Subsequent Hydration Of Cementitious Materials And Its Effect On The Stability Of UHPC

Ultra-high performance concrete（UHPC）has prominent conmpressive strength of more than 120 MPa and excellent durability resulted from reducing the water-binder ratio,optimizing particle size grades and using lots of reactive materials.There exist more than 800 kg/m³cement and 150 kg/m³ silica fume in UHPC respectively.However,due to the limited water and space existed,as well as hyperslow transmission rate,there still remain many unreacted cementitious materils in UHPC after 28-d reaction.In wet environment,especially under the condition of high temperature and pressure,ambient water could penetrate into UHPC,accelerating the subsequent hydration of unreacted cementitious materials.The microstructure and performance of UHPC are thus affected.At present,the congnition of subsequent hydration is not comprehensive,and the long-term stability of cement-based materials with low water-binder ratio（w/b）is in suspense.Cement hydration leads to the expansion of hydrates.Thus,some researchers believe that solid expansion caused by subsequent hydration in the dense matrix would result in expansion stress,the matrix would be damaged when the expansion stress exceeds the permissible tensile stress resulting in the deterioration of the strength and durability.However,other studies indicate that the mechanical properties and durability of cement-based materials with low w/b would be improved by subsequent curing.The effect of w/b,curing method,type and content of cementitious materials on the long-term properties and microstructure of UHPC were systematically studied.The effect of SCM was analyzed and the mechanism of subsequent hydration of cementitious materials was elucidated.The main conclusions of this paper are followed:（1）After 90-d hydrothermal treatment,plain cement paste with low w/b shows obvious volume expansion and strength deterioration.While strength and anti-permeability deterioration do not appear in UHPC.Therefore,it is believed that UHPC would not be damaged even if the paste would be affected by subsequent hydration.（2）The w/b influences the development of strength,volume and permeability of UHPC.UHPC with w/b of 0.16 shows significant strength increase and shrinkage during subsequent hydration.After steam curing,the microstructure of UHPC is too dense to permeate.Therefore,the impact of subsequent hydration in UHPC with steam curing on strength and volume stability is very weak.（3）Cementitious material system affects the long-term performance of UHPC.After subsequent hydration of 180 d,UHPC consisted of cement and slag has the highest compressive strength and volume expansion,while its permeability is relatively high.The long-term performance of cement-silica fume group is affected by silica fume content.When containing 15%silica fume,UHPC with excellent anti-permeability has little increase of compressive strength and shrinkage after 180 d subsequent hydration.When containing 30%silica fume,the initial compressive strength and anti-permeability of UHPC are relatively low.After subsequent curing,the shrinkage ratio and increase ratio of compressive strength of this UHPC are pretty high.The subsequent hydration does not affect the performance of UHPC consisted by cement,silica fume and slag.（4）During subsequent curing,hydrotalcite and hydrogarnet and monosulfoaluminate are produced in plain cement paste and cement-slag paste.The content of ettringite in cement-silica fume paste and cement paste with silica fume and slag are relatively high.However,ettringite decomposite during subsequently hydrothermal curing.The relatively high content of Mg and Al in slag promote the formation of hydrotalcite phase and hydrogarnet,while hydrotalcite adsorbed SO₄^2-in pore solution,hindering the formation of ettringite.（5）During subsequent curing of first 28 d,phase evolution caused by subsequent hydration is obvious.After 28 days,the rate of subsequent hydration decrease.It is most significant in UHPC containing 30%silica fume.Silica fume hinders the subsequent hydration of UHPC,slag promotes long-term hydration,while UHPC consisited of cement,silica fume and slag show no subsequent hydration.（6）During subsequent hydration,hydrates are mainly generated in air pores,which is affected by cementitious material and pore size.A large number of CH fill the air pores in plain cement paste and cement-slag paste.There are little subsequent hydration products in the pores of the cement paste with silica fume and cement paste with silica fume and slag.（7）After subsequent hydration,there is a linear relationship between gas permeability and permeable porosity,but compressive strength has nothing to do with permeable porosity.