水分和二氧化碳引起的水泥陈化对混凝土和砂浆中化学掺合料性能的影响(英文)

Influence of Moisture-and CO_2-Induced Ageing of Cement on Performance of Chemical Admixtures Used in Concrete and Mortar

本工作旨在深入研究水泥的陈化对水泥与化学外加剂相互作用的影响。将Portland水泥分别暴露于35℃和相对湿度为90%的环境中1和3d。选取3种高效减水剂(萘系减水剂NSF、聚羧酸减水剂PCE和干酪素)、1种保水剂(MHEC,甲基纤维素)及两种促凝剂(甲酸钙和无定形氧化铝),研究了水泥与化学外加剂的相互作用。结果表明:水泥的陈化显著改变了新拌浆体的流动性、保水性与凝结行为。由于暴露过程中水泥表面部分水化,水泥的比表面积以及表面电荷均发生变化,进而影响了水泥与化学外加剂的相互作用。具体表现在以下3个方面:1)与新鲜水泥相比,减水剂在陈化水泥中的减水效果减弱,干酪素的分散能力略微降低,而PCE和PNS的分散能力均显著下降,尤其是PNS下降最大;2)陈化水泥呈现较好的保水能力和更高的需水量,因此,为了达到相同的保水效果,其所需的保水剂掺量较新鲜水泥低;3)在陈化水泥中,甲酸钙和无定形氧化铝的促凝效果显著下降,在水泥陈化3d后,这些促凝剂甚至可能延缓水泥水化。

The purpose of this paper was to obtain a more in-depth understanding of the influence of ageing on the interaction of cement with chemical admixtures. The exposure conditions tested were 35 ℃ and 90 % relative humidity for 1 and 3 days, respectively. As admixtures, three superplasticizers （NSF, PCE and casein）, one water-retention agent （MHEC） and two accelerators （Ca formate and amorphous Al2O3） were investigated. The results show that the ageing of cement can lead to a significant change of the flow, water retention and setting behaviour patterns. As a consequence of this partial surface hydration, the specific surface area and the surface charge of cement are altered, substantially affecting the interaction with chemical admixtures. The following correlations were found： 1）The effectiveness of superplasticizers in aged cement is generally less than that in fresh cement. The dispersing power of biopolymer-based superplasticizer like casein is slightly affected by moist storage. The dispersing power of polycarboxylate sample and the NSF admixture is reduced; 2） The aged cement exhibits an improved water retention capacity with a higher water demand to achieve the workability required in practice. Consequently, the dosage of methylcellulose to reach a high water retention in aged cement is lower than that in fresh cement; and 3） The accelerating effect of Ca formate and amorphous Al2O3 in aged cement is decreased. After ageing for 3 d, they may even retard cement hydration.