Single And Combined Effects Of Shrinkagereducing Admixture And Internal Curing On Autogenous Shrinkage Of Hardened Cement Pastes With Low Water-cement Ratio

From both reducing the surface tension of pore solution and maintaining the internal relative humidity of hardened cement pastes(HCP),this paper investigated the single and combined effects of shrinkage reducing admixture(SRA)and internal curing materials(diatomite and superabsorbent polymer(SAP))on autogenous shrinkage of hardened cement pastes with low water-cement ratio(w/c).According to the results,aiming to further reduce the autogenous shrinkage of hardened cement pastes with low w/c,a technical idea on combination use of SRA and internal curing is generated.On the premise of less than 10% strength loss,benefit of up to 40% reduction of autogenous shrinkage has been achieved,which provides a better way to greatly reduce the autogenous shrinkage of hardened cement pastes with low w/c.The experimental results showed that the SRA,if prepared as solution with certain concentration before adding to mix the cement paste with low w/c,can significantly reduce the autogenous shrinkage of the resulting hardened cement pastes,for example,the SRA solution with only 4.0% concentration(the effective content of SRA is 0.72% of cement by weight)reduced the total autogenous shrinkage of hardened cement pastes up to 26.5% when at the age of 28 days;It is also found that the internal relative humidity of the hardened cement pastes with SRA solution is 3% to 11% higher than that of the blank one,which means SRA mitigated the self-desiccation of the hardened cement pastes with low w/c.When the SAP was used as internal curing material,its increasing effect on interior relative humidity of the resulting hardened cement pastes is more remarkable than SRA,the maximum increasing rate is up to 16%,subsequently,the autogenous shrinkage of the corresponding hardened cement pastes with low w/c is greatly reduced,ranging from 31.7% to 51.8%,changing with the content of SAP.Although singly using SRA or SAP,with the increase of SRA or SAP content respectively,the autogenous shrinkage is continuously reduced,the strength of the corresponding hardened cement pastes is also quickly decreased,the strength loss could be up to 30.5%,which is not acceptable in practice.This paper presents a combination use of low concentration SRA solution and low content of SAP,achieving the satisfactory reducing effect on autogenous shrinkage(42.5%),as well as an acceptably small loss of strength(8.75%),which has more reasonably practical value.By studying the shrinkage mechanism,it was found that the shrinkage reducing effect of SRA on hardened cement pastes with low w/c is achieved mainly by reducing the surface tension of the pore solution,secondarily by maintaining the relative humidity inside the hardened cement pastes and by coarsening the pore structure.The internal curing material reduced the autogenous shrinkage of hardened cement pastes mainly by the continuous output of water,which mitigates the self-desiccation caused by the continuous hydration of cement.Because diatomite has the pore structure in the same order as the hardened cement pastes,when used as internal curing material,its own water loss produces similar capillary stress as that of hardened cement pastes,consequently,the reducing effect on the autogenous shrinkage of hardened cement pastes could be very small.By further study,it is found that when the relative humidity inside the hardened cement pastes is located in different range of relative humidity,different linear relationship between the autogenous shrinkage rate and moisture content could be emerged.Therefore,the mechanism of the autogenous shrinkage of hardened cement pastes with low w/c may be different when subject to different internal humidity condition.