Shrinkage And Creep Properties Of Alkali-activated Slag Material And Its Calculation Method

Alkali-activated slag material has the characteristics of relatively high compressive strength,relatively good high-temperature resistance,relatively strong chemical corrosion resistance and freeze-thaw resistance.It has application prospects in salt lake area,freeze-thaw environment,chemical workshop and high-temperature workshop.The shrinkage mechanism and law of alkali-activated slag materials during hardening are not clear;The alkali-activated slag concrete formed by adding coarse and fine aggregates to the alkali-activated slag paste can effectively reduce the shrinkage,but the quantitative law needs to be further investigated;Under the action of load,alkali-activated slag concrete will creep,but it is not clear what the development law of creep is similar to that of cement concrete.Therefore,it is of great theoretical and practical significance to study the shrinkage and creep properties of alkali-activated slag material and its calculation methods in this paper.In this paper,sodium silicate（Na₂O·n Si O₂）solution was used as an activator to carry out the following research work:（1）In order to investigate the influence of different mix proportion of activator on autogenous shrinkage and drying shrinkage during the hardening process of alkali-activated slag paste,the water cement ratio（the mass ratio of water in the activator used for preparing cementitious materials to the slag used for preparing cementitious materials）was carried out between 0.3 and 0.48 The modulus（n in the activator solute Na₂O·n Si O₂ used for the preparation of cementitious materials）is between 0.6 and1.6,and the alkali dosage（the mass ratio of Na₂O·n Si O₂ and slag in the activator solution used for the preparation of alkali activated cementitious materials）is between 5%and 30%,autogenous shrinkage of alkali-activated slag paste with18 mix proportions specimen（100mm×100mm×515mm）within 72h and the drying shrinkage test of 90d after the removal of the formwork on this basis.The test results show that the autogenous shrinkage of alkali-activated slag paste within 72h increases first and then decreases with the increase of water cement ratio,modulus and alkali dosage.The autogenous shrinkage reaches the peak when the water cement ratio is0.35,modulus is 1.2 and alkali dosage is 20%.The silicon content of hydrated calcium silicate from slag hydration product is higher than that of cement hydration product,and the hydrophilicity of hydrated calcium silicate decreases with the increase of its silicon content,so the drying shrinkage of alkali-activated slag is larger due to the loss of water.When the water cement ratio is between 0.3～0.48 and the modulus is between 0.6～1.6,the drying shrinkage of alkali slag paste increases with the increase of water cement ratio and modulus.（2）The alkali-activated slag concrete is formed by adding an appropriate amount of sand and stone into the alkali-activated slag paste.Since the sand and gravel do not shrink,only the alkali-activated slag paste shrinks during the hardening process,adding sand and gravel to the alkali slag slurry will reduce the shrinkage.Adding fiber into the alkali-activated slag concrete will share some of the tensile stress in the shrinkage process of the concrete and also reduce the shrinkage.In order to reveal the shrinkage law of alkali-activated slag concrete under the environment of 20℃and60%relative humidity when adding aggregate and fiber at the same time,taking basalt fiber or PP fiber as the added fiber,the shrinkage test of alkali-activated slag concrete was carried out with 11 mix proportions of aggregate volume content（sand volume percentage of concrete volume）,fiber volume ratio between 0.3%～0.9%,and fiber length between 6mm and 18mm.The results show that most of the hydration products of slag are gel phase,and the gel phase is more easily deformed and coordinated when the shrinkage is restrained by the aggregate.When the volume content of aggregate is 78.5%,the shrinkage of alkali-activated slag concrete with fiber decreases first and then increases with the increase of fiber volume ratio and fiber length.Too high fiber volume ratio and too long fiber will increase the holes in concrete and reduce the inhibition of fiber on concrete shrinkage.According to the test results,the volume fraction of basalt fiber or PP fiber with better shrinkage inhibition effect on alkali-activated slag concrete is 0.6%,and the fiber length is 12mm.（3）In order to study the effect of adding fiber on the mechanical properties of alkali-activated slag concrete,the mechanical properties tests of alkali-activated slag concrete with 11 mix proportions were carried out with basalt fiber and PP fiber as added fibers.The results show that basalt fiber has higher elastic modulus and higher tensile strength than PP fiber,and adding basalt fiber has better effect on improving the mechanical properties of alkali-activated slag concrete under the same fiber volume ratio and fiber length.When the fiber volume ratio is 0.6%and the fiber length is 12mm,the cube（side length 100mm）compressive strength and prismatic compressive strength of alkali-activated slag concrete mixed with basalt fiber are 10.5%and 8.9%higher than that of alkali-activated slag concrete,respectively;When the fiber volume ratio is 0.9%and the fiber length is 12mm,the splitting tensile strength of alkali-activated slag concrete mixed with basalt fiber is 38.4%higher than that of alkali-activated slag concrete.The calculation method for determining the optimal fiber volume fraction and fiber length in alkali-activated slag concrete is established.（4）When the age of alkali-activated slag concrete is 28d and the axial compressive strength is 43.23MPa,the prism（100mm×100mm×400mm）compression creep test.The test results show that when the compressive stress level is between 0.15 and 0.6,the creep coefficient of alkali-activated slag concrete is basically the same.In view of the shortcomings of ACI 209R-92,B3,CEB-FIP 2010and GL2000 in predicting the compressive creep of alkali-activated slag concrete,two methods for predicting the compressive creep of alkali-activated slag concrete are proposed:the secant modulus determined according to the stress-strain relationship replaces the elastic modulus in the existing model,and a prediction model for the compressive creep of alkali-activated slag concrete is proposed;Considering the characteristics of slag hydration products with less crystal composition and more gel phase composition,the shape of the time function in the compression creep coefficient is adjusted reasonably,and the prediction model for the compression creep of alkali-activated slag concrete is obtained by fitting.The modulus of alkali-activated slag concrete is lower than that of cement concrete.When the compression creep coefficient is about the same as that of cement concrete,the compression creep of alkali-activated slag concrete is relatively large.（5）In order to study the effect of adding fiber on the compressive creep of a alkali-activated slag concrete,the compressive creep tests of alkali-activated slag concrete with 10 mix proportions were carried out with basalt fiber and PP fiber as the added fiber.The fiber volume ratio is between 0.3%～0.9%,the fiber length is between 6mm～18mm,and the compressive stress level is between 0.15～0.6.The results show that the anti creep effect of fiber becomes stronger and then weaker with the increase of fiber volume ratio;When the compressive stress level is between 0.15and 0.6,the inhibition effect of fiber on creep increases with the increase of compressive stress level.When the compressive stress level is relatively high,at large fiber volume ratio（0.9%）and long fiber length（18mm）,the inhibition effect of fiber on concrete creep will be offset or more than offset by the internal hole cracking.In this paper,the shrinkage mechanism of alkali-activated slag cementitious material in the hardening process is studied;The quantitative shrinkage law of alkali-activated slag cementitious material was investigated;The difference of creep performance between alkali-activated slag concrete and cement concrete is revealed;The creep development law of alkali-activated slag concrete is quantified.