NOVEL CEMENTITIOUS COMPOSITION

Abstract

The present invention relates to the field of cementitious compositions. Particularly, the invention concerns an alkaline-activated fly ash cementitious composition and the use of this composition as a binder in concrete production.

Claims

1. A cementitious composition comprising: a binder consisting of a source of fly ash and a source of amorphous (reactive) silica; and a chemical activator, wherein the chemical activator comprises a combination of Na.sub.2CO.sub.3 and a source of calcium, wherein the source of calcium is selected from the group consisting of Ca(OH).sub.2, CaO, and mixtures thereof; and the composition is cured at ambient or elevated temperature and used for in situ or precast applications.

2. The cementitious composition of claim 1, wherein the source of amorphous (reactive) silica present in the composition is from 1 to 20 wt % of fly ash mass.

3. The cementitious composition of claim 1, wherein the Na.sub.2CO.sub.3 present in the composition is between 3 and 15 Na.sub.2O.sub.N wt % of fly ash mass.

4. The cementitious composition of claim 1, wherein the source of calcium present in the composition is between 3.5 and 20 CaO wt % of fly ash mass.

5. The cementitious composition of claim 1, wherein the composition is cured at ambient temperature in situ.

6. A method for manufacturing concrete, morter or grout, wherein the method comprises the use of the cementitious composition of claim 1.

7. A process for the preparation of a dry mixed ready-to-use cementitious composition, the process comprising the steps of: (i) providing a binder consisting of a source of fly ash and a source of amorphous (reactive) silica; (ii) providing a chemical activator comprising a combination of Na.sub.2CO.sub.3 and a source of calcium, wherein the source of calcium is selected from Ca(OH).sub.2, CaO, and a mixture thereof; and (iii) mixing the binder and chemical activator in one step.

8. A concrete, mortar or grout product or the like prepared according to the process of claim 7.

9.-12. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0035] The invention will now be described in more detail, by way of example only, with reference to the accompanying figures in which:

[0036] FIG. 1: represents a 2-level CCD design space;

[0037] FIG. 2: represents a 3-level face-centred CCD design space;

[0038] FIG. 3: is a graph of 28-day compressive strength of samples;

[0039] FIG. 4: depicts the main effect of variables;

[0040] FIG. 5: is an interaction plot between Ca(OH).sub.2 and Na.sub.2CO.sub.3;

[0041] FIG. 6: represents the interaction between Na.sub.2CO.sub.3 and silica fume;

[0042] FIG. 7: represents the interaction between silica fume and Ca(OH).sub.2;

[0043] FIG. 8: is a normal probability plot;

[0044] FIG. 9: is a graph of residuals versus fits;

[0045] FIG. 10: is a graph of residuals versus order;

[0046] FIG. 11: shows surface and contour plots of Na.sub.2CO.sub.3 and Ca(OH).sub.2 with silica fume held at a) 5 wt %, b) 10 wt % and c) 15 wt %;

[0047] FIG. 12: is X-ray diffractograms of chosen samples at 3 and 28 days;

[0048] FIG. 13: shows SEM images of chosen samples at 3 and 28 days;

[0049] FIG. 14: is a FTIR spectrum for SC12|CH20|SF5 at different curing ages;

[0050] FIG. 15: is a FTIR spectrum for SC12|CH10|SF5 at different curing ages;

[0051] FIG. 16: is a FTIR spectrum for SC6|CH20|SF5 at different curing ages;

EXAMPLES OF THE INVENTION

[0052] The invention will now be described with reference to the following non-limiting examples.

[0053] 1.1. Experimental Setup

[0054] The experimental setup dealt with performing a detailed analysis of the novel composition of the present invention by mathematical design of experiments (MDE). Central composite designs (CCD), also known as Box-Wilson designs, are highly efficient and flexible. MDE allows for modelling and analysis of the response of interest as well as providing satisfactory information on the experimental variables and their effects and error with minimal number of runs.

[0055] CCD included an embedded factorial or fractional factorial design with centre points that is augmented with a group of axial/star points that allows curvature estimation. This type of design can be used to estimate the first- and second-order terms/effects as well as model a response variable. Using a 2-level CCD example, it will contain a centre point (circle), four factorial points at its corners (the +1 and −1 coded variable levels shown by triangles) and four axial/star points (diamond) that lie a distance +α and −α on each axis. This is shown in FIG. 1.

[0056] There are three kinds of CCD (inscribed, circumscribed, faced) where each type uses a different a value. For this experimental design, a face-centred CCD (α=±1) was chosen as the region of interest encompasses the extremities and a 3.sup.k level CCD was chosen. A visual representation of the box is shown in FIG. 2.

[0057] As mentioned earlier, a 3-level CCD with 3 centre point runs was chosen for the experimental program. Table 1 states the chosen control and response variables and the constant parameters. Table 2 gives the coded values of the control variables and Table 3 states the values of the constant parameters.

TABLE-US-00001 TABLE 1 Chosen parameters for CCD Control variables Response variable Constant parameters Ca(OH).sub.2 content 28-day compressive Curing condition Na.sub.2CO.sub.3 content strength W/B ratio* Silica Fume content Mixing time *Water-to-binder ratio, where binder is sum of all dry materials (fly ash, silica fume, calcium hydroxide and sodium carbonate)

TABLE-US-00002 TABLE 2 Coded and uncoded control variables Control variables* −1 0 +1 Ca(OH).sub.2, wt % 10 15 20 Na.sub.2CO.sub.3**, wt % 6 9 12 Silica Fume, wt % 5 10 15 *wt % = calculated as a percentage of fly ash content **Calculated as Na.sub.2O.sub.eq

TABLE-US-00003 TABLE 3 Constant parameter values Parameter Value Curing condition Ambient temperature W/B ratio 0.27 Mixing time 5 minutes

[0058] The choice of the response variable is dependent on the system being studied. Mechanical strength provides a satisfactory indication of the degree of reaction of the system's final product, alkali aluminosilicate gel. For this experimentation, the average of three compressive strength tests done on different days in accordance to SANS 50196-1. Control variables calcium hydroxide, sodium carbonate and silica fume were chosen to be the significant factors to affect the strength gain.

[0059] Unlike GBFS, which has latent hydraulic properties due to its high calcium content, fly ash is shown to have pozzolanic activity and will exhibit cementitious properties in the presence of lime, hence the addition of Ca(OH).sub.2 as a control variable. However, the activation of fly ash with Ca(OH).sub.2 has low strength development. Shi & Day (2000) used different chemical activators to improve the strength development of FA-Ca(OH).sub.2 binder system. The authors incorporated Na.sub.2SO.sub.4 to increase the pH of the solution as the addition of Na.sub.2SO.sub.4 with Ca(OH).sub.2 gives the chemical reaction:

Na.sub.2SO.sub.4+Ca(OH).sub.2+2H.sub.2O.Math.CaSO.sub.4.2H.sub.2O↓+2NaOH  (2)

[0060] The formation of NaOH is responsible for increasing the pH of the solution, promoting an increase in the degree of early activation. However, the use of Na.sub.2SO.sub.4 produces durability problems such as sulfate attack; and thus Jeon et al. (2015) incorporated the use of Na.sub.2CO.sub.3. The authors found that the same pH increasing effect was present and there was a noticeable improvement in the compressive strength. Jeon et al. (2015) state the strength was approximately 4-5 times higher than samples containing no Na.sub.2CO.sub.3. Thus, to further quantify the effect, this experimentation used different amounts of Ca(OH).sub.2 and Na.sub.2CO.sub.3 to see the variability in the results.

[0061] In Jeon et al. (2015) investigation, the system was thermally activated. However, the aim of this research is to produce a binder than can be cured at ambient conditions on site. Thus, curing conditions were limited to ambient temperature of 25±1° C. and relative humidity of 85±5%.

[0062] 1.2. Materials

[0063] Fly ash, sodium carbonate, hydrated lime and silica fume were procured from local distributors in South Africa. Table 4 represents the chemical composition of fly ash and silica fume.

TABLE-US-00004 TABLE 4 Chemical composition of source materials % Fly ash Silica fume SiO.sub.2 56.23 88.86 TiO.sub.2 1.57 0.01 Al.sub.2O.sub.3 30.67 0.60 Fe.sub.2O.sub.3 4.45 4.63 MnO 0.04 0.13 MgO 0.49 1.03 CaO 4.54 1.86 Na.sub.2O 0.27 0.28 K.sub.2O 0.81 2.03 P.sub.2O.sub.5 0.24 0.08 Cr.sub.2O.sub.3 0.03 0.01 SO.sub.3 0.37 — LOI 0.28 4.58 TOTAL 99.99 99.54

[0064] 1.3. Sample Preparation and Testing Procedures

[0065] 1.3.1. Design Matrix and Sample Preparation

[0066] The binder components (FA, SF, Ca(OH).sub.2 and Na.sub.2CO.sub.3) were intermixed in a ball mill for approximately 5 minutes to limit the effect of grinding on performance of the binders.

[0067] The design matrix of the face-centred CCD with 3 centre points is shown in Table 5.

TABLE-US-00005 TABLE 5 Coded variables for CCD Design Points Run Order Mix ID SC CH SF 1 2 SC12|CH20|SF15 +1 +1 +1 2 5 SC12|CH20|SF5 +1 +1 −1 3 17 SC12|CH10|SF15 +1 −1 +1 4 9 SC12|CH10|SF5 +1 −1 −1 5 3 SC6|CH20SF|15 −1 +1 +1 6 10 SC6|CH20|SF5 −1 +1 −1 7 11 SC6|CH10|SF15 −1 −1 +1 8 6 SC6|CH10|SF5 −1 −1 −1 9 12 SC12|CH15|SF10 +1 0 0 10 4 SC6|CH15|SF10 −1 0 0 11 16 SC9|CH20|SF10 0 +1 0 12 15 SC9|CH10|SF10 0 −1 0 13 1 SC9|CH15|SF15 0 0 +1 14 7 SC9|CH15|SF5 0 0 −1 15 12 SC9|CH15|SF10 0 0 0 16 14 SC9|CH15|SF10 0 0 0 17 8 SC9|CH15|SF10 0 0 0 SC - Sodium Carbonate; CH - Calcium Hydroxide; SF - Silica Fume

[0068] The replication of the centre point runs provides an estimate of the experimental error. The face-centred CCD was performed in random order to eliminate the possibility that one run depends on the conditions of the previous run or have an influence of subsequent runs.

[0069] Binder pastes were mixed in a Hobart mixer. The dry binder was added to a damp bowl and then water was added with the mixer running. Mixing was done for 5 minutes and then mixer was stopped and all dry material was scrapped off the bowl surface. The mixer was run again for another 5 minutes until the paste was completely mixed.

[0070] Prismatic moulds were used to cast 40×40×160 mm samples. The pastes were placed in the moulds, compacted on a vibrating table until there were no air bubbles, covered in film and left to set overnight at 25±1° C. The samples were then demoulded the following day and cured in a room with an ambient temperature of 25±1° C. and relative humidity of 85±5%. The samples were first weighed in air then weighed in water before strength tests were conducted. Compressive strength tests were carried out on three halves of the prism samples at 3, 7 and 28 days. The test procedure was done in accordance to SANS 50196-1.

[0071] 1.3.2. Statistical Analysis Procedure

[0072] The first stage of statistical analysis deals with interpreting the variable effects. This aids in providing necessary information on which variables and interactions may prove to be important terms to include in the model.

[0073] Thereafter, the initial model is formed which is a full model with all factors and interactions included. For this investigation, a full quadratic model was chosen to include possibility of curvature. Then analysis of variance (ANOVA) is applied to test the significance of the model. This is quantified by looking at the p-value, which is the converted F-statistic that is derived from the mean squares. If the p-value is less than 0.05, the model (and its terms) is deemed significant. If it is greater than 0.1, then the model terms are statistically insignificant. The next stage deals with model refinement where insignificant terms in the model are removed and ANOVA tests run until only significant terms remain in the model.

[0074] Thereafter, residual plots are analysed to confirm model assumptions and check its adequacy. This is an important part of the statistical analysis as it examines whether the model in question provides an adequate representation of the true system and it does not violate the least squares regression assumptions. The least squares regression assumptions are:

[0075] 1. Normality assumption;

[0076] 2. Constant variance assumption;

[0077] 3. Independence assumption.

[0078] The normality assumption is checked with the normality probability residual plot. If the graph follows a straight line then the assumption holds true and the model follows a normal distribution. If there is a s-shape or any abnormal shape then the normality assumption is violated.

[0079] The constant variance assumption is validated by the residuals versus predicted response graph. This plot should show a random scatter to satisfy the assumption. A presence of any sort of trend such as a megaphone shape indicates the assumption does not hold true and suggests an inequality of variance.

[0080] Plotting the residuals versus experimental run validates the independence of assumption, which again should show a random scatter with no noticeable trends. If all the assumptions hold true then the model can be deemed adequate and analysis of response surfaces and contour plots can be performed (Montgomery, 2011).

[0081] 1.3.3. Characterization Techniques

[0082] Characterization techniques of X-Ray Diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) were performed. These techniques were used to evaluate the microstructural and mineralogical changes over time for selected samples at 3 and 28 days. This provides information on the different compressive strength values observed during the study.

[0083] The samples required to be prepared before the characterisation techniques could be conducted. After being tested for strength at 3 and 28 days, representative samples for selected binders were collected, then they were dehydrated to prevent further material evolution. This was achieved by placing the samples in acetone to stop the hydration and then transferred to a vacuum chamber to filter out the acetone. Thereafter, the samples were placed in a desiccator at 25° C. to completely dry the sample until testing date (Ismail, et al., 2013). Samples for XRD and FTIR require the material to be in powdered form. This was attained by hand milling the dried samples using a pestle and mortar.

[0084] X-Ray diffraction analysis (XRD) allows for crystalline phase determination. The samples were prepared according to the standardized PANalytical back-loading system, which provides nearly random distribution of the particles. The samples were analyzed using a PANalyticalX′Pert Pro powder diffractometerin θ-θ configuration with an X′Celeratordetector and variable divergence- and fixed receiving slits with Fe filtered Co-Kα radiation (λ=1.789 Å). The phases were identified using X′PertHighscore Plus software. The data was recorded in the angular range 5°<2Θ<90°. The relative phase amounts (wt %) were estimated using the Rietveld method (X′PertHighscore Plus software).

[0085] Microstructure evolution can be further studied with Fourier-Transform Infrared Spectroscopy (FTIR). This characterisation technique provides information on the vibrations generated by the chemical bonds in a material. Lee and Van Deventer (2002) comment that it is necessary to follow the progressive change of chemical bonds within a structure and then correlate that with the physical characteristics observed. Criado et al. (2012) discovered a correlation between the pendulum movement of the main band of the FTIR spectra and compressive strength development of alkali-activated fly ash, over time. The FTIR of the selected samples was recorded on a solid state by a VERTEX 70v spectrometer equipped with the Golden Gate diamond ATR cell (Bruker). FTIR was recorded on the 4000-400 cm.sup.−1 spectral region, with 32 acquisitions at a 4 cm.sup.−1 resolution. Chemical bonds that correspond to the spectra bands were identified according to literature.

[0086] The SEM analysis was conducted on Zeiss Ultra plus SEM at an accelerating voltage of 1 kV. The SEM images were captured for fractured samples.

[0087] 1.4. Results and Discussion

[0088] 1.4.1. Face-Centred CCD Results and Analysis

[0089] The computer software, Minitab® 17.1.0, was used to assist in the design, analysis and interpretation of the 3.sup.k face centred central composite design. The model was analysed first as a full quadratic model which includes first- and second-order terms as well as two-factor interaction terms. This was done to incorporate any presence of curvature in the system.

[0090] It is often required that the impact of different factors be expressed as effect sizes and represented graphically. This helps in understanding which factor did or did not significantly influence the response, which was the 28-day compressive strength. The main factor effects can be calculated as the difference between the factor average and the grand mean. This is represented graphically in FIG. 4.

[0091] The graph in FIG. 4 is known as a main effects plot. It gives a visual representation of the relationship between the response and the independent variables. The dashed horizontal line represents the grand mean and the dots show the response mean at each factor level. The slope of each line visually indicates the impact. A horizontal line (parallel to the x-axis) implies no main factor effect and the response mean is the same across all factor levels. Non-horizontal lines imply there are main factor effects. The steeper the slopes of the line, the larger the effect and thus a greater impact on the response.

[0092] From FIG. 4, it can be noticed that Na.sub.2CO.sub.3 and Ca(OH).sub.2 have a positive impact on the compressive strength. Silica fume, on the other hand, has a negative impact at one factor level but a positive effect at another. Sodium carbonate has increasing slope at each factor level showing that higher quantity of Na.sub.2CO.sub.3 provides a beneficial effect towards strength development. Calcium hydroxide has a large effect on the strength when increasing from 10 to 15 wt % but a much smaller yet still positive effect from 15 to 20 wt %. Silica fume shows a negative impact when amount is increased from 5 to 10 wt % but it is counteracted with a larger positive effect when increased from 10 to 15 wt %.

[0093] However, if there are significant interaction terms then it is not possible to fully interpret the main effects without looking at the interaction plots.

[0094] An interaction plot shows the relationship of how one factor and the response depends on the value of another factor. For interaction plots, analysis is conducted by looking at both lines of an interaction and compared. If both lines are parallel then the interaction effect is concluded to be zero. However, the more different and steeper the slopes become, the greater the influence it has on the results. Furthermore, if the lines cross each other, it is known as disordinal interactions and if they do not then they are called ordinal interactions (Stevens, 2000). Each criterion alters how the factors are interpreted.

[0095] From FIG. 5, it can be noticed that none of the lines are parallel. Sodium carbonate (Na.sub.2CO.sub.3) at 12 wt % gives higher mean strength at two factor levels, 15 and 20 wt %, of Ca(OH).sub.2 except at 10 wt %, where Na.sub.2CO.sub.3 at 6 wt % gave a higher mean value. Sodium carbonate at 12 wt % factor level is always higher than at 9 wt % factor level which is higher than 6 wt % except 10 wt % Ca(OH).sub.2 factor level and even crosses the other lines. This shows that there is a possibility of a significant interaction as the relationship between Ca(OH).sub.2 and strength is dependent on the value of Na.sub.2CO.sub.3.

[0096] As there are interaction effects, it is possible to deduce simple main effects and check whether it is significant or not. This is evaluated by looking at the average of the circle, diamond and square points at each factor level on the x-axis and comparing them. This is shown by the purple line in FIG. 5. If there is a large enough change between each factor level, then the main effect can be deemed possible. The average of the points of Ca(OH).sub.2 at each factor level show that it generally increases, especially between 10 and 15 wt % but less between 15 and 20 wt %. This is confirmed with the main effects plot in FIG. 4 and shows Ca(OH).sub.2 as a possible main effect.

[0097] From FIG. 6, it can be again noticed that none of the lines are parallel. Silica fume at 15 wt % gives higher mean strength at two factor levels, 9 and 12 wt %, of Na.sub.2CO.sub.3 except at 6 wt %, where SF at 5 wt % gave a higher mean value. In fact, the 15 wt % of SF gives the lowest mean value at 6 wt % Na.sub.2CO.sub.3. This shows that there is a possibility of a significant interaction as the relationship between Na.sub.2CO.sub.3 and strength is dependent on the value of SF.

[0098] Evaluating the simple main effect of Na.sub.2CO.sub.3 by taking average of the square, diamond and star points at each factor level (line in FIG. 6) of Na.sub.2CO.sub.3 shows a large increase between 6 and 9 wt % but a very slight increase between 9 and 12 wt %, which is in correlation with the main effects plot in FIG. 4. The main effect of Na.sub.2CO.sub.3 is a possibility.

[0099] From FIG. 7, it can be again noticed parallel lines do not exist. Calcium hydroxide at 15 wt % gives higher mean strength at two factor levels, 10 and 15 wt %, of SF except at 5 wt %, where Ca(OH).sub.2 at 20 wt % gave a higher mean value. This shows that there is a possibility of a significant interaction as the relationship between SF and strength is dependent on the value of Ca(OH).sub.2. Calcium hydroxide at 10 wt % showed a lower mean value except at 15 wt % of SF where the mean value was similar to 20 wt % Ca(OH).sub.2 with 10 wt % Ca(OH).sub.2 being slightly higher. Moreover, it can be noticed that at 5 and 10 wt % SF factor levels for 10 wt % Ca(OH).sub.2, there was almost no change in the mean values.

[0100] Evaluating the simple main effect of SF by taking averages of the points at each factor level (line in FIG. 7) shows that it decreases between 5 and 10 wt % but increases between 10 and 15 wt %, which is in correlation with the main effects plot in FIG. 4. The main effect of SF is also another possibility.

[0101] The possible significance of the interactions and simple main effects will be checked via analysis of variance (ANOVA).

[0102] The analysis of variance (ANOVA) was computed first to test the adequacy of the model as shown in Table 8.

TABLE-US-00006 TABLE 7 Initial full quadratic model ANOVA result Source DF Adj SS Adj MS F-Value P-Value Model 9 970.647 107.850 52.41 <0.0001 Linear 3 303.355 101.118 49.14 <0.0001 Na.sub.2CO.sub.3 1 158.484 158.484 77.02 <0.0001 Ca(OH).sub.2 1 129.096 129.096 62.74 <0.0001 Silica Fume 1 15.775 15.775 7.67 0.028 Square 3 148.133 49.378 24.00 <0.0001 Na.sub.2CO.sub.3*Na.sub.2CO.sub.3 1 0.443 0.443 0.22 0.657 Ca(OH).sub.2*Ca(OH).sub.2 1 101.561 101.561 49.35 <0.0001 Silica Fume*Silica Fume 1 95.432 95.432 46.38 <0.0001 2-Way Interaction 3 519.158 173.053 84.10 <0.0001 Na.sub.2CO.sub.3*Ca(OH).sub.2 1 263.122 263.122 127.87 <0.0001 Na.sub.2CO.sub.3*Silica Fume 1 131.058 131.058 63.69 <0.0001 Ca(OH).sub.2*Silica Fume 1 124.978 124.978 60.73 <0.0001 Error 7 14.404 2.058 Lack-of-Fit 5 12.590 2.518 2.78 0.286 Pure Error 2 1.815 0.907 Total 16 985.051

[0103] As seen from Table 7, the F-value of the model is 52.41 implying that the model is significant and the probability that this value was due to noise is less than 0.01%. Furthermore, the lack-of-fit P-value is 0.286 which indicates that it is insignificant as p>0.05. The insignificance of the lack-of-fit implies that the model fits the experimental data and the factors under consideration have a considerable impact on the response. However, it can be noticed that there is an insignificant term in the model (Na.sub.2CO.sub.3*Na.sub.2CO.sub.3). This was removed and the reduced model ANOVA results are shown in Table 8.

TABLE-US-00007 TABLE 8 Reduced model ANOVA result Source DF Adj SS Adj MS F-Value P-Value Model 8 970.203 121.275 65.34 <0.0001 Linear 3 303.355 101.118 54.48 <0.0001 Na.sub.2CO.sub.3 1 158.484 158.484 85.39 <0.0001 Ca(OH).sub.2 1 129.096 129.096 69.56 <0.0001 Silica Fume 1 15.775 15.775 8.50 0.019 Square 2 147.690 73.845 39.79 <0.0001 Ca(OH).sub.2*Ca(OH).sub.2 1 120.013 120.013 64.66 <0.0001 Silica Fume*Silica Fume 1 102.938 102.938 55.46 <0.0001 2-Way Interaction 3 519.158 173.053 93.24 <0.0001 Na.sub.2CO.sub.3*Ca(OH).sub.2 1 263.122 263.122 141.77 <0.0001 Na.sub.2CO.sub.3*Silica Fume 1 131.058 131.058 70.61 <0.0001 Ca(OH).sub.2*Silica Fume 1 124.978 124.978 67.34 <0.0001 Error 8 14.848 1.856 Lack-of-Fit 6 13.033 2.172 2.39 0.324 Pure Error 2 1.815 0.907 Total 16 985.051

[0104] The reduced model has a F-value of 65.34 which is greater than the initial model and still implies the model is significant. Moreover, the lack-of-fit is still insignificant (p=0.324>0.05) which again shows the adequacy of the model. Table 9 gives a comparison of the summary statistics for ANOVA of the initial model vs. reduced model.

TABLE-US-00008 TABLE 9 Summary statistic comparison between initial and reduced model S R-sq R-sq(adj) R-sq(pred) Initial Model 1.43450 98.54% 96.66% 86.27% Reduced Model 1.36235 98.49% 96.99% 88.12%

[0105] The R.sup.2 of the reduced model indicates it can explain 98.49% of the model's variability. A large R.sup.2 is usually a good indicator of the model's fit. However, the value of R.sup.2 can be artificial increased as more predictors get included in the model without any real improvement to the system. Hence it is beneficial to compare the adjusted R.sup.2 to the original R.sup.2. The adjusted R.sup.2 is a modified version of the normal R.sup.2 and will only increase if the new term improves the model, if it has a strong correlation to the response. It decreases when the predictor does not have a strong correlation to the response. The adjust R.sup.2 will always be less than R.sup.2. From Table 9, the adjusted R.sup.2 is 96.99% which is relatively high and further emphasises the model's adequacy and that non-significant terms have not been included.

[0106] The predicted R.sup.2 is used, in conjunction with the adjusted R.sup.2, to determine if there are too many predictors in the model and if it has been over-fit and is modelling random noise. An over-fit model will usually have a high R.sup.2 value and low predicted R.sup.2. From Table 9, the predicted R.sup.2 is 88.12% which shows the models high predictive capability and confirms that the model has not been over-fit. It is also within the acceptable margin of 0.2 from the adjusted R.sup.2.

[0107] To further confirm the adequacy of the model, it is required to assess the residual plots. A residual is the difference between the observed value and its fitted value. Analysis of residual plots assists in confirming the assumption that the errors are approximately normally distributed with constant variance and whether additional terms would need to be added to the model (Montgomery, 2011).

[0108] The graph in FIG. 9 represents the normal probability plot. Probability plots are used to assess whether the model has a fixed distribution. Most models are in the form,

Response=deterministic+stochastic

where the deterministic part is the fit and stochastic part is the error. The error part is most commonly assumed to be normally distributed. Therefore, a graph of a normal probability plot is generated from the residuals of the fitted model. FIG. 8 shows that the residuals follow a straight line, which verifies the assumption that the residuals are normally distributed.

[0109] The graph in FIG. 9 shows the residuals versus fits graph, which aids in confirming the assumption that the residuals constant variance and a mean of zero. As noticed, there are no visible trends in FIG. 9 as the plots are scattered randomly around zero so the assumption that errors have a mean of zero is valid. Furthermore, there are no visible trends in and the vertical width of the scatter does not change significantly when moving along the fitted values, which shows that the assumption of residuals having a constant variance to be reasonable.

[0110] Another assumption required to be validated is where the errors are said to independent of each other. A plot of residual vs. order plot is required to validate this assumption, which is a plot of the residuals versus the order in which the data was collected. This is shown in FIG. 10 for the model. For the assumption to hold true, it is required that the plots fluctuate randomly across the centreline of the graph, which is observed in FIG. 10 showing the residuals are uncorrelated and the model is adequate.

[0111] With the model statistics and assumptions satisfied after analysis of the residuals, the final model can be used to navigate the design space.

[0112] The final empirical model equation is given in Eq. 9 and Eq. 10 as coded and actual units, respectively. Coded variables help with interpretation of the model and variable effects as the magnitude of the coefficients are given on a common scale.

Y=30.200+3.981.Math.A+3.593.Math.B+1.256.Math.C−6.295.Math.B.sup.2+5.830.Math.C.sup.2+5.735.Math.A.Math.B+4.047.Math.A C−3.952.Math.B.Math.C  (3)

[0113] where,

[0114] Y—Compressive strength (MPa)

[0115] A—Na.sub.2CO.sub.3 (wt %), −1≤A≤1

[0116] B—Ca(OH).sub.2 (wt %), −1≤B≤1

[0117] C—Silica Fume (wt %), −1≤C≤1

Y=23.82−7.106.Math.A+6.41.Math.B−4.471.Math.C−0.2518.Math.B.sup.2+0.2332.Math.C.sup.2+0.3829.Math.A.Math.B+0.2698.Math.A.Math.C−0.1581.Math.B.Math.C  (4)

where,

[0118] Y—Compressive strength (MPa)

[0119] A—Na.sub.2CO.sub.3 (wt %), 6≤A≤12

[0120] B—Ca(OH).sub.2 (wt %), 10≤B≤20

[0121] C—Silica Fume (wt %), 5≤C≤15

[0122] 1.4.2. Compressive Strength

[0123] The surface and contour plots are given in FIG. 11. The general trend noticed from the compressive strength values was that when the strength was high at 3 days, it only increased by a small factor at 28 days. For examples, Mix SC6|CH20|S5, gained 9.81 MPa at 3 days and 29.9 MPa at 28 days. The opposite was true for other samples. The highest strength mix, SC12|CH20|SF15 gained 1.65 MPa at 3 days and ended with 44.2 MPa at 28 days. In general, samples that contained the lowest amount of Na.sub.2CO.sub.3 were responsible for the first trend regardless of the level of Ca(OH).sub.2 and SF.

[0124] The first trend of high early strength is in correlation with Jeon et al. (2015) where the author's samples gained considerable strength at 3 days (28 MPa) but did not develop as aggressively and only increased in strength by 29% at 28 days (36 MPa). The second trend was noticed by other authors (Abdalqader et al., 2016; Wang et al., 1994; Fernández-Jiménezet al., 1999; Li and Sun, 2000) and that is due to the lower pH of Na.sub.2CO.sub.3. It can demonstrate a lower strength in the beginning but a higher strength at later stages due to the effect of CO.sub.3.sup.2− ions, which causes the formation of carbonated compounds that improves mechanical strength.

[0125] Huang and Cheng (1986) mentions that in a FA-Ca(OH).sub.2—H.sub.2O system, the hydration rate is low. The system's hydration rate increased by 1.5% in 7 days and was less than 20% after 180 days. The reasoning stated was that unlike slag, FA has a lower content of CaO; higher aluminium and silicon content, which requires a higher degree of polymerization. Thus, the hydration activation is much less than that of slag. Furthermore, in a system of FA-Ca(OH).sub.2, the pH is less than 13. Fraay and Bejen (1989) demonstrated that a pH value of 13.3 is required for appropriate dissolution of alumina and silica species. Therefore, it is required that other additives be added to the system to increase the alkalinity (Li et al., 2000).

[0126] Sodium carbonate, Na.sub.2CO.sub.3, compared to other conventional activators such as NaOH and Na.sub.2SiO.sub.3 has a lower pH. Thus Na.sub.2CO.sub.3, having a pH of 12.5, does not contain sufficient dissolution power in a FA based system.

[0127] However, by combining Na.sub.2CO.sub.3 with Ca(OH).sub.2 the following chemical balanced equation is formed:

Na.sub.2CO.sub.3+Ca(OH).sub.2.fwdarw.2NaOH.sub.(aq)+CaCO.sub.3  (5)

[0128] From Equation 11 it can be noticed there is formation of NaOH which increases the alkalinity of the mixture, although not a dominant factor, which promotes dissolution of the silica and alumina species in the FA. Furthermore, the CaCO.sub.3 can act as filler material to reduce porosity and promote higher mechanical strength. However, considering the Gibbs energy of the chemical equation, it can be argued that such a reaction does not occur under ambient conditions. But the complexity of the system could cause the reaction to occur or other reactions due to the inclusion of SF which has a high reactivity due to its surface area. Jeon et al. (2015) had a similar blend (FA+Ca(OH).sub.2+Na.sub.2CO.sub.3) but was cured thermally at 60° C., which attained 36 MPa at 28-days. The highest strength sample in this study attained 44.2 MPa at 28 days under ambient conditions.

[0129] Samples that contained a higher content of sodium carbonate had a higher 28-day strength value than those with a lower content. This improvement could be explained by the fact that Na.sub.2O content increases which increases the pH. An increased pH aids in the dissolution of the silicon and alumina species, which is responsible for strength development due to increased gel formation in the samples (Wang et al, 1994; Li and Sun, 2000). Overall both Na.sub.2CO.sub.3 and Ca(OH).sub.2 and their interaction (line in FIG. 5) had a positive impact on the mechanical strength.

[0130] Silica fume, on the other hand, had a positive and negative impact on the mechanical strength depending on the quantity. Silica fume's interaction with Na.sub.2CO.sub.3 was overall positive (line FIG. 6). However, the interaction with Ca(OH).sub.2 is unusual whereby there is a switch in effect from negative to positive with increasing SF quantity. Generally, the particles of SF are spherical and very fine. This implies it has a large surface area and reacts very readily with alkaline solution in a polymerisation reaction. Improved strength is noticeable due to the packing effect of fine SF that acts as a filler material for voids. This creates a more compact microstructure. Furthermore, they also act as nucleation sites for alkali reactions and thus will promote the formation of alkali-activated products (Nurrudin et al., 2010; Rashad & Khalil, 2013; Sayed & Zedan, 2013; Songpiriyakij et al., 2011). This was found to be true for most of the mixes. Samples containing larger quantities of SF produced higher mechanical strength.

[0131] The following sections deal with the characterization techniques used to evaluate the microstructural and mineralogical changes over time for selected samples at 3 and 28 days. The selected samples were SC12|CH20|SF5, SC12|CH10|SF5 and SC6|CH20|SF5, which gained high strength, low strength and moderate strength, respectively.

[0132] 1.4.3. X-Ray Diffraction (XRD) Analysis

[0133] The graph in FIG. 12 represents the diffractograms for samples SC12|CH20|SF5, SC12|CH10|SF5 and SC6|CH20|SF5 at 3 and 28 days.

[0134] Fly ash consists of stable but un-reactive crystalline phases such as mullite and quartz as well as reactive amorphous phases. Therefore, during reaction, the reactive part undergoes alkali activation whilst the un-reactive phases act as micro-aggregate in the final matrix (Kumar et al. 2017; Duxson et al. 2005). The major peak at approximately 31° is due to quartz that was present in the fly ash.

[0135] Portlandite, which is calcium hydroxide, gets consumed and lower amounts are present at 28 days as seen in the XRD results. Sample SC12|CH20|SF5 at 3 days (FIG. 12) the peaks at 21° and 40° represent portlandite and it can be observed that the intensity of the peaks at 28 days is lower than at 3 days, showing the full consumption has not completed. Moreover, for mix SC12|CH10|SF5 and SC6|CH20|SF5 the portlandite peaks, occurring at the same position, occurs in the 3-day samples but disappear completely in the 28-day samples.

[0136] Walkley et al. (2016) suggests that increasing the calcium content in precursors enables greater formation of low-Al, high Ca containing C-(N)-A-S-H with lower mean chain length (MCL) and AFm type phases with little evolution of binder chemistry at later stage. Authors mention that in general, increasing the Ca content in precursors impedes the formation of N-A-S-H and AFm type phases and in Al-rich samples, promotes the formation of portlandite and Al-rich reaction products. However, the diffractograms did not show any zeolitic formation and the hydration product for this type of binder system is most likely to be a C-S-H variant.

[0137] As the peaks of portlandite decrease/disappear with increasing age, there is progressive consumption which can be facilitated by lime-consuming C-S-H formation and also due to the available silicon in the reaction mixtures, forming additional gels (Jeon et al. 2015). The increase in calcite amount over the time period can also decrease the amount of portlandite due to carbonation. This is proven by the calcite peaks noticed in the 28-day samples in sample SC12|CH20|SF5 and SC6|CH20|SF5 at position 34° in both diffractograms. In sample SC12|CH10|SF5 there was no calcite reflection at 28-day but the portlandite reflection at 3-day completely disappears, which can be most likely attributed to its consumption and formation of C-S-H type gels.

[0138] Carbonate salt gaylussite, sodium-calcium carbonate, was identified in the samples which indicate cation exchange reaction between the precursor and activator. In the alkali-activation, the Ca.sup.2+ ions provided by calcium hydroxide must react with the CO.sub.3.sup.2− from sodium carbonate to form carbonate salts such as calcite and gaylussite (Eq. 6) such that the pH can be increased through the release of OH.sup.− ions. After gels start to precipitate, the gaylussite dissolves as a result of decreasing concentration of gaylussite CO ions in the aqueous phase. The carbonate then re-precipitates as CaCO.sub.3 polymorphs such as calcite. Thus, gaylussite is a transient phase that decreases/get consumed as more stable carbonates form at later stages as shown in Eq. 7 (Ke et al., 2016; Yuan et al., 2017; Bernal et al., 2014). This is in correlation with the XRD patterns in FIG. 12. There is a notable decrease in peak intensity from 3 to 28 days for all samples and in the case of SC12|CH20|SF5 and SC6|CH20|SF5, there is formation of calcite. Calcite acts as an inactive filler material and do not contribute to the formation of the gel structure (Aboulayt et al., 2017).

5H.sub.2O+2Na.sup.++Ca.sup.2++2CO.sub.3.sup.2−.fwdarw.Na.sub.2Ca(CO.sub.3).sub.2.5H.sub.2O(gaylussite)  (6)

Na.sub.2Ca(CO.sub.3).sub.2.5H.sub.2O.fwdarw.CaCO.sub.3+2Na.sup.++CO.sub.3.sup.2−+5H.sub.2O  (7)

[0139] In conclusion, the mechanism noticed is the formation of large quantity of gaylussite for all samples which transforms into carbonate phases at later age, noticed by the lower gaylussite peaks and formation of calcite peaks. The portlandite consumption can be tied to the formation of C—S—H type gels. This consumption followed by calcite formation can explain the difference in strength noticed for the three samples. For the high strength sample (SC12|CH20|SF5), portlandite peak intensity was the highest when compared to the other two. Furthermore, unlike the other two samples, portlandite peaks still remained at 28 days showing that more gels could form and the sample can gain more strength. Furthermore, for SC6|CH20|SF5, calcite formation was occurring already at 3rd day and its peak intensity increased at 28 day. This shows the gaylussite transformation was occurring much earlier on when compared to SC12|CH20|SF5 and SC12|CH10|SF5.

[0140] 1.4.4. SEM Analysis

[0141] One of the main trends noticed in the experiment was that samples that gained higher strength at 28-day had lower strengths at early age. The opposite occurred for samples that gained lower strengths at 28-day. FIG. 13 shows SEM samples of SC12|CH20|SF5, SC12|CH10|SF5 and SC6|CH20|SF5, at different curing ages, to further understand the noticed trend.

[0142] For the samples at 3-day, it can already be noticed that there is a large disparity in the amount of unreacted particles. Mix SC12|CH20|SF5, which gained the second highest strength at 28 days but low 3-day strength, has a noticeable amount of unreacted FA and SF particles as well as sodium carbonate crystals. Mix SC12|CH10|SF5, which gained the lowest 28-day strength, showed similar characteristics to the sample SC12|CH20|SF5 SEM image but the amount of unreacted particles were more prominent. However, mix SC6|CH20|SF5, which had one of the highest 3-day strength of all the samples, showed a significantly lower amount of unreacted particles, comparatively. The SEM image of SC6|CH20|SF5 (3D) shows a more developed microstructure with large amorphous area at early age compared to the other two, which can explain the higher strength at 3-day.

[0143] Investigating the 28-day samples, SC12|CH20|SF5 had scarce amount of unreacted particles and mostly consisted of a dense amorphous microstructure. This shows that most of the unreacted particles noticed in the 3-day samples had reacted and could be the reasoning behind on why it gained high strength. Mix SC12|CH10|SF5 which had similar characteristic to mix SC12|CH20|SF5 at 3-day did not, however, show a similar trend. There is still copious amounts of unreacted FA and SF particles as well as sodium carbonate crystals. Sample SC6|CH20|SF5 showed the same characteristic as its 3-day counterpart but contained a larger quantity and size of unreacted particles when compared to mix SC12|CH20|SF5. This shows that it did not undergo a large dissolution phase unlike sample SC12|CH20|SF5, which caused the strength value to be lower in comparison.

[0144] Comparing the microstructure of mix SC12|CH20|SF5 between 3-day and 28-day, it can be noticed the porosity of sample decreased over time. This can be attributed to the formation of calcite and its formation noticed in the XRD diffractograms. This confirms the theory that calcite acts a pore-filling material as mentioned earlier. Similar trend is noticed in the other two samples but the porosity is slightly higher. Moreover, the reduction in porosity can be also be attributed to the inclusion of SF. The particles of SF are spherical and very fine. This implies it has a large surface area and reacts very readily. Improved strength is noticeable due to the packing effect of fine and spherical SF that acts as a filler material for voids. This creates a more compact microstructure. Furthermore, they also act as nucleation sites for reactions and thus will promote the formation of amorphous gels (Nurrudin et al., 2010; Rashad & Khalil, 2013; Sayed & Zedan, 2013; Songpiriyakij et al., 2011).

[0145] Moreover, since no additional adjustments were made to accommodate the increase in SF content, the samples were prone to excessive self-desiccation and cracking due to its large surface area (Khater, 2013; De Silva et al., 2007). Rashad and Khalil (2013) observed similar scenario where a 5% SF replacement created a denser and more homogenous microstructure and had the highest strength. However, with increasing SF, the microstructure deteriorated and caused lower mechanical strength but was still better than with no SF inclusion. Aydin (2013) also had a similar situation where a 20 wt % SF replacement caused a porous structure with disconnected pores. Thus, it can be said that SF could prove to be detrimental at higher levels but beneficial at lower levels.

[0146] In conclusion, the SEM analysis correlated well with the XRD data. The moderate strength sample SC6|CH20|SF5 at 3-day had a lower amount of unreacted particles and a more homogenous microstructure, which can be attributed to the gaylussite transformation into carbonate species as it was the only sample to show calcite peaks at 3-day. This could also imply gel formation took place earlier and it did not gain additional strength unlike SC12|CH20|SF5. SC12|CH20|SF5 gained higher strength due to the denser microstructure at 28-day and this can be connected to the largest portlandite peaks noticed at 3- and 28-day in the XRD data for this sample. SC12|CH6|SF5 still had unreacted after 28-days showing full dissolution had not taken place or was taking place at a slow pace. It was also the most porous sample which can again be tied to no calcite peaks noticed in the XRD data at 28-day in FIG. 12.

[0147] 1.4.5. FTIR Analysis

[0148] Fourier transform infrared spectroscopy (FTIR) enables interpretation of the degree of polymerization of the reaction products and making conclusions about the types of structures present. The vibration of molecular bonds will differ depending on the absorption of photons at appropriate energies. Thus, this allows relating structures to the energy that is absorbed and define the functional groups of the product. Molecular bonds will exhibit narrow peaks if the structure is highly crystalline. If the structure is amorphous, the IR spectra will show wider/broader bands/peaks (Dakhane et al., 2017).

[0149] FIGS. 14, 15 and 16 represent the IR spectrum from wavelength section 1500 to 500 cm.sup.−1 of SC12|CH20|SF5, SC12|CH10|SF5 and SC6|CH20|SF5 for raw blended materials and at different ages, respectively. There were not any noticeable changes from 4000 to 1500 cm.sup.−1 and thus not included in the Figures, except some minor broad humps noticed in the region of 3600 to 3500 cm.sup.−1, which can be attributed to the strength vibration of —OH (Jang & Lee, 2016). There is a significant peak occurring in the region 1440 to 1410 cm.sup.−1 in all samples as well as a smaller peak around 880 to 865 cm.sup.−1 region. This can be associated with stretching vibrations of C═O or CO.sub.3.sup.2− anions, which confirms the formation of carbonate species (Peyne et al., 2017; Kumar et al., 2017; Lee et al., 2017; Abdel-Gawwad & Abo-El-Enein., 2014). The absorbance peaks at these two wavelengths indicate the formation of calcite and with increasing age, the intensity of the peaks increase.

[0150] Noticing the IR spectra of the raw blended mix for all samples, there is an absorbance band around 1080 cm.sup.−1 wavelength. This is attributed to asymmetric stretching vibration mode of Si—O-T (T=tetrahedral Si or Al) caused by the presence of FA in the blended mixture. For all investigated samples, this main band shifted to a lower wavelength with increasing age. This is known to be a common occurrence in FA based activated materials as it represents the extent of the reaction/formation of gels due to the effect of activators on the original material, fly ash (de Vargas et al., 2014; Jang and Lee, 2016; Siyal et al., 2016).

[0151] For SC12|CH20|SF5 the main band in the blended mix appears at 1086 cm.sup.−1 and shifted to 954.7 cm.sup.−1 at 3-day and then to 965.4 cm.sup.−1 at 28-day (FIG. 14). The initial shift to a lower frequency indicates the formation of Al-rich gel but the slight shift to a higher wavelength at a later stage indicates the transformation of the gel into a Si-rich gel. The shift to a lower wavelength is due to the reduced calcium content in the gel due to the incorporation of Al due to the dissolution of FA in the mixture. This shift indicates the material with beneficial mechanical properties (Jang and Lee, 2016; Criado et al., 2012; Abdalqader et al., 2015). Moreover, Garcia-Lodeiro et al. (2011) suggest that bands in the 950-970 cm.sup.−1 region represent C-S-H and C-A-S-H type gels whilst bands in the 1000 cm.sup.−1 represent N-A-S-H gel.

[0152] Therefore, for SC12|CH20|SF5, the reversal in band to a higher wavelength is noticed and is in the range of 950 to 970 cm.sup.−1 which implies it may have a C-A-S-H type gel. Similarly, for SC6|CH20|SF5, the main band appeared at 1076 cm.sup.−1 and moved to 965.6 cm.sup.−1 at 3-day and then to 975.2 cm.sup.−1 at 28-day (FIG. 16). The final wavelength position for SC6|CH20|SF5 is higher than that for SC12|CH20|SF5 (965.4 cm.sup.−1 vs. 975.2 cm.sup.−1) which could be a possible reason for the higher strength gain in SC12|CH20|SF5 than SC6|CH20|SF5 at 28-day. The closer the wavelength is to 950 cm.sup.−1, the gel gets most commonly attributed to C-S-H which gives better mechanical strength (Dakhane et al., 2017). This is evident in the higher amorphous nature of the microstructure as noticed in SEM/EDS analysis discussed herein above.

[0153] For SC12|CH10|SF5, the main band appeared at frequency 1079 cm.sup.−1 which shifted to 1019 cm.sup.−1 at 3-day but then moved to a lower 1003 cm.sup.−1 at 28-day (FIG. 16). Therefore this represents a possible N-A-S-H or N-(C)-A-S-H type gels which can contribute to its lower mechanical strength. It also implies that the polymerization reaction is still continuous and when it completes, the band should move towards a higher wavelength (Siyal et al., 2016).

[0154] The series of small bands appearing around 790 cm.sup.−1 and 660 cm.sup.−1 can be attributed to symmetric stretching of O—Si—O bonds and the strong peaks appearing around 560 cm.sup.−1 can be associated with symmetric stretching of Al—O—Si bonds. This is related to the presence of quartz and mullite from the raw fly ash in the raw blended samples (Jang and Lee, 2016).

Discussion

[0155] The present invention considers trying to eliminate the use NaOH, which is caustic in nature. The process of producing alkali-activated binders has its drawbacks that prevent the process from being accepted in the industry: the use of highly alkaline activators like NaOH and Na.sub.2SiO.sub.3 to attain alkali activation possess problems pertaining to handling and storage (Dakhane, et al., 2017).

[0156] The novel composition of the present invention uses a low impact activator sodium carbonate (Na.sub.2CO.sub.3) and hydrated lime (Ca(OH).sub.2). Sodium carbonate is a naturally occurring mineral and can be obtained from sodium carbonate-rich brines or chemical processes such as the Solvay process. The worldwide total of natural occurring sodium carbonate amounts to 24 billion tonnes (USGS, 2017). Moreover, Na.sub.2CO.sub.3 is reported to be 2-3 times cheaper than NaOH or sodium silicate. In addition to the low cost of this activator compared to conventional ones, it is safer to handle and has been reported to yield lower drying shrinkage. Thus, the use of Na.sub.2CO.sub.3 as an activator can contribute to the development of more sustainable AAMs (Abdalqader et al., 2016).

[0157] The efflorescence formation of the FA-SF blend reacted with Ca(OH).sub.2 and Na.sub.2CO.sub.3 (FA|SF|CH|SC) was little to none. The FA|SF|CH|SC blend, produces a dry AAM cement powder that can be mixed together and stored in bags just like OPC cement bags.

[0158] The FA|SF|CH|SC blend gained a maximum strength of 44.2 MPa and lowest strength of 14.2 MPa at 28-day.

[0159] Sodium carbonate and Ca(OH).sub.2 were the main factors that positively affected the mechanical strength of the FA|SF|CH|SC blend with increasing content.

[0160] The main trend noticed in the FA|SF|CH|SC blend was that samples containing a lower quantity of Na.sub.2CO.sub.3 gained high strength at 3-day but did not undergo much reaction afterward. The opposite of this was also true. This shows Na.sub.2CO.sub.3 is required to react with Ca(OH).sub.2 to increase the pH of the system and help with the dissolution of the Si and Al precursors in the source materials.

[0161] XRD analysis shows crystalline phases of gaylussite and calcite in the FA|SF|CH|SC blend. Gaylussite is a transient phase that decreases/get consumed as more stable carbonates form at later stages.

[0162] Calcite acts as an inactive filler material and do not contribute to the formation of the gel structure. Samples that gained high strength at 3-day but did not undergo much reaction afterward was because of early reaction due to the lower Na.sub.2CO.sub.3 content. This causes a lower quantity of gel formation but an earlier reaction phase. Therefore, the gaylussite had a longer time to transform to its stable carbonate species, such as calcite, which enabled the mixes to gain better strength as seen by the characterization techniques.

[0163] In conclusion, the novel composition of the present invention proves to be a viable FA-based AAM system that provides a cost effective and efficient building material that can gain sufficient strength under ambient conditions. Furthermore, this type of blend can be created as a dry material that can be stored and distributed in bags and only requires the addition of water to form the building material.

[0164] Whilst only certain embodiments or examples of the instant invention have been shown in the above description, it will be readily understood by a person skilled in the art that other modifications and/or variations of the invention are possible. Such modifications and/or variations are therefore to be considered as following within the spirit and scope of the present invention as defined herein.

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