between strength and cement composition has been studied for 30 Australian cements (containing 46–62 per cent C 3 S and 1–11 per cent C 3 A).77 It was concluded that the most significant association between strength and compound composition lay in the regression involving C 3 A alone.

In contrast, C 4 AF was found to have little or no effect on strength. The contribution of C 2 S matched that of C 3 S only after hydrating for 90 days. In a subsequent work 78 it was found that the significance of the correlation of strength with C 3 S passed through a maximum at 7 days, and the correlation with C 3 A was most significant at 28 days.

Schrämli, 81 in analysing his own data and data from the literature, concluded that C 3 A affected initial strengths positively but the ultimate strengths were affected negatively. C 3 A had a negative effect on strengths at 7 and 28 days (31 cements), while the effect of C 3 S is positive.76 The correlation coefficient between strength and C 3 S and C 3 A content was not significant at 1 and 3 days.

The relationship between strength and phase composition has been studied on 114 cements from New Zealand.75 It was concluded that an equation of the type (8.12) σ t = a 0 + a 1 ⋅ C 3 S + a 2 ⋅ ( C 3 S + C 2 S ) + a 3 ⋅ C 3 A + a 4 ⋅ SA yielded better fits than the simple equation (8.13) σ t = a 0 + a 1 ⋅ C 3 S + a 2 ⋅ C 3 A + a 3 ⋅ SA C 4 AF was found to be influential on strength development.

Alexander and Ivanusec 82 established that the association between strength and C 3 S content did vary greatly with the SO 3 content of the cement. However, the effect of C 3 A on strength was insensitive to the SO 3 content. As expected, the regression coefficient associated with C 3 S was consistently positive, confirming the positive effect of C 3 S on strength.

The coefficient associated with C 3 A was also positive, its magnitude always being greater in the equation expressing 28-day strengths than that for 3-day strengths. The coefficient for the effect of C 2 S after 28 days was either positive, negative or equal to zero. Finally, the coefficient associated with C 4 AF was positive, however, its magnitude was significantly lower than that of the coefficient associated with C 3 A.

Even though multiple linear analysis has yielded reasonably good fits to data for the relationship between strength and clinker composition, doubts have arisen whether the effect of single clinker phases is linear, independent and additive. C 3 A may enhance the contribution of C 3 S to strength either by modifying the hydration products or by accelerating the progress of C 3 S hydration.77, 78 The best fit for the data at 3, 7 and 28 days was obtained by an equation of the type (8.14) σ t = a 0 + a 1 ( 1 + b C 3 A ) C 3 S + a 2 where a 0, a 1, a 2 are empirical constants.

• It was also found 78 that the cement strength increased with increasing C 3 A content (linearly) only up to an amount of 14 per cent, and decreases as the C 3 A content becomes even higher.
• A large number of experimentally made clinkers were investigated, 83 varying widely in phase composition.
• These clinkers were ground with constant amounts of gypsum (SO 3 = 2.5 per cent) to identical finenesses (300 m 2 /kg).

The following conclusions were reached. • The overall correlation of strength properties with phase quantities was never linear. • The positive effect of silicates and particularly of C 3 S was the most important one at all ages. Nevertheless, the strength started to decline when the C 3 S content exceeded 70 per cent.

The contribution of C 3 A and C 4 AF to strength development was relatively small. The strength increased with increasing C 3 A content only to some limiting value, and decreased with higher C 3 A contents. The optimum proportion of C 3 A was not constant and changed with the amount of C 3 S and C 4 AF present.

A catalytic action of C 3 A on C 3 S hydration has been proposed and the following equation for calculating the compressive strength was presented: 84 (8.15) σ t = 1 – p e – bt – ( 1 – p ) e – b 2 1 – p e 90 b t – ( 1 – p ) e – 90 b 2 where σ = strength after t days, σ 90 = strength after 90 days, p = C 3 S content of the cement and b, b 2 = rate parameters which are functions of temperature, C 3 A content and fineness.

A series of laboratory-made cements showed a non-linear effect of the individual clinker phases on cement strength and evidence of an interaction between the phases present.85 The resultant strength increased at all ages if C 3 S was replaced with C 3 A, up to an amount of 10 per cent, and declined at higher degrees of substitution.

Substitution of C 3 S by C 4 AF up to 30 per cent had little effect on 1-day strength, but at longer hydration times the resultant strength increased significantly with increasing amount of C 4 AF in the clinker. At constant C 3 S + C 2 S content, the strength in the presence of both C 3 A and C 4 AF was higher than in the presence of each phase alone.

It may be concluded that the available data on the effect of clinker phase composition on strength are rather contradictory. Even though equations which express the relationship between strength and phase composition of the clinker can be derived from the experimental data using multiple regression analysis, a model that assumes independent and additive contribution of the individual clinker constituents to strength without the existence of any interactions, appears highly unlikely.

These relationships are of little practical importance. Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780750662567500205