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A discussion of the paper “a microstructural study of the effect produced by magnesium sulfate on plain and silica fume-bearing portland cement mortars” by David Bonen
Cement and Concrete Research, Vol. 24, No. 2, pp. 371-372, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0008-8846/94 $6.00 + .00
Pergamon
DISCUSSIONS
A DISCUSSION OF THE PAPER "A MICROSTRUCTURAL STUDY OF THE EFFECT PRODUCED BY MAGNESIUM SULFATE ON PLAIN AND SILICA FUME-BEARING PORTLAND CEMENT MORTARS" by David Bonen
Omar Saeed Baghabra AI-Amoudi Department of Civil Engineering King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia The contribution of Dr. Bonen on the effect of magnesium sulfate (M~) on plain and silica fume blended cement pastes and mortars is interesting, particularly for the sophisticated techniques he used and the findings he reported (1-3). Based on a comprehensive investigation carried out at KFUPM (4) on plain (Type I and V) and blended (fly ash, silica fume and blast furnace slag) cements, I would like to put forward two comments, one is on the mechanisms of deterioration and the other is on the deterioration of pastes, mortars and concretes made with plain and blended cements and exposed to MS environments. Deterioration was observed in all plain and blended cements upon exposure to MS and M~-N~ solutions (5,6). However, the deterioration was more intense in silica fume (SF) and blast furnace slag (BSF) cements. This enhanced deterioration was attributed to the depletion of portlandite (CH) as a result of pozzalanic reaction, which was chemically manifested by massive formation of gypsum (1,2,4,5) and physically by spalling, and softening rather than the deep-seated expansion and cracking due to ettringite formation. As Dr. Bonen reported (1), Biczok (7), based on the work of Kind, attributed the gypsum formation to: (i) reaction of MS and N~ with CH, and (ii) decompositional action of M~ on the cementitious C-S-H. However, the role of CH has not yet been well established, although many authorities correlated the reduced CH content with the enhanced deterioration of SF cements. Portlandite (CH) should be considered as the first defensive substance to react with MS and, therefore, it acts as a buffer and a retarder of the Mg-attack on C-S-H (reaction (ii) above) (4,5). The significant reduction of CH in SF and BSF cements would deflect the attack more directly and extensively towards C-S-H thereby generating ultimately gypsum and non-cementitious magnesium silicate hydrate (M-S-H). The morphology of M-S-H has been identified as non-crystalline and fibrous (5). Its amorphous structure makes it very difficult to be triggered by XRD technique. Two other, and rather less significant, mechanisms for the aggravated deterioration of SF and BSF cements were: (i) the absence of brucite (MH) layer, and (ii) disappearance CCR 23(3) 541-553 (1993). 371
Discussions
372
Vol. 24, No. 2
of primary sulfo-aluminate (monosulfate and ettringite) phases. The absence of protective MH layer enhances the ingress of Mg +÷ and SO 4- -ions into the hardened matrix thereby accelerating the deterioration, while the disappearance of primary sulfoaluminates confirmed its instability in MS environment leading to its decomposition to gypsum, alumina and brucite, thus further augmenting the gypsum content and its deleterious action on C-S-H (4,5). The brucite layer and primary sulfo-aluminate phases did not disappear in plain cements, even after two years of exposure (4,5). Dr. Bonen reported that "incorporation of aggregates into pastes has relatively a higher adverse effect on PC mortar than PC + SF mortar", due to the effect of SF on the transition zone (3). After 44 months of exposure of 15 reinforced concretes to MS-NS environment (8), concretes made with SF cements exhibited an advanced stage of deterioration compared to PC concretes, but they were better than all other blended cement concretes. However, the internal matrix (surrounding the reinforcing steel) of SF concretes was better than that of PC concretes as confirmed by the corrosionresistance performance data (8), which support the findings of Dr. Bonen (3). Such contributions are very important since considerable apprehensions were raised on the deterioration of SF paste specimens in MS environments.
REFERENCES
.
Bonen, D. and Cohen, M.D., Cement and Concrete Research, Vol. 22, No. 1, 1992, pp. 169-180.
.
Bonen, D., and Cohen, M.D., Cement and Concrete Research, Vol. 22, No. 4, 1992, pp. 707-718. Bonen, D., Cement and Concrete Research, Vol. 23, No. 3, 1993, pp. 541-553.
.
A1-Amoudi, O.S.B., Ph.D. Dissertation, Department of Civil Engineering, KFUPM, Dhahran, Saudi Arabia.
4.
.
Rasheeduzzafar, et al., "Mechanisms of Magnesium-Sodium Sulfate Attack in Plain and Blended Cements," ASCE, Materials Journal in Civil Engineering, accepted for publication.
.
A1-Amoudi, et al., "Effect of Magnesium Sulfate and Sodium Sulfate on the Durability Performance of Plain and Blended Cements," to be published.
.
Biczok, I., Concrete Corrosion, Concrete Protection, 8th Ed., Akademiai Kiado, Budapest, 1972.
.
A1-Amoudi, O.S.B., "Durability of Fifteen Reinforced Concretes in MagnesiumSodium Sulfate Environment," under preparation.
Pergamon
DISCUSSIONS
A DISCUSSION OF THE PAPER "A MICROSTRUCTURAL STUDY OF THE EFFECT PRODUCED BY MAGNESIUM SULFATE ON PLAIN AND SILICA FUME-BEARING PORTLAND CEMENT MORTARS" by David Bonen
Omar Saeed Baghabra AI-Amoudi Department of Civil Engineering King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia The contribution of Dr. Bonen on the effect of magnesium sulfate (M~) on plain and silica fume blended cement pastes and mortars is interesting, particularly for the sophisticated techniques he used and the findings he reported (1-3). Based on a comprehensive investigation carried out at KFUPM (4) on plain (Type I and V) and blended (fly ash, silica fume and blast furnace slag) cements, I would like to put forward two comments, one is on the mechanisms of deterioration and the other is on the deterioration of pastes, mortars and concretes made with plain and blended cements and exposed to MS environments. Deterioration was observed in all plain and blended cements upon exposure to MS and M~-N~ solutions (5,6). However, the deterioration was more intense in silica fume (SF) and blast furnace slag (BSF) cements. This enhanced deterioration was attributed to the depletion of portlandite (CH) as a result of pozzalanic reaction, which was chemically manifested by massive formation of gypsum (1,2,4,5) and physically by spalling, and softening rather than the deep-seated expansion and cracking due to ettringite formation. As Dr. Bonen reported (1), Biczok (7), based on the work of Kind, attributed the gypsum formation to: (i) reaction of MS and N~ with CH, and (ii) decompositional action of M~ on the cementitious C-S-H. However, the role of CH has not yet been well established, although many authorities correlated the reduced CH content with the enhanced deterioration of SF cements. Portlandite (CH) should be considered as the first defensive substance to react with MS and, therefore, it acts as a buffer and a retarder of the Mg-attack on C-S-H (reaction (ii) above) (4,5). The significant reduction of CH in SF and BSF cements would deflect the attack more directly and extensively towards C-S-H thereby generating ultimately gypsum and non-cementitious magnesium silicate hydrate (M-S-H). The morphology of M-S-H has been identified as non-crystalline and fibrous (5). Its amorphous structure makes it very difficult to be triggered by XRD technique. Two other, and rather less significant, mechanisms for the aggravated deterioration of SF and BSF cements were: (i) the absence of brucite (MH) layer, and (ii) disappearance CCR 23(3) 541-553 (1993). 371
Discussions
372
Vol. 24, No. 2
of primary sulfo-aluminate (monosulfate and ettringite) phases. The absence of protective MH layer enhances the ingress of Mg +÷ and SO 4- -ions into the hardened matrix thereby accelerating the deterioration, while the disappearance of primary sulfoaluminates confirmed its instability in MS environment leading to its decomposition to gypsum, alumina and brucite, thus further augmenting the gypsum content and its deleterious action on C-S-H (4,5). The brucite layer and primary sulfo-aluminate phases did not disappear in plain cements, even after two years of exposure (4,5). Dr. Bonen reported that "incorporation of aggregates into pastes has relatively a higher adverse effect on PC mortar than PC + SF mortar", due to the effect of SF on the transition zone (3). After 44 months of exposure of 15 reinforced concretes to MS-NS environment (8), concretes made with SF cements exhibited an advanced stage of deterioration compared to PC concretes, but they were better than all other blended cement concretes. However, the internal matrix (surrounding the reinforcing steel) of SF concretes was better than that of PC concretes as confirmed by the corrosionresistance performance data (8), which support the findings of Dr. Bonen (3). Such contributions are very important since considerable apprehensions were raised on the deterioration of SF paste specimens in MS environments.
REFERENCES
.
Bonen, D. and Cohen, M.D., Cement and Concrete Research, Vol. 22, No. 1, 1992, pp. 169-180.
.
Bonen, D., and Cohen, M.D., Cement and Concrete Research, Vol. 22, No. 4, 1992, pp. 707-718. Bonen, D., Cement and Concrete Research, Vol. 23, No. 3, 1993, pp. 541-553.
.
A1-Amoudi, O.S.B., Ph.D. Dissertation, Department of Civil Engineering, KFUPM, Dhahran, Saudi Arabia.
4.
.
Rasheeduzzafar, et al., "Mechanisms of Magnesium-Sodium Sulfate Attack in Plain and Blended Cements," ASCE, Materials Journal in Civil Engineering, accepted for publication.
.
A1-Amoudi, et al., "Effect of Magnesium Sulfate and Sodium Sulfate on the Durability Performance of Plain and Blended Cements," to be published.
.
Biczok, I., Concrete Corrosion, Concrete Protection, 8th Ed., Akademiai Kiado, Budapest, 1972.
.
A1-Amoudi, O.S.B., "Durability of Fifteen Reinforced Concretes in MagnesiumSodium Sulfate Environment," under preparation.