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A discussion of the paper “composition of the solution in the hydration of cement” by W. Rechenberg and S. Sprung
CEMENT and CONCRETE RESEARCH. V o l . 13, pp. 751-752, 1983. Printed in the USA. 0008-8846/83 $3.00 + DO. Copyright (c) 1983 Pergamon Press, Ltd.
DISCUSSION A DISCUSSION OF THE PAPER "COMPOSITION OF THE SOLUTION IN THE HYDRATION OF CEMENT" BY W. RECHENBERGAND S. SPRUNG
S. Chandra Div. of Building Materials Chalmers University of Technology S-412 96 G~teborg, Sweden
I t is now very important to concentrate on the composition of liquid solution in pores and capillaries, specially when different types of admixtures are used in concrete. In the referred paper experiments were done by mixing alkali sulfates to a mixture of 85 parts calcium hydroxide and 15 parts of high alumina cement. Calcium hydroxide equilibrium was reached after one day. I t is inferred that the sulfate was quickly bound by the aluminates of the high alumina cement (p. 123). In the next line i t is mentioned that "thus in the portland cement i t can be expected that equilibrium will occur more quickly, the greater the amount and reactivity of tricalcium aluminate". Basing upon this test I do not think this conclusion can be drawn. High alumina cement and portland cement are not comparable. Sulfate can be quickly bound with aluminates but in high alumina cement mainly monocalcium aluminate is present whereas in portland cement only tricalcium aluminate is present. The hydration process in these two cements are different. Further in portland cement gypsum is present and is directly taking part in hydration reaction. In the tests performed alkali sulfate is mixed which will again influence the process. In the case of portland cement the reaction will be following: 2KOH (NaOH) + CaSO4 • 2H20 = Ca(OH)2 + K2SO4 (Na2S04) whereas in the tests performed i t will be: Ca(OH)2 + K2SO4 (Na2SO4) = 2KOH (NaOH) + CaSO4 • 2H20. Character of hydration of tricalcium aluminate is well described by Feldman and Ramchandran (I) and of high alumina cement by Neville (2). Composition and physical properties of aqueous extracts from portland cement clinker pastes containing added materials are well illustrated by Kalousek et al (3). The defects in the solids also play important role in their chemical behaviour (4, 5). This can be very much different in two cases. Rees (4) experiments showed that dislocations intersecting a surface have a significant influence CCR 13, pp. I19-126 (1983) 751
752
Vol. 13, No. 5 DISCUSSION
on reaction rates. Dislocation densities contribute to the rate of dissolution and dislocations intersecting the surface may also be the s t a r t i n g points for the hydration process. High alumina cements are hardening faster than the portland cement. Consequently the heat evolved in these reactions, known as heat of hydration, is released at higher rate (for example 77.93 and 23.46 Cal/gm a f t e r one day for HAC and normal portland cement respectively). This difference w i l l cause an increase in temperature. Thus temperature during hydration of high alumina cement is higher than portland cement. The hydration of calcium aluminate is very much dependent on the temperature. Tests performed in the paper are a f t e r 2 hours of cement hydration, whereas on p. 122 ( l a s t lines of paragraph I) i t is mentioned that the solution of the hardened cement paste is always saturated with calcium hydroxide and not oversaturated. Of course the question of saturation or oversaturation of the solution is a matter of discussion. But I agree with the hypothesis (6) that the soluble s i l i c a p r e c i p i t a t e (which comes in the f i r s t stage of hydration (7)) is adsorbed on the calcium hydroxide nuclei. This hinders t h e i r growth. Supersaturation is needed to overcome t h i s poisoning effect. The reaction of a l k a l i sulfate with pure C3A or a mixture of C3A and C4AF corresponding to t h e i r proportion in portland cement, in presence of calcium hydroxide, w i l l be more r e a l i s t i c . References ( I ) R.F. Feldman and V.S. Ramchandran, "Character of hydration of 3CaO'AI203". J. American Ceramic Soc., Vol 49, No 5 (1966). (2) A. Neville, "High alumina cement concrete". The Construction Press Ltd, Lancaster (1975). (3) G.L. Kalousek et al, "Composition and physical properties of aqueous extracts from Portland cement c l i n k e r pastes containing added materials". National Bureau of Standards, pp. 215-255, Vol 30 (1943). (4) A.L.G. Rees, "Significance of solid state defects in chemical science and technology". Australian J.Sc. 26 (8), pp. 239-246 (1964). (5) A.R.C. Westwood, "Sensitive mechanical pp. 15-25 (1964).
properties". Ind. Eng. Chem. 56 (9),
(6) J.F. Young, "A review of the mechanism of set retardation in Portland cement pastes containing admixtures". Cem. & Conc. Res.,Vol 2, pp. 415-433, (1972). (7) H.N. Stein et a l , "Hydration of tricalcium s i l i c a t e " . J. App. Chem., Vol 17, pp 246-250 (1967).
DISCUSSION A DISCUSSION OF THE PAPER "COMPOSITION OF THE SOLUTION IN THE HYDRATION OF CEMENT" BY W. RECHENBERGAND S. SPRUNG
S. Chandra Div. of Building Materials Chalmers University of Technology S-412 96 G~teborg, Sweden
I t is now very important to concentrate on the composition of liquid solution in pores and capillaries, specially when different types of admixtures are used in concrete. In the referred paper experiments were done by mixing alkali sulfates to a mixture of 85 parts calcium hydroxide and 15 parts of high alumina cement. Calcium hydroxide equilibrium was reached after one day. I t is inferred that the sulfate was quickly bound by the aluminates of the high alumina cement (p. 123). In the next line i t is mentioned that "thus in the portland cement i t can be expected that equilibrium will occur more quickly, the greater the amount and reactivity of tricalcium aluminate". Basing upon this test I do not think this conclusion can be drawn. High alumina cement and portland cement are not comparable. Sulfate can be quickly bound with aluminates but in high alumina cement mainly monocalcium aluminate is present whereas in portland cement only tricalcium aluminate is present. The hydration process in these two cements are different. Further in portland cement gypsum is present and is directly taking part in hydration reaction. In the tests performed alkali sulfate is mixed which will again influence the process. In the case of portland cement the reaction will be following: 2KOH (NaOH) + CaSO4 • 2H20 = Ca(OH)2 + K2SO4 (Na2S04) whereas in the tests performed i t will be: Ca(OH)2 + K2SO4 (Na2SO4) = 2KOH (NaOH) + CaSO4 • 2H20. Character of hydration of tricalcium aluminate is well described by Feldman and Ramchandran (I) and of high alumina cement by Neville (2). Composition and physical properties of aqueous extracts from portland cement clinker pastes containing added materials are well illustrated by Kalousek et al (3). The defects in the solids also play important role in their chemical behaviour (4, 5). This can be very much different in two cases. Rees (4) experiments showed that dislocations intersecting a surface have a significant influence CCR 13, pp. I19-126 (1983) 751
752
Vol. 13, No. 5 DISCUSSION
on reaction rates. Dislocation densities contribute to the rate of dissolution and dislocations intersecting the surface may also be the s t a r t i n g points for the hydration process. High alumina cements are hardening faster than the portland cement. Consequently the heat evolved in these reactions, known as heat of hydration, is released at higher rate (for example 77.93 and 23.46 Cal/gm a f t e r one day for HAC and normal portland cement respectively). This difference w i l l cause an increase in temperature. Thus temperature during hydration of high alumina cement is higher than portland cement. The hydration of calcium aluminate is very much dependent on the temperature. Tests performed in the paper are a f t e r 2 hours of cement hydration, whereas on p. 122 ( l a s t lines of paragraph I) i t is mentioned that the solution of the hardened cement paste is always saturated with calcium hydroxide and not oversaturated. Of course the question of saturation or oversaturation of the solution is a matter of discussion. But I agree with the hypothesis (6) that the soluble s i l i c a p r e c i p i t a t e (which comes in the f i r s t stage of hydration (7)) is adsorbed on the calcium hydroxide nuclei. This hinders t h e i r growth. Supersaturation is needed to overcome t h i s poisoning effect. The reaction of a l k a l i sulfate with pure C3A or a mixture of C3A and C4AF corresponding to t h e i r proportion in portland cement, in presence of calcium hydroxide, w i l l be more r e a l i s t i c . References ( I ) R.F. Feldman and V.S. Ramchandran, "Character of hydration of 3CaO'AI203". J. American Ceramic Soc., Vol 49, No 5 (1966). (2) A. Neville, "High alumina cement concrete". The Construction Press Ltd, Lancaster (1975). (3) G.L. Kalousek et al, "Composition and physical properties of aqueous extracts from Portland cement c l i n k e r pastes containing added materials". National Bureau of Standards, pp. 215-255, Vol 30 (1943). (4) A.L.G. Rees, "Significance of solid state defects in chemical science and technology". Australian J.Sc. 26 (8), pp. 239-246 (1964). (5) A.R.C. Westwood, "Sensitive mechanical pp. 15-25 (1964).
properties". Ind. Eng. Chem. 56 (9),
(6) J.F. Young, "A review of the mechanism of set retardation in Portland cement pastes containing admixtures". Cem. & Conc. Res.,Vol 2, pp. 415-433, (1972). (7) H.N. Stein et a l , "Hydration of tricalcium s i l i c a t e " . J. App. Chem., Vol 17, pp 246-250 (1967).