One of the material with Class C pozzolan compositions (23) is wollastonite (5). Colloidal n-SiO₂ was also added for enhancing the properties of cement matrix. β-CaSiO₃ used here has a CaO content of 46.8%, SiO₂ 45.8%, and Fe₂O₃ content of 4.61%, with no traces of SO₃, which confers with the class C pozzolana (5). Colloidal n-SiO₂ is 99.2% nano-scale SiO₂. A total of 5 mixes were selected apart from the control mix. The mix design was carried out by the packing density method. The experiments were conducted on fresh and hardened paste mixes. The marsh cone test and mini-slump cone test determined the PCE (Polycarboxyl ether)-based superplasticizer dosage. The superplasticizer was obtained from Fosroc India Pvt Ltd. Cement was 53 grade by IS 12269 (24). Superplasticizers were added with respect to total weight of binder materials. CaSiO₃ was procured from Wolkem India Pvt Ltd. and Nanosilica from Nouryon India Pvt Ltd. β-CaSiO₃ is a triclinic acicular crystalline structure. It has a specific gravity of 2.9 and bulk density of 1.5 g/ml. The hardness in Moh’s scale is 4.5. Melting point of β-CaSifO₃ is 1540°C, and co-efficient of expansion is 6.5 X 10^-6. Aspect ratios of the β-CaSiO₃ microfibres has not been defined in this paper, yet with help of FESEM micrographs few dimensions have been found (Figure 9g).

The mix design of various mixes by weight is given in Table 1a, and oxides present in the raw materials are given in Table 1b. From the literature review, we concluded that the CaSiO₃ as a partial replacement to portland cement could be replaced at 15% with significant results. Also, the n-SiO₂ replaced up to 5% of OPC and showed excellent mechanical and durability properties.

CaSiO₃ and cement were premixed by hand until a uniform colour was obtained. The samples are machine mixed with the help of a machine mixer. The mixing was done by ASTM C305 (25). For mixes with colloidal n-SiO₂, the mixing time at high speed has been increased by 5 mins for proper dispersion of nanoparticles in the paste mix (26).

Table 2 shows the mini slump cone test and marsh cone flow time values. The optimum superplasticizer dosage for all paste mixes for 0.25 and 0.4 w/b ratios were determined. The wollastonite microfibres and nanosilica decreases the fluidity of the paste mix and hence demands more superplasticizer dosage than the reference mix.

Figure 2 shows that the reinforcement effect of CaSiO₃ and colloidal n-SiO₂ was significant even though the compressive strength was lower than the control mix-CaSiO₃ and colloidal n-SiO₂ show better strength at 28 days, representing their reinforcement effect. After 28 days, mix NS3 shows the highest strength at 0.25 and 0.40 w/b ratios. It suggests a high secondary reaction by CaSiO₃ and colloidal n-SiO₂.

The secondary reaction will reduce the amount of ettringite and calcium hydroxide and increase the dense CSH gel (5, 22, 26, 37). The strength development can also be attributed to matrix densification and pore size refinement. Strength improved with a decrease in w/b ratios; the highest compressive strength was observed in the 0.25 w/b ratio and the lowest in the 0.55 w/b ratio. Free water, available for reaction, reacts with the cement, CaSiO₃, and colloidal n-SiO₂ to form different hydrates responsible for strength development. The excess water goes and settles in different pores and evaporates after some time, leaving voids in the paste matrix. It results in the weakening of the paste matrix (38). The increase of nano silica after 3% causes a reduction in strength, which can be attributed to the amount of n-SiO₂ particles greater than the amount of free lime in the hydration process. It results in excess silica leaching; hence, the n-SiO₂ acts as a pore filler and does not take part in the hydration process at this point (22, 39). Figure 3 shows the parallel cracks inclined at a slight angle to the applied load, formed in the specimen suggests axial compression caused due to localised tensile stress acting normal to the applied load. The cracks formed are shown in Figure 3a and 3b. In Figure 3c, CaSiO₃ and n-SiO₂ provide reinforcement to the hardened cement matrix, which hinders the crack propagation as described by Neville (40).

Initially, the paste mix was wet; hence the CaSiO₃ and colloidal n-SiO₂ reaches the voids by passing through the pore solution in the paste mix. This mechanism helps the SCMs to have secondary hydration reactions in the future, thereby forming CSH gel and reducing the number of capillary pores. The rate of secondary hydration varies with respect to the chemical and physical properties of admixtures, and also the filler effect increases the compressive strength of the paste matrix. When using an admixture, it was favourable if improvement by filling voids and secondary hydration was more significant than the strength development imparted by the control mix at all ages of curing (41). The n-SiO₂, along with CaSiO₃, gives a collabrated effect of secondary hydration and reinforcement, making them an early age strength developer.

Flexural strength depends more on binding between the various particles than the density of the paste mix (11). Therefore, wollastonite’s reinforcement and colloidal n-SiO₂ strong pozzolanic action will have more effect than pozzolanity of CaSiO₃ alone.

The flexural strength at 3% n-SiO₂ replacement shows the highest values of flexure at 28 days, as seen in Figure 4. The n-SiO₂ will fill the cement pores, thereby increasing the flexural strength. Nanosilica contributes to flexural strength by producing more CSH gel by reacting with CH (22, 42). The CaSiO₃ reinforces the paste matrix and thus shows better results than the control mix, about 54.4%* (0.25 w/b ratio) and 55.3% (0.40 w/b ratio) at 28 days of curing. At 90 days, NS3 showed the highest flexural strength at both 0.25 w/b ratio (29.77%*) and 0.40 w/b ratio (43.2%*). At higher replacement levels of n-SiO₂, the flexural strength decreased, and the increase in flexural strength at 0.25 w/b ratio than 0.40 w/b ratio at all ages wasthe same as discussed above. “*” indicates the percentage change with respect to control mix.

DYE can be determined from the UPV test when the specimen’s density was known. DYE of 0.25 w/b ratio and 0.40 w/b ratio was determined for 28 and 90 days and plotted as a graph in Figure 5. The highest DYE value and transmission time value was observed in NS3 at all w/b ratio and all curing ages. NS3 shows 40% and 58% improvement in DYE to the control mix at 0.25 and 0.40 w/b ratios, respectively. DYE increase can be associated with an increase in the density of the matrix. The increase can also be explained by the reinforcing effect of CaSiO₃ microfibres which acts as a bridge for microcracks (6). The reduction of porosity in the matrix enhances the DYE, shown in the water absorption experiment. DYE was measured in GPA, whereas pulse velocity was measured in km/s.

In this study, we need predictive models from the experimental output. Therefore, we have used regression analysis. The residual plot shown in Figure 6 indicates that the models are sufficient and do not infringe the three assumptions of the ANOVA test (43).

The results obtained from the experiments conducted on the specimens were fitted into a linear regression model to develop mathematical relations between mechanical properties and various parameters. We recommend that Equation [1] be used as a regression model, as it incorporates all continuous variables or parameters. The parameters which affect the compressive strength, flexural strength, and DYE are w/b ratios, curing age, amount of n-SiO₂, and CaSiO₃ (44, 45).

Different parameters can affect the properties of individuals or together in combinations. Variance inflation factor (VIF) can help us find the existence of multicollinearity in the model obtained. While equations have good fil and correlations, the models must be constantly updated to accurately quantify these mechanical properties (46, 47).

The ANOVA model identified significant parameters, and interactions between parameters and response variables were studied.

Equation [2] shows the regression model between compressive strength ad other parameters. The least square method was used to find the coefficients of the model. P-value was considered to find the significant (p ≤ 0.05) and insignificant (p > 0.05) paramters. The statistically insignificant parameters can be omitted from the final Equation [2] (45).

Similarly, flexural strength and DYE were statistically analysed with identical parameters, and the following equations were found.

Different parameters were found according to ASTM C642, and the data was tabulated and analysed (49). Figure 7 shows the percentages of the volume of permeable pores, absorption after immersion, and absorption after boiling for various mixes. The NS3 mix showed the lowest values at 0.25 w/b ratio 28 days result.

Figure 7 shows the least volume of permeable pores and, subsequently, the most negligible water absorption among the paste mixes. At 28 days, a 0.25 w/b ratio of 3% n-SiO₂ and 15% CaSiO₃ (NS3) replacement has almost 20.9% less permeable pores than the control mix. Similarly, for a 0.40 w/b ratio, the difference is 18%, and for a 0.55 w/b ratio, the difference is 18%. At all ages and w/b ratios, NS3 has shown better results than any other paste mixes. At 90 days, 0.25 w/b ratio NS3 has almost 35% less porosity than the control mix, in 0.40 w/b ratio, porosity is reduced by 33% than the control mix, and in 0.55 w/b ratio porosity decreases by 30% than the control mix. Adding CaSiO₃ alone (W15) increases the volume of permeable pores, thereby increasing water absorption. The increase in permeable pores is significantly less in a low w/b ratio, suggesting a reduction in capillary pores and densification compared to higher w/b ratios (1, 50). High n-SiO₂ replacements NS4.5, and NS6 showed better water absorption and volume of pores than the control mix and W15 mix, suggesting that the permeable pores reduce and water absorption through the paste mix also reduce. This can be explained by presence of unreacted n-SiO₂ in the voids as shown in micrographs (Figure 9a), thereby, densifying the blended cement matrix. A higher w/b ratio means higher initial porosity for the voids to be filled by hydrates. Superplasticizers give better compaction to the powder and result in a better hydration reaction as it deflocculates the cement and SCMs (51).

Figure 8 shows the sorptivity values at 28 days and 90 days. The absorption within the first 6 h, i.e., initial sorptivity has the maximum ingress of water and after which the secondary sorptivity happens till 8 days. Here, it can be observed that the sorptivity value increases with the w/b ratio and decreases as curing age increases. At 0.25 w/b ratio, the initial sorptivity and secondary sorptivity is very low compared to 0.40 w/b and 0.55 w/b ratios. This is due to very low porosity at 0.25 w/b ratio. Control mix has the highest sorptivity whereas, the NS3 mix has the lowest sorptivity. The n-SiO₂ present in the mix blocks the pores of present in the matrix and effectively blocks the interconnectivity of the pore network (52). The lowest sorptivity values can be seen in NS3 in all w/b (0.25, 0.40, 0.55) and all curing ages (28 and 90 days). The sorptivity generally happens due to capillary pores prensent in the cement paste matrix. The capillary suction by interconnected pores is the least in the NS3 mix due to matrix densification and pore discontinuity due to mineral admixtures added. At a low w/b ratio, the superplasticizers help in dispersing the n-SiO₂ and CaSiO₃ to the pores and act as fillers, and form hydrates after primary and secondary reactions (3, 5, 22, 39, 42). The highest sorptivity values were found in NS6 and control paste mixes at different curing ages and w/b ratios. This can be explained by high capillary pores present in these mixes enabling transport of water through the paste matrix.

From table 5, we have found the values of permeable pore radius with help of Hagen-Poiseuille formula and Lucas-Washburn model (53). A relationship between pore radius, sorptivity, and porosity which is as follows:

Where, C= constant = (γcosϴ/2η), γ- surface tension of water, ϴ- contact angle of water, η- dynamic viscosity of the liquid, - porosity, I-sorptivity.

As suggested earlier, the pore radius increases as the w/b ratio increases. The pore radius is minimum in NS3 at all w/b ratios, and curing ages and the pore radius is maximum for NS6 and control mix. These results corroborate with the mechanical and durability properties.

Table 6 shows that at a 0.25 w/b ratio, the paste mix NS3 has the lowest change in compressive strength and mass compared to other mixes. It signifies that the NS3 mix at 0.25 w/b has a dense matrix with low capillary pores, which does not permit the ingress of a harmful chemical such as sodium sulphate, thereby increasing the durability of structures. However, table 6 has different paste mixes representing the lowest values, i.e., NS1.5. The change in optimum value is due to an increase in water content; the higher water content increases the porosity, increasing the ingress of harmful chemicals. In table 6, the optimum found at a 0.55 w/b ratio is NS4.5. The increased n-SiO₂ at higher water content resulted in the formation of hydrates at pores. It resulted in an increased density of the matrix and a reduction in the ingress of harmful solutions (5, 22, 37).

After 90 days curing, loss of compressive strength and mass was observed in high w/b ratios of 0.40 and 0.55. Hence, we can characterize these as sulphate attacks (31). At 28 days and a 0.25 w/b ratio, there was no significant change in compressive strength or mass. At 28 days and 0.40 w/b ratio and 0.55 w/b ratio, there is an increase in compressive strength and mass. It suggests that the sodium sulphate solution at 28 days has no significant or negative effect on the paste mixes at 28 days. Thus, there is no need to conduct experiments for the sulphate treatment test at 28 days. However, at 90 days, we can see sodium sulphate attacking the paste mix. Though the decrease is significantly smaller in the 0.25 w/b ratio, the increase is significantly higher at the 0.40 w/b ratio and 0.55 w/b ratio. Especially at a 0.55 w/b ratio, due to the high amount of voids, the decrease in compressive strength and mass is higher. At 28 days of curing of 0.40 w/b ratio and 0.55 w/b ratio, we can see an increase in compressive strength and mass, suggesting that there is an ingress of water with salts into the pores of the paste matrix. However, salts do not affect the paste matrix (i.e., the formation of gypsum and ettringite has not begun yet). There is no significant change in CaSiO₃ only mix W15 with respect to control mix in all w/b ratios and curing ages.

FESEM analysis of the paste mixes was performed to study the influence of CaSiO₃ and n-SiO₂ on the surface morphology of the hydrated cement matrix. At 90 days, unreacted n-SiO₂ was found in NS4.5, and NS6 paste mixes indicating stoppage of the hydration process due to the unavailability of lime (Figure 9a). The n-SiO₂ was found filling the voids. At 0.40 and 0.55 w/b ratios, micro-sized pores were observed in the paste mixes (Figure 9b). CSH gel formations were found in almost all the paste mixes at 28 and 90 days, and the dense CSH formations were found in NS1.5 and NS3 paste mixes (Figure 9c and 9d). The n-SiO₂ formed a very dense CSH gel, increasing the mechanical strength, and it filled the voids, which will keep forming CSH gel filling the voids and increasing the mechanical properties (Figure 9d).

We also observed the CaSiO₃ microfibres reinforcing microvoids and cracks, and n-SiO₂ was found filling the voids of the cement matrix (Figure 9a and 9c). In control mix, presence of calcium oxide and ettringite was determined by EDS after 28 days curing (Figure 9e). The formation of stratlingite (CASH) was observed, which happens when belite reacts with aluminium hydroxide (AH) in the presence of water (Figure 9f) (54). The chemical equation is as given in Equation [6]. This chemical reaction only takes place in the absence of CH.

Stratlingite and CSH are the primary hydration products of blended cement. Stratlingite usually forms in blended cement as in a control cement matrix; there is always a presence of CH (55).