Effect of Ca(OH)_2 and CaSO_4 2H_2O on the Strength of Po tland-Fly Ash Cement Pastes Wang Shanba R.L.BergerThe effect of Ca(OH)_2 and CaSO_4·2H_2O added to the Portland-fly ash cement pastes on the strength was studied by strength test combining.with QXDA, thermal analysis and SME as well as TMS method,The results were as fo- llows; The addition of Ca(OH)_2 to Portland-fly ash cement decreased the stre- ngth at early age.Althongh it increased slightly the reaction degree of fly ash,the degree of reaction was so low(7-12%)(depending upon the amounts of Ca(OH)_2 added to the pastes) that the strengths at 90-deys age were little lower than that of Portland-fly ash cement paste without Ca(OH)_2 addition except the paste with 5% Ca(OH)_2 addition. The addition of CaSO_4·2H_2O also reduced the strength of Portland-fly ash cement at early age,but the strength of the pastes with 2% and 4% addi- tion of CaSO_4·2H_2O could catch up with that of Portland-fly ash cement without the addition of CaSO_4·2H_2O at 14 to 28 days,and the strengths of all pastes with CaSO_4·2H_2O addition surpassed Portland-fly ash cement at 90 days.CaSO_4·2H_2O addition was consumed up at 28 days to form ettringite in the capillaries and to dense the structure of the pastes,so that the strength was increased at later age. The strengths of Portland-fly ash cement pastes,whether Ca(OH)_2 and CaSO_4·2H_2O were added or not,were lower than that of Portland cement at 90 days if wafer/solid ratio by volume was numerically equal and might ca- tch up with or surpassed Portland cement at later age in case of using equal water/solid ratio by weight.In later case,the contribution was not comple- tely stemmed from the pozzolanic reaction of fly ash and the formation of ettringite,and one of the important reasons was lower total porosity at the begining than Portland cement paste because of low specific gravities of fly ash,Ca(OH)_2 and CaSO_4·2H_2O which were added to Portland cement. TMS method test results showed that there existed only 8% monomer in the silica acid of fly ash,while a large amount was dimer and polymer.A small amount of monomer might be responsible for lower reactivity of fly ash. After addition of Ca(OH)_2 and CaSO_4·2H_2O,the monomer decreased and polymer increased in fly ash cement pastes compared with the pastes without Ca(OH)_2 and CaSO_4·2H_2O addition at 60 days age.