Na+和K+对水泥基材料早期收缩性能的影响

利用压汞、纳米压痕以及27Al固体核磁共振方法,系统研究了不同碱金属离子(Na~+和K~+)与含量对水泥基材料早期自收缩和干燥收缩性能的影响规律。结果表明,超量的Na~+或K~+均促进了水泥基材料早期自收缩和干燥收缩,但K~+的促进作用明显大于Na~+,其机制在于:(1)K~+比Na~+更易促进水泥基材料中小于50纳米孔数量的增加,这使得超量K~+水泥基材料在干燥时具有更大的收缩驱动力;(2)虽然K~+比Na~+更易促进水泥基材料生成高密度水化硅酸钙(HD C-S-H),但同时超量K~+水泥基材料中C-S-H总量也较高,大量的C-S-H是导致较高早期收缩的根源;(3)与Na~+相比,K~+更明显地阻碍了钙矾石(AFt)晶体的形成,削弱了AFt的骨架作用,且K~+可促使AFt向单硫型水化硫铝酸钙(AFm)转变,因而导致超量K~+水泥基材料更易发生收缩变形。

Influence of Na+ and K+ on the early-age shrinkage properties of cement-based materials

The influence of the different metal ions (K⁺ and Na⁺) and alkali content on autogenous shrink-age and drying shrinkage at early age has been investigated through mercury intrusion porosimetry (MIP), nanoindentation and ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR). The results in-dicate that the autogeneous and drying shrinkage of cement-based materials are increased by K⁺ or Na⁺, whereas the effect of K⁺ is much more obvious. The mechanisms of these phenomena are attributed to three reasons:(1) the capillary pores less than 50nm in cement-based material with high K⁺ content are more than those with high content Na⁺, leading to a higher driving force when shrinkage occurs;(2) K⁺ tends to promote the formation of high density calcium silicate hydrate (HD C-S-H), but the total content of C-S-H in cement-based material with high K⁺ content is higher than the one with high Na⁺ content, and the large amount of C-S-H is primary cause of high early-age shrinkage; and (3) K⁺ is easier to hinder the formation of AFt and accelerate the transformation of AFt to AFm than Na⁺ at early age, leading to the weak framework function of AFt. The cement-based material with high K⁺ content has more shrinkage values at early-age.