Expansive Mechanism Of MgO Expansive Agent And Its Application In The Shrinkage-compensating Cementitious Materials

Expansive agents have been proved to be effective in compensating the shrinkage of concrete,or even causing expansion by reacting with water and cementitious components.MgO expansive agent has the advantages of less requirements of water supply,higher thermal stability of hydration product and controllable expansive rate.MgO expansive agent can be adopted to compensate both the early-age and the late-age shrinkage of concrete.A great number of problems,e.g.the over-expansion in elevated temperature and the continuous expansion in the long term,still remain unsolved due to the lack of mechanism research.These defects induced the dispute to the safety of concrete components containing MgO expansive agents and hindered its promotion.The mechanism of the expansion caused by the hydration of MgO has been figured out in this research,thus the problems found in the application of MgO expansive agents can be explained accordingly.The properties and performances in practical engineering of MgO expansive agent were investigated in this research.Its production process,application in high-temperature high-pressure(HTHP)environment and reactivity-evaluating method were explored.(1)It has been proved that the Mg(OH)₂ crystals formed through the hydration of MgO in the pore solution of cement binder,which is a high alkali environment,appear to be hexagonal lamellas.These crystals were then attached on the surface of MgO particles or on the pore walls in the surrounding area.The crystal growth pressure is large enough to expand the capillary pores,thus generating the expansion.The total volume of capillary pores with diameter range from 0.1 to 1?m increased with the incorporation of MgO expansive agents.The expansion still continued even after MgO had completely hydrated.(2)The compressive strengths of the mortars and concretes varied little for those with an MgO dosage of less than 8%,but their volume shrinkage was sufficiently compensated.MgO with higher activity hydrated more rapidly in the early age,but the expansion stopped earlier and the ultimate expansion is smaller.Increasing the water to binder(w/b)ratio appropriately accelerated the expansion rate and increased the ultimate expansion,but excessive w/b ratio hindered the expansion.Higher curing temperature accelerated the expansion rate and increased the ultimate expansion.Supplementary cementitious materials(SCM)like fly ash has controversial effects on the expansion behavior of MgO by both promoting hydration and hindering the growth of Mg(OH)₂.MgO expansive agents significantly decrease the workability of binder paste.(3)A new evaluating method:Excessive ethylic acid buffer solution reacting and weighing method,has been proposed to quantitate the distribution of the reactivity.The reactivity of MgO is divided into 4 zones:The high zone,the medium zone,the low zone and the inactive zone.(4)A new method was proposed to produce a compound MgO expansive agent(CMEA)by calcining the mixture of dolomite(Ca Mg(CO₃)₂)and serpentine(Mg6[Si₄O₁₀](OH)₈),both of which are cheap tailings.The major constituent of CMEA is C₂S and MgO.Incorporating CMEA into cement decreases its mechanical strength slightly,but generates a volume expansion significantly.The expansion rate and speed of mortars containing CMEA is controlled by the calcining temperature of CMEA clinker,and the regularity of CMEA is similar to ordinary MgO expansive agents.(5)High temperature high pressure(HTHP)condition accelerated the expansion rate and increased the ultimate expansion significantly.It facilitated Mg²⁺to immigrate further from MgO particles.Except for expanding mircocracks,Mg(OH)₂crystals also mix with CSH gels and crystals,the hydration product of cement,thus expanding the gap between them.The reactivity of MgO expansive agent adopted in oil-well cement cured in HTHP condition shall be lower than 200s and the proportion shall be lower than 4%.(6)Admixing 6.2%M-type MgO expansive agent compensated the shrinkage of mass concrete effectively during the descending period of temperature and the expansion remained stable in the long term.Meanwhile,the mechanical strength of concrete meted the strength requirements.The thermal-shrinkage of concrete was compensated by the expansion,forming compressive pre-stress thus reducing the probability of shrinkage-cracking.