Abstract
During a digital fabrication of cement based materials,each layer should support the self-weight of the upper layer,where the shape stability,workability,and thixotropy are important factors for successful fabrication[1].
The cement based materials for a digital fabrication mainly consist of water,cement binder,and fine aggregates,which equals mortar,and fine additions such as nano-silica and microsilica are additionally used to control the shape stability,workability and thixotropy of a mortar.Nano-silica and micro-silica increase the water requirement and decrease workability due to their higher specific surface area[2,3].
The effect of fine additions on the performance of mortar for a digital fabrication is usually evaluated based on rheological properties,which are measured by shear test using standard rheometer[4,5].Compressive method,which is called squeeze flow test,can be used for performance evaluation[6].Squeeze flow test measures the resistance of samples to the compressive loading.This test shows load-displacement curves,which are generally classified into three stages:elastic,plastic,and hardening stages[7].If the applied load is larger than yielding point,the sample starts to have cracks on their surface and fails to serve properly as a supporting material.Yielding point exists between the elastic stage and plastic(or hardening)stage,which is,together with stiffness,a critical property to describe the shape stability of mortar.
In this study,the effect of nano-silica and micro-silica on the shape stability of mortar was analyzed through a squeeze flow test.Also,their workability was measured by flow table test[8].The behavior of mortar over time(9,30,60,90,and 120 minutes after mixing start)were compared to examine the effect of thixotropy.
TypeⅠPortland cement was used to produce mortar samples.A standard sand specified in ISO 679[9]was used as fine aggregates.Water to cement ratio is 0.5 and fine aggregates to cement ratio is 3,which is specified mix proportion in ISO 679.
Colloidal silica was used as nanomaterials,where its average particle size is 80-90 nm and solid content is 0.41.Colloidal silica replaced 1%,2%,3%and 4% of cement by weight,respectively.Micro-silica was used to replace 10%,20% and 30% of fine aggregates by weight,respectively.Its particle size ranges from 0.1 to 0.5 mm.
Mortar samples were mixed during 5 minutes.For squeeze flow test,the same mortar samples were filled into five cylindrical molds,which has 60 mm diameter and 45 mm height.Five molded samples belong to the mortar sample over 9,30,60,90 and 120 minutes after start of mixing,respectively.Each sample is placed between parallel plates and the mold is removed,where bottom plate is fixed and the upper plate moves down applying compressive load to the sample at the rate of 1.5 N/s.
Figure 1 Load-displacement curves for mortar with 10%replacement of microsilica
Figure 1 shows the compressive loadvertical displacement curves for mortar sample where 10%of fine aggregates are replaced by micro-silica by weight.At low displacement range,load is a linear function of displacement;stiffness is constant.At intermediate displacement range,plastic behavior is observed and hardening occurs at large displacement.
The vertical displacement was set as 0 mm,when the compressive load is 3 N.The tangent point of the loaddisplacement curves passing through(3 N,0 mm)is defined as the point of elastic limit.It is assumed elastic limit serves as yielding point and represents the shape stability because the homogeneity of materials starts to break right after elastic limit.Compared to samples that have passed over time,the mortar has a higher stiffness and yield point due to thixotropy.
The workability of mortar according to the replacement of micro-silica was measured through flow table test.25 drops of compaction were applied to each mortar sample with time.As time passed,the workability of mortar decreased.
Figure 2 and Figure 3 show a trend of yielding point and 25 drops according to the replacement of micro-silica,respectively.As the portion of micro silica increases,both yielding point and workability increases.Since micro-silica induces water requirement,the connection between every particle is enhanced,which results in a higher shape stability.In terms of workability,micro-silica fills the void between relatively larger particles and the continuity between particles and homogeneity are increased,which results in a higher workability.
The replacement of nano-silica shows similar results with the replacement of micro-silica,but the ef fect on workability is relatively small.This reason seems to relate to the particle size.That is,micro sized particles contribute more to the increase of homogeneity of mortar.
Figure 2 Trend of yielding point with the replacement of micro-silica
Figure 3 Trend of 25 drops of mortar with the replacement of micro-silica