低活性氧化镁膨胀剂对混凝土微结构、收缩和耐久性的影响

Influence of S-type Magnesia Expansive Agent on Micro-structure,Shrinkage and Durability of Concrete

混凝土中掺入氧化镁膨胀剂(MEA)是解决混凝土收缩开裂引起性能劣化的有效方式之一。S型MEA由于水化活性较低,可以长时间提供稳定的微膨胀补偿收缩,因此被较多地应用于工程中。将S型MEA以4%、5%、6%(质量分数)的掺量掺入混凝土中,系统研究了S型MEA对混凝土的微观结构与力学性能、收缩性能及耐久性能等宏观性能的影响。结果表明:水化产物Mg(OH)_(2)的结晶生长使混凝土微观孔隙产生膨胀,S型MEA掺量5%时混凝土28d孔隙率由7.93%增大到10.63%,因此水化早期混凝土强度降低,抗氯离子渗透能力降低;同时,混凝土10~50 nm范围内孔隙数量降低,混凝土的收缩性能得到改善,S型MEA掺量为4%、5%、6%时自收缩分别降低了25.66%、18.58%、46.02%。随着S型MEA的不断水化,Mg(OH)_(2)逐渐的填充作用降低混凝土孔隙率,提高了混凝土后期强度;然而随着S型MEA掺量提高,未水化的MgO引起的膨胀与冻融作用或硫酸盐侵蚀作用耦合时,则对混凝土耐久性能存在不利影响。最后,给出了S型MEA混凝土的收缩模型和氯离子渗透系数模型,可结合短龄期试验用于预测混凝土长期性能,对工程应用具有一定指导作用。

Incorporating magnesia expansive agents(MEA)into concrete is one of the effective ways to solve the deterioration of concrete performance due to cracking caused by shrinkage.S-type MEA can provide a stable micro-expansive to compensate for shrinkage of concrete over a long period of time because of the low activity of hydration,which is widely used in engineering.In this paper,S-type MEA was mixed with concrete at different MEA contents(i.e.,4%,5%and 6%,mass fraction).The effect of MEA on the microstructure and macroscopic properties(i.e.,mechanical,shrinkage and durability)of concrete was investigated.The results show that the crystal growth of the hydration product magnesium hydroxide expands the microscopic pores of concrete with MEA,and the porosity of concrete increases from 7.93%to 10.63%at 28 d when the MEA content is 5%.Therefore,the concrete strength before 28 d and the resistance to chloride ion penetration are reduced.Also,the content of pores in the range of 10–50 nm decreases,thus improving the shrinkage performance of the concrete.The autogenous shrinkage is reduced by 25.66%,18.58%and 46.02%when the MEA content is 4%,5%,and 6%,respectively.As the MEA hydrates continuously,the gradual filling effect of magnesium hydroxide reduces the porosity and improves the strength of concrete with MEA.However,swelling from un-hydrated MEA is coupled with the effect of either freeze-thaw cycles or sulfate attack with increasing the MEA content,thus having a negative effect on the durability of concrete.In addition,the shrinkage model and chloride ion permeability coefficient model of concrete with MEA were also proposed to predict the long-term performance of concrete,thus having a certain guiding value on the engineering applications.