The effect of retarders on the microstructure and mechanical properties of magnesia-phosphate cement mortar

Abstract Magnesia–phosphate cement (MPC) mortars based on magnesia and ammonium dihydrogen orthophosphate (ADP) were prepared with additions of sodium triphosphate (STP), boric acid, and borax as set-retarders. The resulting materials were characterised in terms of their setting characteristics, mechanical properties, and microstructures. The retarders were found to increase the setting time and to reduce the intensity of the exothermic reactions during the initial setting and hardening stages. The effectiveness of STP as a retarder was limited by its solubility in the saturated ammonium phosphate solution, determined as 340 g dm −3 , which meant that the maximum setting time obtained was approximately 15 min. In contrast, successive additions of boric acid and borax led to a cumulative increase in setting time to a maximum of 1 h for the compositions investigated. The use of retarders also led to an extension of the initial hardening period, as indicated by reductions in the early values of Young's modulus and flexural strength. The microstructural characteristics of the materials produced were very dependent on the type and level of additive employed; for STP, the binding phase was relatively fine grained ( 10 μm) structures were observed as a result of a reduction in the supersaturation of the aqueous solution. Well-aged mortar specimens exhibited a high incidence of microcracks on the fracture surfaces, which were related to the occurrence of a fracture process zone ahead of the main crack tip during fracture.

[1]  A. Regazzoni,et al.  Boric acid adsorption on magnetite and zirconium dioxide , 1984 .

[2]  B. El-Jazairi.  RAPID REPAIR OF CONCRETE PAVINGS , 1982 .

[3]  Sandor Popovics,et al.  EARLY AGE PROPERTIES OF MAGNESIUM PHOSPHATE-BASED CEMENTS UNDER VARIOUS TEMPERATURE CONDITIONS , 1987 .

[4]  Sandor Popovics,et al.  RAPID HARDENING CEMENTS FOR REPAIR OF CONCRETE , 1987 .

[5]  John H. Sharp,et al.  The chemical composition of mortars made from magnesia-phosphate cement , 1988 .

[6]  S. A. Rodger,et al.  The chemistry of hydration of high alumina cement in the presence of accelerating and retarding admixtures , 1984 .

[7]  R. C. Weast.  CRC Handbook of Chemistry and Physics , 1973 .

[8]  J. Pera,et al.  Mechanism of setting reaction in magnesia-phosphate cements , 2000 .

[9]  N. L. Thomas,et al.  The effect of lead nitrate on the early hydration of portland cement , 1981 .

[10]  E. Fuller,et al.  Microstructural Aspects of the Fracture of Hardened Cement Paste , 1989 .

[11]  L. E. Kukacka,et al.  Characteristics of magnesium polyphosphate cements derived from ammonium polyphosphate solutions , 1983 .

[12]  K Asgar,et al.  Reaction of Cobalt-Chromium Casting Alloy with Investment , 1966, Journal of dentistry research.

[13]  Quanbing Yang,et al.  Factors influencing properties of phosphate cement-based binder for rapid repair of concrete , 1999 .

[14]  L. E. Kukacka,et al.  Magnesium monophosphate cements derived from diammonium phosphate solutions , 1983 .

[15]  G. J. Arquette,et al.  Precipitation of Pyrophosphate and Triphosphate with Tris(ethylenediamine)cobalt(III) Chloride and Hexamminecobalt(III) Chloride , 1955 .

[16]  B. El-Jazairi.  THE PROPERTIES OF HARDENED MPC MORTAR AND CONCRETE RELEVANT TO THE REQUIREMENTS OF RAPID REPAIR OF CONCRETE PAVEMENTS , 1987 .

[17]  R Neiman,et al.  Setting and Thermal Reactions of Phosphate Investments , 1980, Journal of dental research.

[18]  K. Utsumi,et al.  Deterioration of phosphate-bonded investment on exposure to 100% relative humidity atmosphere. , 1982, The Bulletin of Tokyo Medical and Dental University.

[19]  S. Igarashi,et al.  Detection of Failures around Main Cracks and Their Relation to the Fracture Toughness of Cement Pastes , 1995 .

[20]  John H. Sharp,et al.  The microstructure and mechanical properties of mortars made from magnesia-phosphate cement , 1989 .