Compatibility Assessment of Commercial Cements with superplasticizers

. In the present study, compatibility assessment of commercially available cements (OPC and PPC) and SNF & PCE based super plasticizers are made by evaluating the workabilities, setting times and compressive strengths of M40 grade concrete mixes made with two different brands of cement and with two different chemical admixtures. The dosage of SNF and PCE based chemical admixtures to be used in M40 grade concrete is determined using marsh cone test. It was found that M40 concrete made with SNF based superplasticizer has achieved desired compressive strength than the concrete made with PCE based superplasticizer. This shows that SNF based superplasticizer is more compatible for M40 grade concrete than PCE based superplasticizer. So it is suggested that for the development of PPC based M40 grade concrete SNF superplasticizers can be used.


Introduction
Cementsuperplasticizer interaction in concrete is a complex blend of chemical and physical mechanisms that are interdependent. The complicated nature of the problem prevents the development of simple solutions to address the field related issues of application of super plasticizers. Studies on cement-water reducer interactions in India have been limited to the workability evaluation of concretes containing these chemicals, in specific regions where rapid slump loss has been observed in concreting operations. There have not been any investigations to understand the physico-chemical nature of this interaction. Thus, the results from these studies are not broad-based, i.e. they apply to a small group of cements and/or chemical admixtures. There is a distinct need for the characterization of Indian cement and admixture properties, in order to understand the nature of their interactions.

Marsh Cone Method for Determining Optimum Dosage of Admixture
Marsh cone test is dependable and simple method to study the rheological properties of cements and mortars. Flow time of cement through marsh cone is indicator of viscosity, which depends upon cement-admixture compatibility. It is widely used to study cement-admixture compatibility and to determine optimum admixture dosage of a specific cement-admixture combination. The main principle behind the test is to find the time required for a certain quantity of material (cement slurry) to flow out the cone through a particular size of aperture. The parameters flow time and fluidity are inversely proportional. As flow time increases fluidity decreases. Graphs were plotted with % admixture dosage on X axis and flow time on Y axis for the two brands of PPC cement at 0 minute, 30 minutes and 60 minutes retention. The dosage after which there is no change in flow time for 0 minute retention is noted as optimum dosage of admixture.

Workability
The slump is determined as. Per IS 1199: (1959) using standard slump cone apparatus.

Setting times of concrete
The setting times of concrete are determined as per IS 8142 (1976) using penetration resistance apparatus. Mortar passing through 4.75 mm sieve is separated from concrete this test. According to IS 8142 (1976), the initial setting time is the elapsed time, after initial contact of cement and water, required for the mortar (sieved from the concrete) to reach a penetration resistance of 3.43 N/mm 2 and final setting time is the elapsed time, after initial contact of cement and water, for the mortar (sieved from the concrete) to reach a penetration resistance of 26.97 N/mm 2

Preparation of sample for setting times of concrete
The concrete produced is sieved through a 4.75 mm sieve into a non-absorptive surface. The mortar which is passed through this sieve is remixed by hand and placed in the test container or three cube moulds of size150 mm to a depth of 140 mm in layers of 50 mm duly compacted by rodding each layer by means of tamping rod with uniformly distributed strokes. The top surface of concrete is kept 13 mm below the top of mould to remove the free water from surface. The top surface of specimen is covered and protected with a suitable tight fitting, water impermeable cover for duration of the test, except when free surface water is being removed or penetration tests are being made. The free surface water is removed with the help of a pipette

Apparatus for determining setting time of concrete
A penetration resistance apparatus of spring reaction type, graduated from 0 kgf to 60 kgf in increments of 1 kgf is used. Removable needles of 645, 323, 161, 65, 32 and 16mm 2 bearing areas are used. Each needle shank is scribed peripherally at a distance of 25mm above the bearing face. The photograph of apparatus is shown in Fig 4. A needle of appropriate size depending upon the state of hardening of the mortar is fixed to the apparatus. The bearing surface of the needle is brought into contact with the mortar surface and a vertical force is applied gradually and uniformly on the apparatus until the needle penetrates the mortar to a depth of 25 mm as indicated by the scribe mark. The force required and the times of application are recorded. Sufficient care is taken in subsequent tests to avoid area where the mortar has been disturbed by previous tests. As per IS 8142 (1976), the clear distance between two needle impressions shall be at least two diameters of the needle being used, but not less than 13 mm and the clear distance between any needle impression and the side of the container shall be not less than 25mm. Photographs taken during the test are shown in

Super plasticizer
In the present work, water-reducing admixtures conforming to IS: 9103-1999 used are (1) Conplast SP 430 is a Sulphonated Naphthalene based Formaldehyde (SNF) super plasticizer manufactured by Fosroc.

Mix Proportions
The grade of concrete used in the present investigation is standard grade (M40). The mix proportions of M40 grade concrete are designed using IS: 10262-2009. The mix proportions and materials required for one cubic meter of concrete in standard grade (M40) is given in table below:

March Cone Test Results
Saturation dosage varies with the type of the superplasticizer and cement. Flow time decreases with increase in dosage of superplasticizer. The saturation dosage of admixture can be defined as that point beyond which there is no significant decrease in the flow time. Flow time after reaching saturation dosage is either same as saturation dosage or more or less constant after saturation point. Saturation point can be taken as the maximum admixture content to be used in concrete because there after no significant change will occur in flow time. SNF based superplasticizer optimum dosage for two brands of cement varies. Brand 1 showed 0.9% and brand 2 showed 1.1% saturation dosages at 0 minute retention. Flow time and % dosage for brand 1 is lower than brand 2. With the addition of admixture beyond 1.5%, flow time has no variation. It may be due to the variation in chemical properties. From the results obtained it is observed that for both the selected cements their saturation dosage differs. Fineness of cement also plays a role in the variation of admixture dosage. As fineness increases surface area increases and resulting in more adsorption of admixture. Fineness is directly proportional to saturation dosage. As fineness increases saturation dosage also increases. Consistency, setting time etc. also depends on fineness. As fineness increases, setting time also increases. Flow time decreases as retention period increases and dosage at which flow time is lowest differ from 0 minute retention to 60 minute retention. Optimum dosage of brand 1 cement is 1.1% and that of brand 2 is 0.7% at 30 minute retention whereas optimum dosage of brand 1 cement is 0.9% and that of brand 2 is 0.7% at 60 minute retention. Similarly for PCE based the optimum dosage for Brand 1 showed 0.9% and brand 2 showed 0.7%. Due to compatibility problems before application at fields compatibility check between cement and admixture should be done. So to reduce incompatibility problems check the compatibility whenever cement and admixture are changed.

Workability
The slump is determined as. Per IS 1199: (1959) using standard slump cone apparatus. It was understood from the literature that the increased rate of slump in the case of the concrete with admixture/superplasticizer is due to the effect of the gradual absorption of the water reducing agent by the product of hydration.
In SNF based concretes made with two brands of cement, desired workability is achieved in brand 2 than in brand 1. Whereas in PCE based concretes made with two brands of cement, desired workability is not achieved in both the brands. So as far as workability required is concerned brand 2 performed better.

Setting Times of Cement Concrete
The trial mixes with mix proportions are prepared and the initial and final setting times of cement concrete are determined as per IS 8142:1976. The time interval for which the concrete remain in plastic condition is known as the setting time. The addition of superplasticizer to the concrete increases the setting times of the concrete.
Four M40 grade concrete samples were prepared with 2 brands of cement and 2 types of admixtures and tested for setting times of concrete. The setting time of concrete are determined as per IS: 8142(1976) using penetration resistance apparatus. Mortar passing through 4.75mm sieve is separated from concrete for this test According to IS 8142 (1976), the initial setting time is the elapsed time, after initial contact of cement and water, required for the motor (sieved from the concrete) to reach a penetration resistance of 3.43 N/mm 2 and the final setting time is the elapsed time, after initial contact of cement and water, required for the mortar (sieved from the concrete) to reach a penetration resistance of 26.97 N/mm 2 We tested the initial and final setting times of concretes by the trial mixes with addition of superplasticizer. The penetration resistances of concrete mixes with superplasticizer are shown in Tables 3 to 6. M40 concrete made with SNF based superplasticizer has shown better initial and final setting times of concrete than in concrete made with PCE based superplasticizer. This shows that SNF based superplasticizer is more compatible for M40 grade concrete than PCE based superplasticizer. Same observations are made in concretes made with both brands of cement but Brand 2 showed better performance in setting times of concrete.

Compressive Strength
Compressive strengths of cement concrete at ages 28 days through normal curing is determined as per IS 516(1959).The experimental results and discussion of results are presented below.

Conclusions
Based on experiments conducted on 2 types of M40 concrete made with brans 1 and brand 2 cements and admixed with SNF and PCE based superplasticizers, the following conclusions are drawn: 1. The optimum dosage of superplasticizer for SNF based is 0.9% and 0.7% for concretes made with brand 1 and brand 2 respectively 2. In SNF based concretes made with two brands of cement, desired workability is achieved in brand 2 than in brand 1. Whereas in PCE based concretes made with two brands of cement, desired workability is not achieved in both the brands. So as far as workability required is concerned brand 2 performed better. 3. M40 concrete made with SNF based superplasticizer has shown better initial and final setting times of concrete than the concrete made with PCE based superplasticizer. This shows that SNF based superplasticizer is more compatible for M40 grade concrete than PCE based superplasticizer. Same observations are made in concretes made with both brands of cement but brand 2 showed better performance in setting times of concrete. 4. M40 concrete made with SNF based superplasticizer has achieved desired compressive strength than the concrete made with PCE based superplasticizer. This shows that SNF based superplasticizer is more compatible for M40 grade concrete than PCE based superplasticizer. Same observations are made in concretes made with both brands of cement but brand 2 showed better performance in achieving strength of concrete. 5. So based on achievement of desired setting times and compressive strengths, it can be concluded that brand 2 cement based M40 concrete (ie, PPC based) is more compatible with SNF based superplasticizer than PCE based superplasticizer. So it is suggested that for the development of PPC based M40 grade concrete SNF superplasticizers can be used. 6. The mix proportion is approximately same in all the mixes even though the workability, compressive strengths and setting times of cement concretes depending on the ingredients of the cements.