advance cementitious material

Advanced cementitious materials

Dr.V.V.Deshmukh

Introduction :

Recent years have seen the development of many new advances in cementitious materials as ceramic like materials formed as the result of chemical reactions occurring at or near ambient temperatures. These new advances have occurred as a result of manipulating the microstructure and controlling the chemistry or both, of cements. These advances have led to the development of new families of high performance cementitious materials including very high strength materials. Some of these materials cross the boundaries of what has been defined as traditional cementitious materials, and the term chemically bonded ceramics has been used to classify these new materials. CBCs are defined at or near ambient temperatures. These new novel cements or CBCs follow the general rules of behavior of cementitious materials in certain respect The strength of hardened paste increases as the ratio of water to cement is reduced. Because the residual porosity, its distribution and the excess uncombined molecular water are responsible for most of the limitations on the properties of conventional hardened cement paste. Many attempts have been made to reduce the amount of water used in processing. The situation has changed beginning in about 1970 as new approaches have led to the development of more advanced cement matrix composites.


1) Densification by pressure and heat


The bonds limiting the strength of cement paste are normally thought to be weak van der waals forces Before 1970,the potential strength of cement paste at theoretical density had never been approached because considerable porosity (20 to 30% or more total porosity) always remain after complete hydration of cement. Following this research resulted in achieving very high strengths by warm pressing. Compressive strengths up to 650 Mpa (compared to more typical 30 Mpa) tensile strength up to 68 Mpa and values of youngs modulus up to 40 Gpa were attained. Enormous increases in strength resulted from the removal of most of the porosity and the generation of very homogenous fine microstructure with porosity's as low as 2%.


2) Micro defect free cement.

The warm pressed cements discussed in the previous section were successful but not easy to produce in large amounts due to the high pressure used. The next step was to develop more easily processed materials. .Another innovation was the engineering of a new class of high strength materials, the MDF cement. MDF refers to the absence of relatively large voids or defects which are usually present in conventionally mixed cement paste because of entrapped air and inadequate dispersion. In the MDF process 4 to 7% of one of several water soluble polymer is added as a rheological aid to permit cement to be mixed with very small amount of water., subsequent high shear mixing produces a plastic cohesive mixture which can be shaped by extrusion or other forming technique and which sets in times ranging from minutes to hours. The highest strength materials have been prepared with calcium aluminate cement. Control of the particle size distribution for optimum particle packing was also considered important for generating strength. A final processing stage in which entrapped air is removed by applying modest pressure or heating at 80 0C resulted in a paste that is free of large defects. with excellent mechanical properties. Very low porosity's were achieved <1% as well as flexural strength of 150 Mpa. compressive strength of 300 Mpa and a young’s modulus of 50 Gpa. When MDF material is exposed to moisture, the polymer phase which MDF cement where constitute 30% on a volume basis, swells and softens. This property of MDF cement has restricted the use of its in application where water is used.


3) DSP and other densely packed systems

An important class of new materials termed DSP (Densified systems containing homogeneously arranged ultrafine particles) was first elucidated in detail by Bache The new class of materials is defined as materials with matrix comprising of 1) Densely packed particles of a size ranging from 0.5 to 100micron usually cement 2) Homogeneously arranged ultra-fine particles ranging in size from 50 A0 to 0.5 microns usually silica fume, arranged in the spaces between the larger particles. The combination of densely packed silicafume and cement was found to benefit for a combination of reasons


1) The silica particles are smaller than even the finest cement produced by grinding and therefore pack more easily into the spaces between the cement particles.

2) The silica particles are spherical in shape

3) The particles are chemically less reactive than cement, which eliminates the problem of too rapid hardening encountered with very fine cement

4) Finally with added dispersing agents a low water requirement may be achieved.

Numerous investigations have contributed to the understanding of the effects of fine particles in densely packed cementitious materials. With 15% silica fume replacement of cement thwere are 2000000 particles of silica fume for each grain of portland cement in a concrete mixture. Concrete containing 5 to 15% silica fume have high compressive strengths up to 100 Mpa flexural strength up to 12 Mpa and young’s moduli (up to 34 Gpa)and have very low permeability to water (10-9 um).The microstructure of the critical interfacial zone between cement paste and the aggregates in concrete is more dense and uniform than when conventional pastes are used and the bond between paste and other embedded materials such as aggregates and fibers appears to be improved. The most striking results have been found with silica fume substituted pastes and DSP systems. Compressive strengths of up to 270 Mpa or higher with young’s moduli up to 80 Gpa were achieved in preparations with up to 20 to 25% silicafume at a water to solid ratio of 0.12 to 0.22 through mechanical compaction. Such materials are used to resist mechanical erosion in impeller screws for moving coal and fly ash and in flooring to industrial area. This material retains a compressive strengths of 300 Mpa up to about 500 0c and 200 Mpa at about 700 0C.



Silica fume hydration reactions.

Properly dispersed silica fume particles when used in propertions to replace up to 10 % of cement significantly reduces the bleeding and segregation of the mixtures and may be used in higher proportions. Silica fume contains particles as fine as 0.1 microns or less which partially dissolve in saturated Ca(OH)2 solution in a time as early as 5 to 15 minutes, and a SiO2 rich hydrates is deposited in layers or films on the silica fume particles. Despite the early rapid reaction much silica fume is remained for later slow reaction. The fume particles play an important role in various composites when they surround each cement grain, densifying the matrix, filling the voids with the strong hydration products and improve the bonding with aggregates, and reinforcing materials such as fibers. Silica fume by reacting with alkali also affords a protection against the alkali aggregate type reaction occurring between a cement pore solution and glass fiber.


Particle packing in concrete.


The Toufar/Aim model of dry particle has been verified as adequately modeling the dry packing of mixers of powder, each with a different size distribution. Typically in the model ,materials with three different size distributions may be mixed .Furthermore, the characterization of the size distribution can be modeled by a commonly used procedure describe by Rosin-Rammler. Input to this PC-based algorithm consists of the experimentally determined tap density of each component and the characteristic diameter of the distribution as described by D in Rosin-Rammler fitting equation. The results of applying this algorithm to concrete systems have provided the mathematical basis for formulating concrete mixtures which were developed through field experience in the concrete placement. Its applications should prove useful in monitoring the quality of concrete in the design stages and to maximize performance.


Discussion and summary


The particle packing and hydration reactions in DSP cement pastes are responsible for the fine microstructural development. These complex reactions involve phase solubility ,accelerating and retarding effects of a multiphase, multiparticle size distribution material, and surface effects at the solid-liquid interface. This particle packing combined with chemical reaction is extremely important for developing strength. The initial degree of dispersion of cement and fume in the paste strongly influences the development of the final hardened paste microstructure. The ultra fine particles can fill the intergranular interstices and produce a dense paste structure. Reflected in a high strength. Superplasticizers should be used to minimize the water demand and adequately disperse the fine particles, resulting in dense products with fine pore size, very low permeability and low ionic diffusivity. Despite the rapid early hydration, much silica fume remains unreacted until a later stage. Physical and chemical characteristics together influence the hydration kinetics. Silica fume ordinaraly accelerates the early portland cement hydrtion, largely because of its very high surface area, increasing the heat development and resembling a high early strength cement. Fume also disperses the hydration product, provides for deposition of C-S-H and there by fills the pore interstices with fine hydration products. The mechanical properties of some highstrength DRC -type materials have been summarised in table given below