Strength Optimization of Novel Binder Containing Plasterboard Gypsum Waste

INTRODUCTION

Gypsum plasters are fast setting and are used mostly as plasterboard for aesthetic works, rapid construction, or fireretarding purposes for indoor applications. The majority of plasterboard waste has traditionally been landfilled, as it was classified as a nonhazardous inert waste able to be codisposed of with other wastes. In July 2005, however, the EU Landfill Directive required that plasterboard and other waste gypsum products be reclassified as hazardous noninert wastes. Detailed statistics on waste plasterboard arising are currently scarce, but it is estimated that some 300,000 tonnes (330,693 tons) of waste plasterboard are generated each year in the UK from new construction activity (largely as offcuts). The amount of plasterboard waste arising from demolition projects is more difficult to quantify, but maybe in the range 500,000 tonnes (551,155 tons) to more than 1 million tonnes (1,102,311 tons) annually in the UK (Mtpa).1

The application of cementitious materials with lower strengths than portland cement is not limited to backfills. They can be used in many applications such as structural fills, insulation, and isolation fills, pavement bases, conduit bedding, erosion control, void filling, nuclear facilities, and bridge reclamation.2

Various research workers have investigated cementitious systems based on gypsum from different sources. Investigations on the mixture of phosphogypsum anhydrite and blastfurnace slag (BFS)3 reported that a mixture of 24 to 70% of BFS with admixtures of Ca(OH)2, Na[sub]2[/sub]SO[sub]4[/sub], and FeSO4 reached mechanical strengths of approximately 23 MPa (3.33 ksi) after 28 days of standard curing. The incorporation of metakaolin in gypsum plasters has also been studied.4 The replacement of 5 and 10% of the plaster resulted in higher or similar strengths relative to pure gypsum. The work of Martinez-Aguilar et al.5 reported that a mixture of fluor-gypsum-ordinary portland cement (OPC)-geothermal waste showed strengths similar to neat OPC pastes. BFS and metakaolin are commonly used cementitious materials that require activating agents to undergo hydration reactions. When mixed with OPC, they react with the Ca(OH)2 that results from the OPC hydration; they can also be activated by chemicals such as Na[sub]2[/sub]SO[sub]4[/sub] and CaSO[sub]4[/sub]. The resulting cementitious products are calcium silicate hydrate (C-S-H); ettringite and other compounds such as aluminate-ferrite phases can be formed.6-8

Sulfate activation of slag, such as occurs in super-sulfated cement (SSC), represents another interesting variant for achievement of environmentally relevant objectives. By avoiding clinker burning, the emission of CO2 will be clearly reduced by using SSC.9 Furthermore, other sources of industrial wastes can be incorporated as activators to make sulfatealkaline activated slag in this type of binder. Cement by-pass dust (BPD) is a by-product of cement manufacturing and therefore is considered an industrial waste. Approximately 15 million tonnes (16,534,669 tons) of BPD are produced annually by the American cement industry.10 Previous studies conducted on BPD (also known as cement kiln dust [CKD]) indicated the potential use of this material in many applications such as pozzolanic base stabilization,11,12 a filler in asphalt concrete mixtures,13 wet soil conditioning, and waste stabilization and solidification.14

RESEARCH SIGNIFICANCE

The increasing importance of the environmental impact of building materials production makes it necessary to develop reasonable alternatives in the production of OPC. The amount of waste produced by construction and demolition activities has been growing greatly all around the world. Recycling and using this kind of waste as alternative construction materials appears as an interesting choice to minimize the problems caused by rising amounts of construction and industrial wastes. The objective of this paper is to provide information for the optimization of the compressive strength in composite binders containing cement by-pass dust (BPD), plasterboard gypsum waste (PG), and basic oxygen slag waste (BOS).

EXPERIMENTAL INVESTIGATION

The binary and ternary combination of BOS, PG, and BPD were prepared in which the water content was fixed at a water-to-solid ratio of 0.3. Results of the flow test (ASTM C 109) of studied pastes varied from 88 to 185 mm (3.464 to 7.283 in.) showing that both material proportions and their combinations affect the water demand of mixtures. Pastes were cast in cubes of 50 mm (1.968 in.) per side and mechanically fully compacted in three layers. After casting, molds were covered with a plastic sheet to prevent water loss and maintained in a laboratory environment for 24 hours. At this age, specimens were demolded and cured at 20 ± 1 °C (33.8 °F) and 98% relative humidity (RH) until the test time.

At 3, 7, and 28 days, the compressive strength was determined on 50 mm (1.968 in.) cubes. The data reported represent the average values obtained from three compressive tests at each age.

Materials

Crushed PG from a plasterboard recycling plant was obtained in the form of a mixture of medium-sized pieces/powder and paper. It required sieving and grinding and was conditioned to form a powder with particle size distribution shown in Fig. 1. BOS was ball milled and particle size analysis indicated the mean particle size was 500 micron (Fig. 1). Cement BPD from a local cement works was used. This had a maximum particle size of 200 micron (Fig. 1). The chemical composition of the starting materials is presented in Table 1. It should be mentioned that no portland cement was used in any of the mixtures.

Experimental plan

In this study, the experimental plan for determining the proportioning of pastes used was designed to optimize the mixture ingredients to achieve the highest compressive strength. To include the chemical interaction of each starting material in binary systems leading to the highest compressive strength in the ternary combinations, it was appropriated to test the possible binary combinations against compressive strength at the first stage. Then, at the second stage, ternary combinations of raw materials were considered by choosing the optimum proportions obtained in the first stage corresponding to the highest compressive strength in each binary group of materials. The location of points representing the mixture proportions evaluated in this study is shown in Fig. 2. The resulting paste mixture proportions are presented in Tables 2 to 4. In Tables 2 and 3, the sequence of materials notation corresponds to the order of optimization of binary and ternary systems. As the optimization of binary system of PG-BPD in Table 4 resulted in no optimum compressive strength, the optimization of ternary system at second stage was not considered for this particular binary system.

TEST RESULTS AND DISCUSSION

The experimental results of compressive strength of each group of mixtures are reported in Tables 5 to 7. The strength development of paste mixtures using a range of BPD and BOS with the same water content indicated that Mixture BPD40/BOS60 (40% of BPD and 60% of BOS) showed the highest strength at 7 days; however, the optimum compressive strength at 3 days corresponded to Mixture BPD60/BOS40 (Fig. 3). This could be because the rapid-reacting cementitious components of the BPD used caused higher early-strength gain. At 28 days, the highest compressive strength was still achieved by Mixture BPD40/BOS60, showing that the longterm strength development of the binary system of BOS-BPD is related to the BOS content in the mixture. In contrast, in Fig. 3, it can be observed that the higher BPD content above the threshold of 40% in the mixture resulted in lower compressive strength. The addition of PG into the binary system led to slightly less compressive strength corresponding to an optimum percentage of PG (Fig. 4). From Table 5, it can be seen that the highest strength at 3, 7, and 28 days belongs to the mixture containing 10% of PG. Figure 4 shows that the strength decreases for ternary composite binder containing more than 10% PG and there are not significant changes for ternary mixtures containing 20 and 30% PG after 28 days. Therefore, it can be observed that Mixture PG10/BPD36/BOS54 presents the optimum strength in this order of optimization. Strength development of PG-BPD-BOS mixtures (Fig. 5) also confirmed the greater strength gain for the mixture containing 10% PG from 7 to 28 days.

For binary compositions of PG-BOS, Table 6 shows that the mixture of 20% PG and 80% BOS achieved the highest strength at all test ages. The catalyst effect of PG in the binary combination of PG-BOS was considerable-it reduced the amount of BOS from 90 to 80%, increased the PG content by 10%, and resulted in an increase in strength of 16%. Glasser15 stated that in the combination of gypsum and BOS, sulfate such as gypsum, hemi-hydrate, and phosphogypsum performs as an activator and forms crystalline phases of C-S-H, AFt, and Al(OH)3. Also, the sulfur in slag acts to some extent as an autoactivator to trigger the pozzolanic reactions. Figure 6 depicts the optimum percentage of 20% for PG in the binary combination and the dramatic decrease in strength by increasing PG from 20 to 60%. The effect of BPD in the ternary system shown in Fig. 7 is quite remarkable. It can be seen that the compressive strength reduced by up to 50% as a result of increasing the BPD content from 5 to 20%. Then the strength was slightly improved by increasing the BPD content from 20 to 30% and then remained the same in mixtures containing more than 30% BPD. At 28 days, the maximum compressive strength shown in Fig. 7 was obtained for the combination of 5% BPD, 19% PG, and 76% BOS. The maximum value obtained is 10.5 MPa (1.52 ksi) and it is higher than the highest compressive strength achieved for the combination of PG10/BPD36/BOS54 in which PG was added to the optimum proportion of the binary system of BPD-BOS. Strength development of PG-BPD-BOS mixtures are also shown in Fig. 8. It can be seen that the increase of BPD content had no significant effect on compressive strengths at 3 and 7 days; however, at 28 days, mixtures with less BPD content showed greater compressive strength. Also, the rate of strength gain in mixtures incorporating 5% BPD was greater than mixtures containing more BPD.

Results obtained from the binary combination of PG-BPD are shown in Table 7. Figure 9 depicts the effect of BPD content on strength of pastes made with PG and BPD range from 40 to 80%. The compressive strength development shows a similar trend at all tests ages. It can be observed that the strength increases when the BPD content increases and the highest value (12.55 MPa [1.82 ksi]) was obtained from the binary mixture containing 80% BPD and 20% PG at 28 days. The rate of strength gain was also greater in mixtures containing more BPD (Fig. 10). This is due to the cementitious properties of BPD that act as the main binder in this combination. BPD is not considered to be a pozzolanic material, therefore, no further reaction between gypsum and BPD took place and no pozzolanic interaction was established. As a result, no optimum proportion can be obtained from PG-BPD mixtures to incorporate BOS at the second stage.

Figures 11 and 12 illustrate the isoresponse curves for compressive strength of ternary systems of PG-BPD-BOS and BPD-PG-BOS, respectively. It should be noted that in these figures, the remaining percentage of the mixture is the BPD percentage. By analyzing the place where iso-lines cross the coordinate axes (x and y), the contribution of each material (BOS or PG, respectively) on compressive strength can be easily observed. For instance, starting from a 10% PG content point to a maximum PG content used, that is, 30% in a PG-BPD-BOS ternary system (Fig. 11(a) and (b)), it can be observed that incorporation of PG up to 30% produces an increase of compressive strength. At 28 days (Fig. 11(c)), iso-lines cross the y axis from 0 to approximately 6%, indicating a similar strength level compared with those obtained for a high percentage of BOS, but higher replacement levels produce lower compressive strength values.

When the effect of BOS content is analyzed running from 10 to 55% on the x axis, curves show that compressive strength increases when BOS content increases at all test ages. It can be noticed that at 28 days, the maximum compressive strength corresponds to the mixture with 54% BOS and 10% PG. This is in the line with generally accepted behavior of pozzolanic materials as BOS is the main reacting part of the mixture. The role of the PG and BPD in ternary systems of PG-BPD-BOS is to maintain the pH of a pore solution within values between 11.7 and 12.0 to favor the pozzolanic reaction of slag.16

The development of compressive strength of the BPD-PGBOS system (Fig. 12) shows a similar trend to PG-BPD-BOS composition. The parabolic iso-response curve depicts a maximum of compressive strength up to 28 days. At early ages (Fig. 12(a) and (b)), this is obtained for PG contents of up to 10% and BOS contents of up to 40%. At 28 days (Fig. 12(c)), the contour of iso-response curves changes markedly and the maximum compressive strength corresponds to pastes with large percentage of BOS and a low percentage of PG (<20%). As seen in Table 6, the maximum compressive strength corresponding to an optimum percentage of PG and BOS was obtained for the combination of 5% BPD, 19% PG, and 76% BOS, which is different from the one obtained in the ternary system of PG-BPD-BOS. However, the zone delimited by relatively higher strengths of 8.5 to 10.5 MPa (1.23 to 1.52 ksi), iso-lines show that the same strength level can be achieved by several PG/BOS combinations. The maximum value obtained is 10.5 MPa (1.52 ksi) and it is located in the small iso-response zone including up to 22% PG and more than 75% BOS.

The effect of BPD content on the compressive strength in the binary and ternary combinations of PG, BPD, and BOS in Fig. 3, 7, and 9 is remarkable. Figure 3 shows that the BPD content has a great influence on compressive strength of binary combinations of BPD-BOS pastes. It can be seen that an increase of BPD content up to 40% leads to an increase of the strength from 3.58 to 9.5 MPa (0.52 to 1.38 ksi) after 28 days of curing. In contrast, compressive strength showed a decrease of 32.6% when BPD content increased from 40 to 90%. In general, slag is typically hydrated after mixing with portland cement or other alkali materials such BPD providing a source of alkalinity with which the slag reacts to form cement hydration products.17 The slag surface is first covered by hydration products from portland cement or BPD, then attacked by Ca^sup 2+^ ions from the supersaturated solution, producing the inner hydrate. The dissolution of Ca^sup 2+^ and Al^sup 3+^ ions from slag leaves a skeleton hydrated layer, which transform gradually into inner hydrate by the supply of Ca^sup 2+^. The skeleton hydrate is a porous solid containing equant grains of Type III C-S-H.18 In the presence of BPD or portland cement, various crystalline phases of C-S-H, AFm, AFt, hydrogarnet, and hydrotalcite-like phase can be formed as a main component responsible for early and long-term strength. The excessive amount of alkali in the system has a detrimental effect on the hydration of alkali-activated slag causing a delay of setting and low strength. The precise causes of this behavior have not been clarified. The formation of monosulfate due to instability of ettringite at high pH was suggested to be the main reason.19

In the ternary system of BPD-PG-BOS (Fig. 7), it can be observed that the highest compressive strength corresponds to the mixture containing 5% BPD at 28 days and then an increase of BPD from 5 to 20% resulted in a decrease of 44.8% in strength. As shown in Fig. 7, there are not significant changes for mixtures when the percentage of BPD increases up to 50%. Hydration, initial setting and hardening of ternary binder of BPD, PG, and BOS are associated with the formation of calcium sulphoaluminate. The final setting and strength gaining of these kinds of cements are attributed to ettringite, hydrated alumina together with a hydrated calcium silicate.18 The formation of monosulfate and instability of ettringite at a pH of the pore solution was suggested as the cause of this phenomenon.19 Matschei et al.,9 however, proposed that the increased alkali content is associated with a high sulfate concentration in the pore solution from the beginning of hydration. Due to the high sulfate content, a preferred precipitation and growth of ettringite crystals on the surface of slag grains led to a higher content of capillary pores, which were not filled with C-S-H and hindered strength development. This hydration behavior in conjunction with a spatial isolation of the slag grains due to rapid carbonation and recrystalization of C-S-H to calcium carbonate modifications led to observed strength loss.20

With respect to the effect of gypsum on compressive strength, as stated by Matschei et al.,9 the early compressive strength of supersulfated cements was enhanced by sulfate activation of the slag in comparison with slag mixtures without any addition of calcium sulfate. This phenomenon can be observed in current results by comparing the binary and ternary combinations of BPD-BOS and PG-BPD-BOS (Tables 5 and 6). It can be seen that the compressive strength obtained from Mixture BPD5/PG19/BOS76 was higher than binary Mixture BPD10/BOS90, which contained a higher level of slag.

In the binary compositions of PG-BPD, it can be observed that no optimum percentage for PG was found as the strength showed a marked increase, when BPD content increases from 40 to 90%. This confirms the cementitious properties of BPD used in this investigation as a by-product of cement manufactures. This also implies the absence of any pozzolanic or enhancing interaction between PG and BPD to produce a stronger cementitious matrix. In addition, due to a continuous decrease in strength with an increase of BPD content, it can be postulated that PG showed no beneficial effect such as filler effect on the matrix of hardened binder to enhance the pore structure of paste and increase the strength or the hydration rate. In other words, an increase in PG content results in lower compressive strength due to reduction of potential cementitious materials in the mixture known as dilution effect.

Figures 13 and 14 summarize the optimum percentage of BOS and PG needed to obtain the highest compressive strength at different ages. At early ages for the BPD-PG-BOS system, the maximum strength is achieved by a low percentage of BOS and PG, whereas at 28 days, the BOS and PG content should be increased. In PG-BPD-BOS, there are no significant changes in BOS content at all ages; however, the highest compressive strength was obtained with relatively lower PG content at 28 days. Figures 13 and 14 show similar patterns for changes in optimum BOS, and PG content changes the PD-PG-BOS system, whereas these patterns in the PG-BPDBOS system are different. It can be observed that the optimum percentage of BOS and PG is affected not only by the type and characteristics of materials used, but also by the order of optimization in which binary and ternary mixtures were designed to optimize the combined material proportions. These results indicate that an addition of PG into the optimized mixture of BPD-BOS results in the highest compressive strength that is different from where BPD is added into the optimum combination of PG-BOS. In other words, the interaction of BOS with any sulfate or alkali activator plays a significant role in the final result of the optimization process. This also implies that the optimization of a ternary combination of pozzolanic materials such as BOS in the presence of various activators is not a linear task; therefore, a proper experimental plan should be designed for indicating the location of the point representing the optimum combination of materials.

CONCLUSIONS

For binary and ternary systems of BPD, PG, and BOS studied, containing various percentages of BPD, crushed PG, and BOS, the following conclusions can be drawn:

1. Crushed PG can be used as a source of sulfate together with slag and BPD to form a novel sulfate-activated pozzolan binder. At all ages, there is a ternary blend of BPD, PG, and BOS that present an optimum compressive strength. It is attributable to the activating effect of BPD and PG on hydration of BOS. These mixtures make a valuable engineering cementitious material without any portland cement;

2. It was observed that the optimum percentage of BOS and PG is affected not only by the type and characteristics of materials used, but also by the order of optimization in which the binary and ternary mixtures were designed to optimize the material proportions. In the ternary system of PG-BPD-BOS, the highest compressive strength corresponds to Mixture PG10/BPD36/BOS54, whereas in the BPD-PG-BOS system, the highest strength obtained was for Mixture BPD5/PG19/BOS76;

3. The BPD content has a great influence on compressive strength of binary and ternary combinations of BPD, PG, and BOS pastes. Results showed a delay of setting and a decrease of compressive strength due to excessive dosage of BPD. The precise cause of this behavior has not been clarified. However, the formation of monosulfate or spatial isolation of the slag grains, leading to a higher content of capillary pores, was assumed to be the main reason;

4. In the binary compositions of PG-BPD, it can be observed that no optimum percentage for PG was found as the strength showed a marked increase when the BPD content increased from 40 to 90%. This implies the absence of any pozzolanic or enhancing interaction between PG and BPD to produce a stronger cementitious matrix; and

5. The iso-response method highlights the significant effect of experimental plan to optimize the mixture proportions and contribution of each material and their interactions on compressive strength.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support of the Waste and Resources Action Programme (WRAP) and the contractor, Skanska. Materials were kindly provided by Lafarge plasterboard, Castle Cement, and Corus Limited.